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1 **THIS DOCUIMENT IS A WORK IN PROGRESS.** As long as this notice exists, you sho uld probably ignore everything below it. 1 # Mojo in Chromium
2 2
3 **THIS DOCUIMENT IS A WORK IN PROGRESS.** As long as this notice exists, you
4 should probably ignore everything below it.
3 5
6 This document is intended to serve as a Mojo primer for Chromium developers. No
7 prior knowledge of Mojo is assumed, but you should have a decent grasp of C++
8 and be familiar with Chromium's multi-process architecture as well as common
9 concepts used throughout Chromium such as smart pointers, message loops,
10 callback binding, and so on.
4 11
5 # Introduction 12 [TOC]
6
7 This document is intended to serve as a Mojo primer for Chromium developers. No prior knowledge of Mojo is assumed, but you should have a decent grasp of C++ an d be familiar with Chromium's multi-process architecture as well as common conce pts used throughout Chromium such as smart pointers, message loops, callback bin ding, and so on.
8 13
9 ## Should I Bother Reading This? 14 ## Should I Bother Reading This?
10 15
11 If you're planning to build a Chromium feature that needs IPC and you aren't alr eady using Mojo, you probably want to read this. **Legacy IPC** -- _i.e._, `foo_ messages.h` files, message filters, and the suite of `IPC_MESSAGE_*` macros -- * *is on the verge of deprecation.** 16 If you're planning to build a Chromium feature that needs IPC and you aren't
17 already using Mojo, you probably want to read this. **Legacy IPC** -- _i.e._,
18 `foo_messages.h` files, message filters, and the suite of `IPC_MESSAGE_*` macros
19 -- **is on the verge of deprecation.**
12 20
13 ## Why Mojo? 21 ## Why Mojo?
14 22
15 Mojo provides IPC primitives for pushing messages and data around between transf errable endpoints which may or may not cross process boundaries; it simplifies t hreading with regard to IPC; it standardizes message serialization in a way that 's resilient to versioning issues; and it can be used with relative ease and con sistency across a number of languages including C++, Java, and `JavaScript` -- a ll languages which comprise a significant share of Chromium code. 23 Mojo provides IPC primitives for pushing messages and data around between
24 transferrable endpoints which may or may not cross process boundaries; it
25 simplifies threading with regard to IPC; it standardizes message serialization
26 in a way that's resilient to versioning issues; and it can be used with relative
27 ease and consistency across a number of languages including C++, Java, and
28 `JavaScript` -- all languages which comprise a significant share of Chromium
29 code.
16 30
17 The messaging protocol doesn't strictly need to be used for IPC though, and ther e are some higher-level reasons for this adoption and for the specific approach to integration outlined in this document. 31 The messaging protocol doesn't strictly need to be used for IPC though, and
32 there are some higher-level reasons for this adoption and for the specific
33 approach to integration outlined in this document.
18 34
19 ### Code Health 35 ### Code Health
20 36
21 At the moment we have fairly weak separation between components, with DEPS being the strongest line of defense against increasing complexity. 37 At the moment we have fairly weak separation between components, with DEPS being
38 the strongest line of defense against increasing complexity.
22 39
23 A component Foo might hold a reference to some bit of component Bar's internal s tate, or it might expect Bar to initialize said internal state in some particula r order. These sorts of problems are reasonably well-mitigated by the code revie w process, but they can (and do) still slip through the cracks, and they have a noticeable cumulative effect on complexity as the code base continues to grow. 40 A component Foo might hold a reference to some bit of component Bar's internal
41 state, or it might expect Bar to initialize said internal state in some
42 particular order. These sorts of problems are reasonably well-mitigated by the
43 code review process, but they can (and do) still slip through the cracks, and
44 they have a noticeable cumulative effect on complexity as the code base
45 continues to grow.
24 46
25 We think we can make a lasting positive impact on code health by establishing mo re concrete boundaries between components, and this is something a library like Mojo gives us an opportunity to do. 47 We think we can make a lasting positive impact on code health by establishing
48 more concrete boundaries between components, and this is something a library
49 like Mojo gives us an opportunity to do.
26 50
27 ### Modularity 51 ### Modularity
28 52
29 In addition to code health -- which alone could be addressed in any number of wa ys that don't involve Mojo -- this approach opens doors to build and distribute parts of Chrome separately from the main binary. 53 In addition to code health -- which alone could be addressed in any number of
54 ways that don't involve Mojo -- this approach opens doors to build and
55 distribute parts of Chrome separately from the main binary.
30 56
31 While we're not currently taking advantage of this capability, doing so remains a long-term goal due to prohibitive binary size constraints in emerging mobile m arkets. Many open questions around the feasibility of this goal should be answer ed by the experimental Mandoline project as it unfolds, but the Chromium project can be technically prepared for such a transition in the meantime. 57 While we're not currently taking advantage of this capability, doing so remains
58 a long-term goal due to prohibitive binary size constraints in emerging mobile
59 markets. Many open questions around the feasibility of this goal should be
60 answered by the experimental Mandoline project as it unfolds, but the Chromium
61 project can be technically prepared for such a transition in the meantime.
32 62
33 ### Mandoline 63 ### Mandoline
34 64
35 The Mandoline project is producing a potential replacement for `src/content`. Be cause Mandoline components are Mojo apps, and Chromium is now capable of loading Mojo apps (somethings we'll discuss later), Mojo apps can be shared between bo th projects with minimal effort. Developing your feature as or within a Mojo app lication can mean you're contributing to both Chromium and Mandoline. 65 The Mandoline project is producing a potential replacement for `src/content`.
66 Because Mandoline components are Mojo apps, and Chromium is now capable of
67 loading Mojo apps (somethings we'll discuss later), Mojo apps can be shared
68 between both projects with minimal effort. Developing your feature as or within
69 a Mojo application can mean you're contributing to both Chromium and Mandoline.
36 70
37 # Mojo Overview 71 ## Mojo Overview
38 72
39 This section provides a general overview of Mojo and some of its API features. Y ou can probably skip straight to [Your First Mojo Application](#Your_First_Mojo_ Application.md) if you just want to get to some practical sample code. 73 This section provides a general overview of Mojo and some of its API features.
74 You can probably skip straight to
75 [Your First Mojo Application](#Your-First-Mojo-Application) if you just want to
76 get to some practical sample code.
40 77
41 The Mojo Embedder Development Kit (EDK) provides a suite of low-level IPC primit ives: **message pipes**, **data pipes**, and **shared buffers**. We'll focus pri marily on message pipes and the C++ bindings API in this document. 78 The Mojo Embedder Development Kit (EDK) provides a suite of low-level IPC
79 primitives: **message pipes**, **data pipes**, and **shared buffers**. We'll
80 focus primarily on message pipes and the C++ bindings API in this document.
42 81
43 _TODO: Java and JS bindings APIs should also be covered here._ 82 _TODO: Java and JS bindings APIs should also be covered here._
44 83
45 ## Message Pipes 84 ### Message Pipes
46 85
47 A message pipe is a lightweight primitive for reliable, bidirectional, queued tr ansfer of relatively small packets of data. Every pipe endpoint is identified b y a **handle** -- a unique process-wide integer identifying the endpoint to the EDK. 86 A message pipe is a lightweight primitive for reliable, bidirectional, queued
87 transfer of relatively small packets of data. Every pipe endpoint is identified
88 by a **handle** -- a unique process-wide integer identifying the endpoint to the
89 EDK.
48 90
49 A single message across a pipe consists of a binary payload and an array of zero or more handles to be transferred. A pipe's endpoints may live in the same proc ess or in two different processes. 91 A single message across a pipe consists of a binary payload and an array of zero
92 or more handles to be transferred. A pipe's endpoints may live in the same
93 process or in two different processes.
50 94
51 Pipes are easy to create. The `mojo::MessagePipe` type (see [//third\_party/mojo /src/mojo/public/cpp/system/message\_pipe.h](https://code.google.com/p/chromium/ codesearch#chromium/src/third_party/mojo/src/mojo/public/cpp/system/message_pipe .h)) provides a nice class wrapper with each endpoint represented as a scoped ha ndle type (see members `handle0` and `handle1` and the definition of `mojo::Scop edMessagePipeHandle`). In the same header you can find `WriteMessageRaw` and `Re adMessageRaw` definitions. These are in theory all one needs to begin pushing th ings from one endpoint to the other. 95 Pipes are easy to create. The `mojo::MessagePipe` type (see
96 `/third_party/mojo/src/mojo/public/cpp/system/message_pipe.h`) provides a nice
97 class wrapper with each endpoint represented as a scoped handle type (see
98 members `handle0` and `handle1` and the definition of
99 `mojo::ScopedMessagePipeHandle`). In the same header you can find
100 `WriteMessageRaw` and `ReadMessageRaw` definitions. These are in theory all one
101 needs to begin pushing things from one endpoint to the other.
52 102
53 While it's worth being aware of `mojo::MessagePipe` and the associated raw I/O f unctions, you will rarely if ever have a use for them. Instead you'll typically use bindings code generated from mojom interface definitions, along with the pub lic bindings API which mostly hides the underlying pipes. 103 While it's worth being aware of `mojo::MessagePipe` and the associated raw I/O
104 functions, you will rarely if ever have a use for them. Instead you'll typically
105 use bindings code generated from mojom interface definitions, along with the
106 public bindings API which mostly hides the underlying pipes.
54 107
55 ## Mojom Bindings 108 ### Mojom Bindings
56 109
57 Mojom is the IDL for Mojo interfaces. When given a mojom file, the bindings gene rator outputs a collection of bindings libraries for each supported language. Mo jom syntax is fairly straightforward (TODO: Link to a mojom language spec?). Con sider the example mojom file below: 110 Mojom is the IDL for Mojo interfaces. When given a mojom file, the bindings
111 generator outputs a collection of bindings libraries for each supported
112 language. Mojom syntax is fairly straightforward (TODO: Link to a mojom language
113 spec?). Consider the example mojom file below:
58 114
59 ``` 115 ```
60 // frobinator.mojom 116 // frobinator.mojom
61 module frob; 117 module frob;
62 interface Frobinator { 118 interface Frobinator {
63 Frobinate(); 119 Frobinate();
64 }; 120 };
65 ``` 121 ```
66 122
67 This can be used to generate bindings for a very simple `Frobinator` interface. Bindings are generated at build time and will match the location of the mojom so urce file itself, mapped into the generated output directory for your Chromium b uild. In this case one can expect to find files named `frobinator.mojom.js`, `fr obinator.mojom.cc`, `frobinator.mojom.h`, _etc._ 123 This can be used to generate bindings for a very simple `Frobinator` interface.
124 Bindings are generated at build time and will match the location of the mojom
125 source file itself, mapped into the generated output directory for your Chromium
126 build. In this case one can expect to find files named `frobinator.mojom.js`,
127 `frobinator.mojom.cc`, `frobinator.mojom.h`, _etc._
68 128
69 The C++ header (`frobinator.mojom.h`) generated from this mojom will define a pu re virtual class interface named `frob::Frobinator` with a pure virtual method o f signature `void Frobinate()`. Any class which implements this interface is eff ectively a `Frobinator` service. 129 The C++ header (`frobinator.mojom.h`) generated from this mojom will define a
130 pure virtual class interface named `frob::Frobinator` with a pure virtual method
131 of signature `void Frobinate()`. Any class which implements this interface is
132 effectively a `Frobinator` service.
70 133
71 ## C++ Bindings API 134 ### C++ Bindings API
72 135
73 Before we see an example implementation and usage of the Frobinator, there are a handful of interesting bits in the public C++ bindings API you should be famili ar with. These complement generated bindings code and generally obviate any need to use a `mojo::MessagePipe` directly. 136 Before we see an example implementation and usage of the Frobinator, there are a
137 handful of interesting bits in the public C++ bindings API you should be
138 familiar with. These complement generated bindings code and generally obviate
139 any need to use a `mojo::MessagePipe` directly.
74 140
75 In all of the cases below, `T` is the type of a generated bindings class interfa ce, such as the `frob::Frobinator` discussed above. 141 In all of the cases below, `T` is the type of a generated bindings class
142 interface, such as the `frob::Frobinator` discussed above.
76 143
77 ### `mojo::InterfacePtr<T>` 144 #### `mojo::InterfacePtr<T>`
78 145
79 Defined in [//third\_party/mojo/src/mojo/public/cpp/bindings/interface\_ptr.h](h ttps://code.google.com/p/chromium/codesearch#chromium/src/third_party/mojo/src/m ojo/public/cpp/bindings/interface_ptr.h). 146 Defined in `/third_party/mojo/src/mojo/public/cpp/bindings/interface_ptr.h`.
80 147
81 `mojo::InterfacePtr<T>` is a typed proxy for a service of type `T`, which can be bound to a message pipe endpoint. This class implements every interface method on `T` by serializing a message (encoding the method call and its arguments) and writing it to the pipe (if bound.) This is the standard way for C++ code to tal k to any Mojo service. 148 `mojo::InterfacePtr<T>` is a typed proxy for a service of type `T`, which can be
149 bound to a message pipe endpoint. This class implements every interface method
150 on `T` by serializing a message (encoding the method call and its arguments) and
151 writing it to the pipe (if bound.) This is the standard way for C++ code to talk
152 to any Mojo service.
82 153
83 For illustrative purposes only, we can create a message pipe and bind an `Interf acePtr` to one end as follows: 154 For illustrative purposes only, we can create a message pipe and bind an
155 `InterfacePtr` to one end as follows:
84 156
85 ``` 157 ```
86 mojo::MessagePipe pipe; 158 mojo::MessagePipe pipe;
87 mojo::InterfacePtr<frob::Frobinator> frobinator; 159 mojo::InterfacePtr<frob::Frobinator> frobinator;
88 frobinator.Bind( 160 frobinator.Bind(
89 mojo::InterfacePtrInfo<frob::Frobinator>(pipe.handle0.Pass(), 0u)); 161 mojo::InterfacePtrInfo<frob::Frobinator>(pipe.handle0.Pass(), 0u));
90 ``` 162 ```
91 163
92 You could then call `frobinator->Frobinate()` and read the encoded `Frobinate` m essage from the other side of the pipe (`handle1`.) You most likely don't want t o do this though, because as you'll soon see there's a nicer way to establish se rvice pipes. 164 You could then call `frobinator->Frobinate()` and read the encoded `Frobinate`
165 message from the other side of the pipe (`handle1`.) You most likely don't want
166 to do this though, because as you'll soon see there's a nicer way to establish
167 service pipes.
93 168
94 ### `mojo::InterfaceRequest<T>` 169 #### `mojo::InterfaceRequest<T>`
95 170
96 Defined in [//third\_party/mojo/src/mojo/public/cpp/bindings/interface\_request. h](https://code.google.com/p/chromium/codesearch#chromium/src/third_party/mojo/s rc/mojo/public/cpp/bindings/interface_request.h). 171 Defined in `/third_party/mojo/src/mojo/public/cpp/bindings/interface_request.h`.
97 172
98 `mojo::InterfaceRequest<T>` is a typed container for a message pipe endpoint tha t should _eventually_ be bound to a service implementation. An `InterfaceRequest ` doesn't actually _do_ anything, it's just a way of holding onto an endpoint wi thout losing interface type information. 173 `mojo::InterfaceRequest<T>` is a typed container for a message pipe endpoint
174 that should _eventually_ be bound to a service implementation. An
175 `InterfaceRequest` doesn't actually _do_ anything, it's just a way of holding
176 onto an endpoint without losing interface type information.
99 177
100 A common usage pattern is to create a pipe, bind one end to an `InterfacePtr<T>` , and pass the other end off to someone else (say, over some other message pipe) who is expected to eventually bind it to a concrete service implementation. `In terfaceRequest<T>` is here for that purpose and is, as we'll see later, a first- class concept in Mojom interface definitions. 178 A common usage pattern is to create a pipe, bind one end to an
179 `InterfacePtr<T>`, and pass the other end off to someone else (say, over some
180 other message pipe) who is expected to eventually bind it to a concrete service
181 implementation. `InterfaceRequest<T>` is here for that purpose and is, as we'll
182 see later, a first-class concept in Mojom interface definitions.
101 183
102 As with `InterfacePtr<T>`, we can manually bind an `InterfaceRequest<T>` to a pi pe endpoint: 184 As with `InterfacePtr<T>`, we can manually bind an `InterfaceRequest<T>` to a
185 pipe endpoint:
103 186
104 ``` 187 ```
105 mojo::MessagePipe pipe; 188 mojo::MessagePipe pipe;
106 189
107 mojo::InterfacePtr<frob::Frobinator> frobinator; 190 mojo::InterfacePtr<frob::Frobinator> frobinator;
108 frobinator.Bind( 191 frobinator.Bind(
109 mojo::InterfacePtrInfo<frob::Frobinator>(pipe.handle0.Pass(), 0u)); 192 mojo::InterfacePtrInfo<frob::Frobinator>(pipe.handle0.Pass(), 0u));
110 193
111 mojo::InterfaceRequest<frob::Frobinator> frobinator_request; 194 mojo::InterfaceRequest<frob::Frobinator> frobinator_request;
112 frobinator_request.Bind(pipe.handle1.Pass()); 195 frobinator_request.Bind(pipe.handle1.Pass());
113 ``` 196 ```
114 197
115 At this point we could start making calls to `frobinator->Frobinate()` as before , but they'll just sit in queue waiting for the request side to be bound. Note t hat the basic logic in the snippet above is such a common pattern that there's a convenient API function which does it for us. 198 At this point we could start making calls to `frobinator->Frobinate()` as
199 before, but they'll just sit in queue waiting for the request side to be bound.
200 Note that the basic logic in the snippet above is such a common pattern that
201 there's a convenient API function which does it for us.
116 202
117 ### `mojo::GetProxy<T>` 203 #### `mojo::GetProxy<T>`
118 204
119 Defined in [//third\_party/mojo/src/mojo/public/cpp/bindings/interface\_request. h](https://code.google.com/p/chromium/codesearch#chromium/src/third_party/mojo/s rc/mojo/public/cpp/bindings/interface_request.h). 205 Defined in
206 `/third_party/mojo/src/mojo/public/cpp/bindings/interface`_request.h`.
120 207
121 `mojo::GetProxy<T>` is the function you will most commonly use to create a new m essage pipe. Its signature is as follows: 208 `mojo::GetProxy<T>` is the function you will most commonly use to create a new
209 message pipe. Its signature is as follows:
122 210
123 ``` 211 ```
124 template <typename T> 212 template <typename T>
125 mojo::InterfaceRequest<T> GetProxy(mojo::InterfacePtr<T>* ptr); 213 mojo::InterfaceRequest<T> GetProxy(mojo::InterfacePtr<T>* ptr);
126 ``` 214 ```
127 215
128 This function creates a new message pipe, binds one end to the given `InterfaceP tr` argument, and binds the other end to a new `InterfaceRequest` which it then returns. Equivalent to the sample code just above is the following snippet: 216 This function creates a new message pipe, binds one end to the given
217 `InterfacePtr` argument, and binds the other end to a new `InterfaceRequest`
218 which it then returns. Equivalent to the sample code just above is the following
219 snippet:
129 220
130 ``` 221 ```
131 mojo::InterfacePtr<frob::Frobinator> frobinator; 222 mojo::InterfacePtr<frob::Frobinator> frobinator;
132 mojo::InterfaceRequest<frob::Frobinator> frobinator_request = 223 mojo::InterfaceRequest<frob::Frobinator> frobinator_request =
133 mojo::GetProxy(&frobinator); 224 mojo::GetProxy(&frobinator);
134 ``` 225 ```
135 226
136 ### `mojo::Binding<T>` 227 #### `mojo::Binding<T>`
137 228
138 Defined in [//third\_party/mojo/src/mojo/public/cpp/bindings/binding.h](https:// code.google.com/p/chromium/codesearch#chromium/src/third_party/mojo/src/mojo/pub lic/cpp/bindings/binding.h). 229 Defined in `/third_party/mojo/src/mojo/public/cpp/bindings/binding.h`.
139 230
140 Binds one end of a message pipe to an implementation of service `T`. A message s ent from the other end of the pipe will be read and, if successfully decoded as a `T` message, will invoke the corresponding call on the bound `T` implementatio n. A `Binding<T>` must be constructed over an instance of `T` (which itself usua lly owns said `Binding` object), and its bound pipe is usually taken from a pass ed `InterfaceRequest<T>`. 231 Binds one end of a message pipe to an implementation of service `T`. A message
232 sent from the other end of the pipe will be read and, if successfully decoded as
233 a `T` message, will invoke the corresponding call on the bound `T`
234 implementation. A `Binding<T>` must be constructed over an instance of `T`
235 (which itself usually owns said `Binding` object), and its bound pipe is usually
236 taken from a passed `InterfaceRequest<T>`.
141 237
142 A common usage pattern looks something like this: 238 A common usage pattern looks something like this:
143 239
144 ``` 240 ```
145 #include "components/frob/public/interfaces/frobinator.mojom.h" 241 #include "components/frob/public/interfaces/frobinator.mojom.h"
146 #include "third_party/mojo/src/mojo/public/cpp/bindings/binding.h" 242 #include "third_party/mojo/src/mojo/public/cpp/bindings/binding.h"
147 #include "third_party/mojo/src/mojo/public/cpp/bindings/interface_request.h" 243 #include "third_party/mojo/src/mojo/public/cpp/bindings/interface_request.h"
148 244
149 class FrobinatorImpl : public frob::Frobinator { 245 class FrobinatorImpl : public frob::Frobinator {
150 public: 246 public:
151 FrobinatorImpl(mojo::InterfaceRequest<frob::Frobinator> request) 247 FrobinatorImpl(mojo::InterfaceRequest<frob::Frobinator> request)
152 : binding_(this, request.Pass()) {} 248 : binding_(this, request.Pass()) {}
153 ~FrobinatorImpl() override {} 249 ~FrobinatorImpl() override {}
154 250
155 private: 251 private:
156 // frob::Frobinator: 252 // frob::Frobinator:
157 void Frobinate() override { /* ... */ } 253 void Frobinate() override { /* ... */ }
158 254
159 mojo::Binding<frob::Frobinator> binding_; 255 mojo::Binding<frob::Frobinator> binding_;
160 }; 256 };
161 ``` 257 ```
162 258
163 And then we could write some code to test this: 259 And then we could write some code to test this:
164 260
165 ``` 261 ```
166 // Fun fact: The bindings generator emits a type alias like this for every 262 // Fun fact: The bindings generator emits a type alias like this for every
167 // interface type. frob::FrobinatorPtr is an InterfacePtr<frob::Frobinator>. 263 // interface type. frob::FrobinatorPtr is an InterfacePtr<frob::Frobinator>.
168 frob::FrobinatorPtr frobinator; 264 frob::FrobinatorPtr frobinator;
169 scoped_ptr<FrobinatorImpl> impl( 265 scoped_ptr<FrobinatorImpl> impl(
170 new FrobinatorImpl(mojo::GetProxy(&frobinator))); 266 new FrobinatorImpl(mojo::GetProxy(&frobinator)));
171 frobinator->Frobinate(); 267 frobinator->Frobinate();
172 ``` 268 ```
173 269
174 This will _eventually_ call `FrobinatorImpl::Frobinate()`. "Eventually," because the sequence of events when `frobinator->Frobinate()` is called is roughly as f ollows: 270 This will _eventually_ call `FrobinatorImpl::Frobinate()`. "Eventually," because
271 the sequence of events when `frobinator->Frobinate()` is called is roughly as
272 follows:
175 273
176 1. A new message buffer is allocated and filled with an encoded 'Frobinate' me ssage. 274 1. A new message buffer is allocated and filled with an encoded 'Frobinate'
177 1. The EDK is asked to write this message to the pipe endpoint owned by the `F robinatorPtr`. 275 message.
178 1. If the call didn't happen on the Mojo IPC thread for this process, EDK hops to the Mojo IPC thread. 276 1. The EDK is asked to write this message to the pipe endpoint owned by the
179 1. The EDK writes the message to the pipe. In this case the pipe endpoints liv e in the same process, so this essentially a glorified `memcpy`. If they lived i n different processes this would be the point at which the data moved across a r eal IPC channel. 277 `FrobinatorPtr`.
180 1. The EDK on the other end of the pipe is awoken on the Mojo IPC thread and a lerted to the message arrival. 278 1. If the call didn't happen on the Mojo IPC thread for this process, EDK hops
181 1. The EDK reads the message. 279 to the Mojo IPC thread.
182 1. If the bound receiver doesn't live on the Mojo IPC thread, the EDK hops to the receiver's thread. 280 1. The EDK writes the message to the pipe. In this case the pipe endpoints live
183 1. The message is passed on to the receiver. In this case the receiver is gene rated bindings code, via `Binding<T>`. This code decodes and validates the `Frob inate` message. 281 in the same process, so this essentially a glorified `memcpy`. If they lived
184 1. `FrobinatorImpl::Frobinate()` is called on the bound implementation. 282 in different processes this would be the point at which the data moved
283 across a real IPC channel.
284 1. The EDK on the other end of the pipe is awoken on the Mojo IPC thread and
285 alerted to the message arrival.
286 1. The EDK reads the message.
287 1. If the bound receiver doesn't live on the Mojo IPC thread, the EDK hops to
288 the receiver's thread.
289 1. The message is passed on to the receiver. In this case the receiver is
290 generated bindings code, via `Binding<T>`. This code decodes and validates
291 the `Frobinate` message.
292 1. `FrobinatorImpl::Frobinate()` is called on the bound implementation.
185 293
186 So as you can see, the call to `Frobinate()` may result in up to two thread hops and one process hop before the service implementation is invoked. 294 So as you can see, the call to `Frobinate()` may result in up to two thread hops
295 and one process hop before the service implementation is invoked.
187 296
188 ### `mojo::StrongBinding<T>` 297 #### `mojo::StrongBinding<T>`
189 298
190 Defined in [//third\_party/mojo/src/mojo/public/cpp/bindings/strong\_binding.h]( https://code.google.com/p/chromium/codesearch#chromium/src/third_party/mojo/src/ mojo/public/cpp/bindings/strong_binding.h). 299 Defined in `third_party/mojo/src/mojo/public/cpp/bindings/strong_binding.h`.
191 300
192 `mojo::StrongBinding<T>` is just like `mojo::Binding<T>` with the exception that a `StrongBinding` takes ownership of the bound `T` instance. The instance is de stroyed whenever the bound message pipe is closed. This is convenient in cases w here you want a service implementation to live as long as the pipe it's servicin g, but like all features with clever lifetime semantics, it should be used with caution. 301 `mojo::StrongBinding<T>` is just like `mojo::Binding<T>` with the exception that
302 a `StrongBinding` takes ownership of the bound `T` instance. The instance is
303 destroyed whenever the bound message pipe is closed. This is convenient in cases
304 where you want a service implementation to live as long as the pipe it's
305 servicing, but like all features with clever lifetime semantics, it should be
306 used with caution.
193 307
194 ## The Mojo Shell 308 ## The Mojo Shell
195 309
196 Both Chromium and Mandoline run a central **shell** component which is used to c oordinate communication among all Mojo applications (see the next section for an overview of Mojo applications.) 310 Both Chromium and Mandoline run a central **shell** component which is used to
311 coordinate communication among all Mojo applications (see the next section for
312 an overview of Mojo applications.)
197 313
198 Every application receives a proxy to this shell upon initialization, and it is exclusively through this proxy that an application can request connections to ot her applications. The `mojo::Shell` interface provided by this proxy is defined as follows: 314 Every application receives a proxy to this shell upon initialization, and it is
315 exclusively through this proxy that an application can request connections to
316 other applications. The `mojo::Shell` interface provided by this proxy is
317 defined as follows:
199 318
200 ``` 319 ```
201 module mojo; 320 module mojo;
202 interface Shell { 321 interface Shell {
203 ConnectToApplication(URLRequest application_url, 322 ConnectToApplication(URLRequest application_url,
204 ServiceProvider&? services, 323 ServiceProvider&? services,
205 ServiceProvider? exposed_services); 324 ServiceProvider? exposed_services);
206 QuitApplication(); 325 QuitApplication();
207 }; 326 };
208 ``` 327 ```
209 328
210 and as for the `mojo::ServiceProvider` interface: 329 and as for the `mojo::ServiceProvider` interface:
211 330
212 ``` 331 ```
213 module mojo; 332 module mojo;
214 interface ServiceProvider { 333 interface ServiceProvider {
215 ConnectToService(string interface_name, handle<message_pipe> pipe); 334 ConnectToService(string interface_name, handle<message_pipe> pipe);
216 }; 335 };
217 ``` 336 ```
218 337
219 Definitions for these interfaces can be found in [//mojo/application/public/inte rfaces](https://code.google.com/p/chromium/codesearch#chromium/src/mojo/applicat ion/public/interfaces/). Also note that `mojo::URLRequest` is a Mojo struct defi ned in [//mojo/services/network/public/interfaces/url\_loader.mojom](https://cod e.google.com/p/chromium/codesearch#chromium/src/mojo/services/network/public/int erfaces/url_loader.mojom). 338 Definitions for these interfaces can be found in
339 `/mojo/application/public/interfaces`. Also note that `mojo::URLRequest` is a
340 Mojo struct defined in
341 `/mojo/services/network/public/interfaces/url_loader.mojom`.
220 342
221 Note that there's some new syntax in the mojom for `ConnectToApplication` above. The '?' signifies a nullable value and the '&' signifies an interface request r ather than an interface proxy. 343 Note that there's some new syntax in the mojom for `ConnectToApplication` above.
344 The '?' signifies a nullable value and the '&' signifies an interface request
345 rather than an interface proxy.
222 346
223 The argument `ServiceProvider&? services` indicates that the caller should pass an `InterfaceRequest<ServiceProvider>` as the second argument, but that it need not be bound to a pipe (i.e., it can be "null" in which case it's ignored.) 347 The argument `ServiceProvider&? services` indicates that the caller should pass
348 an `InterfaceRequest<ServiceProvider>` as the second argument, but that it need
349 not be bound to a pipe (i.e., it can be "null" in which case it's ignored.)
224 350
225 The argument `ServiceProvider? exposed_services` indicates that the caller shoul d pass an `InterfacePtr<ServiceProvider>` as the third argument, but that it may also be null. 351 The argument `ServiceProvider? exposed_services` indicates that the caller
352 should pass an `InterfacePtr<ServiceProvider>` as the third argument, but that
353 it may also be null.
226 354
227 `ConnectToApplication` asks the shell to establish a connection between the call er and some other app the shell might know about. In the event that a connection can be established -- which may involve the shell starting a new instance of th e target app -- the given `services` request (if not null) will be bound to a se rvice provider in the target app. The target app may in turn use the passed `exp osed_services` proxy (if not null) to request services from the connecting app. 355 `ConnectToApplication` asks the shell to establish a connection between the
356 caller and some other app the shell might know about. In the event that a
357 connection can be established -- which may involve the shell starting a new
358 instance of the target app -- the given `services` request (if not null) will be
359 bound to a service provider in the target app. The target app may in turn use
360 the passed `exposed_services` proxy (if not null) to request services from the
361 connecting app.
228 362
229 ## Mojo Applications 363 ### Mojo Applications
230 364
231 All code which runs in a Mojo environment, apart from the shell itself (see abov e), belongs to one Mojo **application** or another**`**`**. The term "applicatio n" in this context is a common source of confusion, but it's really a simple con cept. In essence an application is anything which implements the following Mojom interface: 365 All code which runs in a Mojo environment, apart from the shell itself (see
366 above), belongs to one Mojo **application** or another**`**`**. The term
367 "application" in this context is a common source of confusion, but it's really a
368 simple concept. In essence an application is anything which implements the
369 following Mojom interface:
232 370
233 ``` 371 ```
234 module mojo; 372 module mojo;
235 interface Application { 373 interface Application {
236 Initialize(Shell shell, string url); 374 Initialize(Shell shell, string url);
237 AcceptConnection(string requestor_url, 375 AcceptConnection(string requestor_url,
238 ServiceProvider&? services, 376 ServiceProvider&? services,
239 ServiceProvider? exposed_services, 377 ServiceProvider? exposed_services,
240 string resolved_url); 378 string resolved_url);
241 OnQuitRequested() => (bool can_quit); 379 OnQuitRequested() => (bool can_quit);
242 }; 380 };
243 ``` 381 ```
244 382
245 Of course, in Chromium and Mandoline environments this interface is obscured fro m application code and applications should generally just implement `mojo::Appli cationDelegate` (defined in [//mojo/application/public/cpp/application\_delegate .h](https://code.google.com/p/chromium/codesearch#chromium/src/mojo/application/ public/cpp/application_delegate.h).) We'll see a concrete example of this in the next section, [Your First Mojo Application](#Your_First_Mojo_Application.md). 383 Of course, in Chromium and Mandoline environments this interface is obscured
384 from application code and applications should generally just implement
385 `mojo::ApplicationDelegate` (defined in
386 `/mojo/application/public/cpp/application_delegate.h`.) We'll see a concrete
387 example of this in the next section,
388 [Your First Mojo Application](#Your-First-Mojo-Application).
246 389
247 The takeaway here is that an application can be anything. It's not necessarily a new process (though at the moment, it's at least a new thread). Applications ca n connect to each other, and these connections are the mechanism through which s eparate components expose services to each other. 390 The takeaway here is that an application can be anything. It's not necessarily a
391 new process (though at the moment, it's at least a new thread). Applications can
392 connect to each other, and these connections are the mechanism through which
393 separate components expose services to each other.
248 394
249 **`**`**NOTE: This is not true in Chromium today, but it should be eventually. F or some components (like render frames, or arbitrary browser process code) we pr ovide APIs which allow non-Mojo-app-code to masquerade as a Mojo app and therefo re connect to real Mojo apps through the shell. 395 **NOTE##: This is not true in Chromium today, but it should be eventually. For
396 some components (like render frames, or arbitrary browser process code) we
397 provide APIs which allow non-Mojo-app-code to masquerade as a Mojo app and
398 therefore connect to real Mojo apps through the shell.
250 399
251 ## Other IPC Primitives 400 ### Other IPC Primitives
252 401
253 Finally, it's worth making brief mention of the other types of IPC primitives Mo jo provides apart from message pipes. A **data pipe** is a unidirectional channe l for pushing around raw data in bulk, and a **shared buffer** is (unsurprisingl y) a shared memory primitive. Both of these objects use the same type of transfe rable handle as message pipe endpoints, and can therefore be transferred across message pipes, potentially to other processes. 402 Finally, it's worth making brief mention of the other types of IPC primitives
403 Mojo provides apart from message pipes. A **data pipe** is a unidirectional
404 channel for pushing around raw data in bulk, and a **shared buffer** is
405 (unsurprisingly) a shared memory primitive. Both of these objects use the same
406 type of transferable handle as message pipe endpoints, and can therefore be
407 transferred across message pipes, potentially to other processes.
254 408
255 # Your First Mojo Application 409 ## Your First Mojo Application
256 410
257 In this section, we're going to build a simple Mojo application that can be run in isolation using Mandoline's `mojo_runner` binary. After that we'll add a serv ice to the app and set up a test suite to connect and test that service. 411 In this section, we're going to build a simple Mojo application that can be run
412 in isolation using Mandoline's `mojo_runner` binary. After that we'll add a
413 service to the app and set up a test suite to connect and test that service.
258 414
259 ## Hello, world! 415 ### Hello, world!
260 416
261 So, you're building a new Mojo app and it has to live somewhere. For the foresee able future we'll likely be treating `//components` as a sort of top-level home for new Mojo apps in the Chromium tree. Any component application you build shou ld probably go there. Let's create some basic files to kick things off. You may want to start a new local Git branch to isolate any changes you make while worki ng through this. 417 So, you're building a new Mojo app and it has to live somewhere. For the
418 foreseeable future we'll likely be treating `//components` as a sort of
419 top-level home for new Mojo apps in the Chromium tree. Any component application
420 you build should probably go there. Let's create some basic files to kick things
421 off. You may want to start a new local Git branch to isolate any changes you
422 make while working through this.
262 423
263 First create a new `//components/hello` directory. Inside this directory we're g oing to add the following files: 424 First create a new `//components/hello` directory. Inside this directory we're
425 going to add the following files:
264 426
265 **components/hello/main.cc** 427 **components/hello/main.cc**
266 ``` 428 ```
267 #include "base/logging.h" 429 #include "base/logging.h"
268 #include "third_party/mojo/src/mojo/public/c/system/main.h" 430 #include "third_party/mojo/src/mojo/public/c/system/main.h"
269 431
270 MojoResult MojoMain(MojoHandle shell_handle) { 432 MojoResult MojoMain(MojoHandle shell_handle) {
271 LOG(ERROR) << "Hello, world!"; 433 LOG(ERROR) << "Hello, world!";
272 return MOJO_RESULT_OK; 434 return MOJO_RESULT_OK;
273 }; 435 };
274 ``` 436 ```
275 437
276
277 **components/hello/BUILD.gn** 438 **components/hello/BUILD.gn**
278 ``` 439 ```
279 import("//mojo/public/mojo_application.gni") 440 import("//mojo/public/mojo_application.gni")
280 441
281 mojo_native_application("hello") { 442 mojo_native_application("hello") {
282 sources = [ 443 sources = [
283 "main.cc", 444 "main.cc",
284 ] 445 ]
285 deps = [ 446 deps = [
286 "//base", 447 "//base",
287 "//mojo/environment:chromium", 448 "//mojo/environment:chromium",
288 ] 449 ]
289 } 450 }
290 ``` 451 ```
291 452
292 For the sake of this example you'll also want to add your component as a depende ncy somewhere in your local checkout to ensure its build files are generated. Th e easiest thing to do there is probably to add a dependency on `"//components/he llo"` in the `"gn_all"` target of the top-level `//BUILD.gn`. 453 For the sake of this example you'll also want to add your component as a
454 dependency somewhere in your local checkout to ensure its build files are
455 generated. The easiest thing to do there is probably to add a dependency on
456 `"//components/hello"` in the `"gn_all"` target of the top-level `//BUILD.gn`.
293 457
294 Assuming you have a GN output directory at `out_gn/Debug`, you can build the Moj o runner along with your shiny new app: 458 Assuming you have a GN output directory at `out_gn/Debug`, you can build the
459 Mojo runner along with your shiny new app:
295 460
296 ```
297 ninja -C out_gn/Debug mojo_runner components/hello 461 ninja -C out_gn/Debug mojo_runner components/hello
298 ```
299 462
300 In addition to the `mojo_runner` executable, this will produce a new binary at ` out_gn/Debug/hello/hello.mojo`. This binary is essentially a shared library whic h exports your `MojoMain` function. 463 In addition to the `mojo_runner` executable, this will produce a new binary at
464 `out_gn/Debug/hello/hello.mojo`. This binary is essentially a shared library
465 which exports your `MojoMain` function.
301 466
302 `mojo_runner` takes an application URL as its only argument and runs the corresp onding application. In its current state it resolves `mojo`-scheme URLs such tha t `"mojo:foo"` maps to the file `"foo/foo.mojo"` relative to the `mojo_runner` p ath (_i.e._ your output directory.) This means you can run your new app with the following command: 467 `mojo_runner` takes an application URL as its only argument and runs the
468 corresponding application. In its current state it resolves `mojo`-scheme URLs
469 such that `"mojo:foo"` maps to the file `"foo/foo.mojo"` relative to the
470 `mojo_runner` path (_i.e._ your output directory.) This means you can run your
471 new app with the following command:
303 472
304 ```
305 out_gn/Debug/mojo_runner mojo:hello 473 out_gn/Debug/mojo_runner mojo:hello
306 ```
307 474
308 You should see our little `"Hello, world!"` error log followed by a hanging appl ication. You can `^C` to kill it. 475 You should see our little `"Hello, world!"` error log followed by a hanging
476 application. You can `^C` to kill it.
309 477
310 ## Exposing Services 478 ### Exposing Services
311 479
312 An app that prints `"Hello, world!"` isn't terribly interesting. At a bare minim um your app should implement `mojo::ApplicationDelegate` and expose at least one service to connecting applications. 480 An app that prints `"Hello, world!"` isn't terribly interesting. At a bare
481 minimum your app should implement `mojo::ApplicationDelegate` and expose at
482 least one service to connecting applications.
313 483
314 Let's update `main.cc` with the following contents: 484 Let's update `main.cc` with the following contents:
315 485
316 **components/hello/main.cc** 486 **components/hello/main.cc**
317 ``` 487 ```
318 #include "components/hello/hello_app.h" 488 #include "components/hello/hello_app.h"
319 #include "mojo/application/public/cpp/application_runner.h" 489 #include "mojo/application/public/cpp/application_runner.h"
320 #include "third_party/mojo/src/mojo/public/c/system/main.h" 490 #include "third_party/mojo/src/mojo/public/c/system/main.h"
321 491
322 MojoResult MojoMain(MojoHandle shell_handle) { 492 MojoResult MojoMain(MojoHandle shell_handle) {
323 mojo::ApplicationRunner runner(new hello::HelloApp); 493 mojo::ApplicationRunner runner(new hello::HelloApp);
324 return runner.Run(shell_handle); 494 return runner.Run(shell_handle);
325 }; 495 };
326 ``` 496 ```
327 497
328 This is a pretty typical looking `MojoMain`. Most of the time this is all you wa nt -- a `mojo::ApplicationRunner` constructed over a `mojo::ApplicationDelegate` instance, `Run()` with the pipe handle received from the shell. We'll add some new files to the app as well: 498 This is a pretty typical looking `MojoMain`. Most of the time this is all you
499 want -- a `mojo::ApplicationRunner` constructed over a
500 `mojo::ApplicationDelegate` instance, `Run()` with the pipe handle received from
501 the shell. We'll add some new files to the app as well:
329 502
330 **components/hello/public/interfaces/greeter.mojom** 503 **components/hello/public/interfaces/greeter.mojom**
331 ``` 504 ```
332 module hello; 505 module hello;
333 interface Greeter { 506 interface Greeter {
334 Greet(string name) => (string greeting); 507 Greet(string name) => (string greeting);
335 }; 508 };
336 ``` 509 ```
337 510
338 Note the new arrow syntax on the `Greet` method. This indicates that the caller expects a response from the service. 511 Note the new arrow syntax on the `Greet` method. This indicates that the caller
512 expects a response from the service.
339 513
340 **components/hello/public/interfaces/BUILD.gn** 514 **components/hello/public/interfaces/BUILD.gn**
341 ``` 515 ```
342 import("//third_party/mojo/src/mojo/public/tools/bindings/mojom.gni") 516 import("//third_party/mojo/src/mojo/public/tools/bindings/mojom.gni")
343 517
344 mojom("interfaces") { 518 mojom("interfaces") {
345 sources = [ 519 sources = [
346 "greeter.mojom", 520 "greeter.mojom",
347 ] 521 ]
348 } 522 }
349 ``` 523 ```
350 524
351 **components/hello/hello\_app.h** 525 **components/hello/hello_app.h**
352 ``` 526 ```
353 #ifndef COMPONENTS_HELLO_HELLO_APP_H_ 527 #ifndef COMPONENTS_HELLO_HELLO_APP_H_
354 #define COMPONENTS_HELLO_HELLO_APP_H_ 528 #define COMPONENTS_HELLO_HELLO_APP_H_
355 529
356 #include "base/macros.h" 530 #include "base/macros.h"
357 #include "components/hello/public/interfaces/greeter.mojom.h" 531 #include "components/hello/public/interfaces/greeter.mojom.h"
358 #include "mojo/application/public/cpp/application_delegate.h" 532 #include "mojo/application/public/cpp/application_delegate.h"
359 #include "mojo/application/public/cpp/interface_factory.h" 533 #include "mojo/application/public/cpp/interface_factory.h"
360 534
361 namespace hello { 535 namespace hello {
(...skipping 15 matching lines...) Expand all
377 551
378 DISALLOW_COPY_AND_ASSIGN(HelloApp); 552 DISALLOW_COPY_AND_ASSIGN(HelloApp);
379 }; 553 };
380 554
381 } // namespace hello 555 } // namespace hello
382 556
383 #endif // COMPONENTS_HELLO_HELLO_APP_H_ 557 #endif // COMPONENTS_HELLO_HELLO_APP_H_
384 ``` 558 ```
385 559
386 560
387 **components/hello/hello\_app.cc** 561 **components/hello/hello_app.cc**
388 ``` 562 ```
389 #include "base/macros.h" 563 #include "base/macros.h"
390 #include "components/hello/hello_app.h" 564 #include "components/hello/hello_app.h"
391 #include "mojo/application/public/cpp/application_connection.h" 565 #include "mojo/application/public/cpp/application_connection.h"
392 #include "third_party/mojo/src/mojo/public/cpp/bindings/interface_request.h" 566 #include "third_party/mojo/src/mojo/public/cpp/bindings/interface_request.h"
393 #include "third_party/mojo/src/mojo/public/cpp/bindings/strong_binding.h" 567 #include "third_party/mojo/src/mojo/public/cpp/bindings/strong_binding.h"
394 568
395 namespace hello { 569 namespace hello {
396 570
397 namespace { 571 namespace {
(...skipping 33 matching lines...) Expand 10 before | Expand all | Expand 10 after
431 605
432 void HelloApp::Create( 606 void HelloApp::Create(
433 mojo::ApplicationConnection* connection, 607 mojo::ApplicationConnection* connection,
434 mojo::InterfaceRequest<Greeter> request) { 608 mojo::InterfaceRequest<Greeter> request) {
435 new GreeterImpl(request.Pass()); 609 new GreeterImpl(request.Pass());
436 } 610 }
437 611
438 } // namespace hello 612 } // namespace hello
439 ``` 613 ```
440 614
441 And finally we need to update our app's `BUILD.gn` to add some new sources and d ependencies: 615 And finally we need to update our app's `BUILD.gn` to add some new sources and
616 dependencies:
442 617
443 **components/hello/BUILD.gn** 618 **components/hello/BUILD.gn**
444 ``` 619 ```
445 import("//mojo/public/mojo_application.gni") 620 import("//mojo/public/mojo_application.gni")
446 621
447 source_set("lib") { 622 source_set("lib") {
448 sources = [ 623 sources = [
449 "hello_app.cc", 624 "hello_app.cc",
450 "hello_app.h", 625 "hello_app.h",
451 ] 626 ]
452 deps = [ 627 deps = [
453 "//base", 628 "//base",
454 "//components/hello/public/interfaces", 629 "//components/hello/public/interfaces",
455 "//mojo/application/public/cpp", 630 "//mojo/application/public/cpp",
456 "//mojo/environment:chromium", 631 "//mojo/environment:chromium",
457 ] 632 ]
458 } 633 }
459 634
460 mojo_native_application("hello") { 635 mojo_native_application("hello") {
461 sources = [ 636 sources = [
462 "main.cc", 637 "main.cc",
463 ], 638 ],
464 deps = [ ":lib" ] 639 deps = [ ":lib" ]
465 } 640 }
466 ``` 641 ```
467 642
468 Note that we build the bulk of our application sources as a static library separ ate from the `MojoMain` definition. Following this convention is particularly us eful for Chromium integration, as we'll see later. 643 Note that we build the bulk of our application sources as a static library
644 separate from the `MojoMain` definition. Following this convention is
645 particularly useful for Chromium integration, as we'll see later.
469 646
470 There's a lot going on here and it would be useful to familiarize yourself with the definitions of `mojo::ApplicationDelegate`, `mojo::ApplicationConnection`, a nd `mojo::InterfaceFactory<T>`. The TL;DR though is that if someone connects to this app and requests a service named `"hello::Greeter"`, the app will create a new `GreeterImpl` and bind it to that request pipe. From there the connecting ap p can call `Greeter` interface methods and they'll be routed to that `GreeterImp l` instance. 647 There's a lot going on here and it would be useful to familiarize yourself with
648 the definitions of `mojo::ApplicationDelegate`, `mojo::ApplicationConnection`,
649 and `mojo::InterfaceFactory<T>`. The TL;DR though is that if someone connects to
650 this app and requests a service named `"hello::Greeter"`, the app will create a
651 new `GreeterImpl` and bind it to that request pipe. From there the connecting
652 app can call `Greeter` interface methods and they'll be routed to that
653 `GreeterImpl` instance.
471 654
472 Although this appears to be a more interesting application, we need some way to actually connect and test the behavior of our new service. Let's write an app te st! 655 Although this appears to be a more interesting application, we need some way to
656 actually connect and test the behavior of our new service. Let's write an app
657 test!
473 658
474 ## App Tests 659 ### App Tests
475 660
476 App tests run inside a test application, giving test code access to a shell whic h can connect to one or more applications-under-test. 661 App tests run inside a test application, giving test code access to a shell
662 which can connect to one or more applications-under-test.
477 663
478 First let's introduce some test code: 664 First let's introduce some test code:
479 665
480 **components/hello/hello\_apptest.cc** 666 **components/hello/hello_apptest.cc**
481 ``` 667 ```
482 #include "base/bind.h" 668 #include "base/bind.h"
483 #include "base/callback.h" 669 #include "base/callback.h"
484 #include "base/logging.h" 670 #include "base/logging.h"
485 #include "base/macros.h" 671 #include "base/macros.h"
486 #include "base/run_loop.h" 672 #include "base/run_loop.h"
487 #include "components/hello/public/interfaces/greeter.mojom.h" 673 #include "components/hello/public/interfaces/greeter.mojom.h"
488 #include "mojo/application/public/cpp/application_impl.h" 674 #include "mojo/application/public/cpp/application_impl.h"
489 #include "mojo/application/public/cpp/application_test_base.h" 675 #include "mojo/application/public/cpp/application_test_base.h"
490 676
(...skipping 34 matching lines...) Expand 10 before | Expand all | Expand 10 after
525 loop.Run(); 711 loop.Run();
526 } 712 }
527 713
528 } // namespace 714 } // namespace
529 } // namespace hello 715 } // namespace hello
530 ``` 716 ```
531 717
532 We also need to add a new rule to `//components/hello/BUILD.gn`: 718 We also need to add a new rule to `//components/hello/BUILD.gn`:
533 719
534 ``` 720 ```
535 mojo_native_application("apptests") { 721 mojo_native_application("apptests") {
536 output_name = "hello_apptests" 722 output_name = "hello_apptests"
537 testonly = true 723 testonly = true
538 sources = [ 724 sources = [
539 "hello_apptest.cc", 725 "hello_apptest.cc",
540 ] 726 ]
541 deps = [ 727 deps = [
542 "//base", 728 "//base",
543 "//mojo/application/public/cpp:test_support", 729 "//mojo/application/public/cpp:test_support",
544 ] 730 ]
545 public_deps = [ 731 public_deps = [
546 "//components/hello/public/interfaces", 732 "//components/hello/public/interfaces",
547 ] 733 ]
548 data_deps = [ ":hello" ] 734 data_deps = [ ":hello" ]
549 } 735 }
550 ``` 736 ```
551 737
552 Note that the `//components/hello:apptests` target does **not** have a binary de pendency on either `HelloApp` or `GreeterImpl` implementations; instead it depen ds only on the component's public interface definitions. 738 Note that the `//components/hello:apptests` target does **not** have a binary
553 739 dependency on either `HelloApp` or `GreeterImpl` implementations; instead it
554 The `data_deps` entry ensures that `hello.mojo` is up-to-date when `apptests` is built. This is desirable because the test connects to `"mojo:hello"` which will in turn load `hello.mojo` from disk. 740 depends only on the component's public interface definitions.
741
742 The `data_deps` entry ensures that `hello.mojo` is up-to-date when `apptests` is
743 built. This is desirable because the test connects to `"mojo:hello"` which will
744 in turn load `hello.mojo` from disk.
555 745
556 You can now build the test suite: 746 You can now build the test suite:
557 747
558 ``` 748 ninja -C out_gn/Debug components/hello:apptests
559 ninja -C out_gn/Debug components/hello:apptests
560 ```
561 749
562 and run it: 750 and run it:
563 751
564 ``` 752 out_gn/Debug/mojo_runner mojo:hello_apptests
565 out_gn/Debug/mojo_runner mojo:hello_apptests
566 ```
567 753
568 You should see one test (`HelloAppTest.GreetWorld`) passing. 754 You should see one test (`HelloAppTest.GreetWorld`) passing.
569 755
570 One particularly interesting bit of code in this test is in the `SetUp` method: 756 One particularly interesting bit of code in this test is in the `SetUp` method:
571 757
572 ```
573 mojo::URLRequestPtr app_url = mojo::URLRequest::New(); 758 mojo::URLRequestPtr app_url = mojo::URLRequest::New();
574 app_url->url = "mojo:hello"; 759 app_url->url = "mojo:hello";
575 application_impl()->ConnectToService(app_url.Pass(), &greeter_); 760 application_impl()->ConnectToService(app_url.Pass(), &greeter_);
576 ``` 761
577 762 `ConnectToService` is a convenience method provided by `mojo::ApplicationImpl`,
578 `ConnectToService` is a convenience method provided by `mojo::ApplicationImpl`, and it's essentially a shortcut for calling out to the shell's `ConnectToApplica tion` method with the given application URL (in this case `"mojo:hello"`) and th en connecting to a specific service provided by that app via its `ServiceProvide r`'s `ConnectToService` method. 763 and it's essentially a shortcut for calling out to the shell's
579 764 `ConnectToApplication` method with the given application URL (in this case
580 Note that generated interface bindings include a constant string to identify eac h interface by name; so for example the generated `hello::Greeter` type defines a static C string: 765 `"mojo:hello"`) and then connecting to a specific service provided by that app
581 766 via its `ServiceProvider`'s `ConnectToService` method.
582 ``` 767
768 Note that generated interface bindings include a constant string to identify
769 each interface by name; so for example the generated `hello::Greeter` type
770 defines a static C string:
771
583 const char hello::Greeter::Name_[] = "hello::Greeter"; 772 const char hello::Greeter::Name_[] = "hello::Greeter";
584 ``` 773
585 774 This is exploited by the definition of
586 This is exploited by the definition of `mojo::ApplicationConnection::ConnectToSe rvice<T>`, which uses `T::Name_` as the name of the service to connect to. The t ype `T` in this context is inferred from the `InterfacePtr<T>*` argument. You ca n inspect the definition of `ConnectToService` in [//mojo/application/public/cpp /application\_connection.h](https://code.google.com/p/chromium/codesearch#chromi um/src/mojo/application/public/cpp/application_connection.h) for additional clar ity. 775 `mojo::ApplicationConnection::ConnectToService<T>`, which uses `T::Name_` as the
776 name of the service to connect to. The type `T` in this context is inferred from
777 the `InterfacePtr<T>*` argument. You can inspect the definition of
778 `ConnectToService` in `/mojo/application/public/cpp/application_connection.h`
779 for additional clarity.
587 780
588 We could have instead written this code as: 781 We could have instead written this code as:
589 782
590 ``` 783 ```
591 mojo::URLRequestPtr app_url = mojo::URLRequest::New(); 784 mojo::URLRequestPtr app_url = mojo::URLRequest::New();
592 app_url->url = "mojo::hello"; 785 app_url->url = "mojo::hello";
593 786
594 mojo::ServiceProviderPtr services; 787 mojo::ServiceProviderPtr services;
595 application_impl()->shell()->ConnectToApplication( 788 application_impl()->shell()->ConnectToApplication(
596 app_url.Pass(), mojo::GetProxy(&services), 789 app_url.Pass(), mojo::GetProxy(&services),
597 // We pass a null provider since we aren't exposing any of our own 790 // We pass a null provider since we aren't exposing any of our own
598 // services to the target app. 791 // services to the target app.
599 mojo::ServiceProviderPtr()); 792 mojo::ServiceProviderPtr());
600 793
601 mojo::InterfaceRequest<hello::Greeter> greeter_request = 794 mojo::InterfaceRequest<hello::Greeter> greeter_request =
602 mojo::GetProxy(&greeter_); 795 mojo::GetProxy(&greeter_);
603 services->ConnectToService(hello::Greeter::Name_, 796 services->ConnectToService(hello::Greeter::Name_,
604 greeter_request.PassMessagePipe()); 797 greeter_request.PassMessagePipe());
605 ``` 798 ```
606 799
607 The net result is the same, but 3-line version seems much nicer. 800 The net result is the same, but 3-line version seems much nicer.
608 801
609 # Chromium Integration 802 ## Chromium Integration
610 803
611 Up until now we've been using `mojo_runner` to load and run `.mojo` binaries dyn amically. While this model is used by Mandoline and may eventually be used in Ch romium as well, Chromium is at the moment confined to running statically linked application code. This means we need some way to register applications with the browser's Mojo shell. 804 Up until now we've been using `mojo_runner` to load and run `.mojo` binaries
612 805 dynamically. While this model is used by Mandoline and may eventually be used in
613 It also means that, rather than using the binary output of a `mojo_native_applic ation` target, some part of Chromium must link against the app's static library target (_e.g._, `"//components/hello:lib"`) and register a URL handler to teach the shell how to launch an instance of the app. 806 Chromium as well, Chromium is at the moment confined to running statically
614 807 linked application code. This means we need some way to register applications
615 When registering an app URL in Chromium it probably makes sense to use the same mojo-scheme URL used for the app in Mandoline. For example the media renderer ap p is referenced by the `"mojo:media"` URL in both Mandoline and Chromium. In Man doline this resolves to a dynamically-loaded `.mojo` binary on disk, but in Chro mium it resolves to a static application loader linked into Chromium. The net re sult is the same in both cases: other apps can use the shell to connect to `"moj o:media"` and use its services. 808 with the browser's Mojo shell.
616 809
617 This section explores different ways to register and connect to `"mojo:hello"` i n Chromium. 810 It also means that, rather than using the binary output of a
618 811 `mojo_native_application` target, some part of Chromium must link against the
619 ## In-Process Applications 812 app's static library target (_e.g._, `"//components/hello:lib"`) and register a
620 813 URL handler to teach the shell how to launch an instance of the app.
621 Applications can be set up to run within the browser process via `ContentBrowser Client::RegisterInProcessMojoApplications`. This method populates a mapping from URL to `base::Callback<scoped_ptr<mojo::ApplicationDelegate>()>` (_i.e._, a fac tory function which creates a new `mojo::ApplicationDelegate` instance), so regi stering a new app means adding an entry to this map. 814
622 815 When registering an app URL in Chromium it probably makes sense to use the same
623 Let's modify `ChromeContentBrowserClient::RegisterInProcessMojoApplications` (in `//chrome/browser/chrome_content_browser_client.cc`) by adding the following co de: 816 mojo-scheme URL used for the app in Mandoline. For example the media renderer
624 817 app is referenced by the `"mojo:media"` URL in both Mandoline and Chromium. In
625 ``` 818 Mandoline this resolves to a dynamically-loaded `.mojo` binary on disk, but in
819 Chromium it resolves to a static application loader linked into Chromium. The
820 net result is the same in both cases: other apps can use the shell to connect to
821 `"mojo:media"` and use its services.
822
823 This section explores different ways to register and connect to `"mojo:hello"`
824 in Chromium.
825
826 ### In-Process Applications
827
828 Applications can be set up to run within the browser process via
829 `ContentBrowserClient::RegisterInProcessMojoApplications`. This method populates
830 a mapping from URL to `base::Callback<scoped_ptr<mojo::ApplicationDelegate>()>`
831 (_i.e._, a factory function which creates a new `mojo::ApplicationDelegate`
832 instance), so registering a new app means adding an entry to this map.
833
834 Let's modify `ChromeContentBrowserClient::RegisterInProcessMojoApplications`
835 (in `//chrome/browser/chrome_content_browser_client.cc`) by adding the following
836 code:
837
626 apps->insert(std::make_pair(GURL("mojo:hello"), 838 apps->insert(std::make_pair(GURL("mojo:hello"),
627 base::Bind(&HelloApp::CreateApp))); 839 base::Bind(&HelloApp::CreateApp)));
628 ``` 840
629 841 you'll also want to add the following convenience method to your `HelloApp`
630 you'll also want to add the following convenience method to your `HelloApp` defi nition in `//components/hello/hello_app.h`: 842 definition in `//components/hello/hello_app.h`:
631 843
632 ```
633 static scoped_ptr<mojo::ApplicationDelegate> HelloApp::CreateApp() { 844 static scoped_ptr<mojo::ApplicationDelegate> HelloApp::CreateApp() {
634 return scoped_ptr<mojo::ApplicationDelegate>(new HelloApp); 845 return scoped_ptr<mojo::ApplicationDelegate>(new HelloApp);
635 } 846 }
636 ``` 847
637 848 This introduces a dependency from `//chrome/browser` on to
638 This introduces a dependency from `//chrome/browser` on to `//components/hello:l ib`, which you can add to the `"browser"` target's deps in `//chrome/browser/BUI LD.gn`. You'll of course also need to include `"components/hello/hello_app.h"` i n `chrome_content_browser_client.cc`. 849 `//components/hello:lib`, which you can add to the `"browser"` target's deps in
639 850 `//chrome/browser/BUILD.gn`. You'll of course also need to include
640 That's it! Now if an app comes to the shell asking to connect to `"mojo:hello"` and app is already running, it'll get connected to our `HelloApp` and have acces s to the `Greeter` service. If the app wasn't already running, it will first be launched on a new thread. 851 `"components/hello/hello_app.h"` in `chrome_content_browser_client.cc`.
641 852
642 ## Connecting From the Browser 853 That's it! Now if an app comes to the shell asking to connect to `"mojo:hello"`
643 854 and app is already running, it'll get connected to our `HelloApp` and have
644 We've already seen how apps can connect to each other using their own private sh ell proxy, but the vast majority of Chromium code doesn't yet belong to a Mojo a pplication. So how do we use an app's services from arbitrary browser code? We u se `content::MojoAppConnection`, like this: 855 access to the `Greeter` service. If the app wasn't already running, it will
645 856 first be launched on a new thread.
646 ``` 857
647 #include "base/bind.h" 858 ### Connecting From the Browser
648 #include "base/logging.h" 859
649 #include "components/hello/public/interfaces/greeter.mojom.h" 860 We've already seen how apps can connect to each other using their own private
650 #include "content/public/browser/mojo_app_connection.h" 861 shell proxy, but the vast majority of Chromium code doesn't yet belong to a Mojo
651 862 application. So how do we use an app's services from arbitrary browser code? We
652 void LogGreeting(const mojo::String& greeting) { 863 use `content::MojoAppConnection`, like this:
653 LOG(INFO) << greeting; 864
654 } 865 ```
655 866 #include "base/bind.h"
656 void GreetTheWorld() { 867 #include "base/logging.h"
657 scoped_ptr<content::MojoAppConnection> connection = 868 #include "components/hello/public/interfaces/greeter.mojom.h"
658 content::MojoAppConnection::Create("mojo:hello", 869 #include "content/public/browser/mojo_app_connection.h"
659 content::kBrowserMojoAppUrl); 870
660 hello::GreeterPtr greeter; 871 void LogGreeting(const mojo::String& greeting) {
661 connection->ConnectToService(&greeter); 872 LOG(INFO) << greeting;
662 greeter->Greet("world", base::Bind(&LogGreeting)); 873 }
663 } 874
664 ``` 875 void GreetTheWorld() {
665 876 scoped_ptr<content::MojoAppConnection> connection =
666 A `content::MojoAppConnection`, while not thread-safe, may be created and safely used on any single browser thread. 877 content::MojoAppConnection::Create("mojo:hello",
667 878 content::kBrowserMojoAppUrl);
668 You could add the above code to a new browsertest to convince yourself that it w orks. In fact you might want to take a peek at `MojoShellTest.TestBrowserConnect ion` (in [//content/browser/mojo\_shell\_browsertest.cc](https://code.google.com /p/chromium/codesearch#chromium/src/content/browser/mojo_shell_browsertest.cc)) which registers and tests an in-process Mojo app. 879 hello::GreeterPtr greeter;
669 880 connection->ConnectToService(&greeter);
670 Finally, note that `MojoAppConnection::Create` takes two URLs. The first is the target app URL, and the second is the source URL. Since we're not really a Mojo app, but we are still trusted browser code, the shell will gladly use this URL a s the `requestor_url` when establishing an incoming connection to the target app . This allows browser code to masquerade as a Mojo app at the given URL. `conten t::kBrowserMojoAppUrl` (which is presently `"system:content_browser"`) is a reas onable default choice when a more specific app identity isn't required. 881 greeter->Greet("world", base::Bind(&LogGreeting));
671 882 }
672 ## Out-of-Process Applications 883 ```
673 884
674 If an app URL isn't registered for in-process loading, the shell assumes it must be an out-of-process application. If the shell doesn't already have a known ins tance of the app running, a new utility process is launched and the application request is passed onto it. Then if the app URL is registered in the utility proc ess, the app will be loaded there. 885 A `content::MojoAppConnection`, while not thread-safe, may be created and safely
675 886 used on any single browser thread.
676 Similar to in-process registration, a URL mapping needs to be registered in `Con tentUtilityClient::RegisterMojoApplications`. 887
677 888 You could add the above code to a new browsertest to convince yourself that it
678 Once again you can take a peek at //content/browser/mojo\_shell\_browsertest.cc for an end-to-end example of testing an out-of-process Mojo app from browser cod e. Note that `content_browsertests` runs on `content_shell`, which uses `ShellCo ntentUtilityClient` as defined [//content/shell/utility/shell\_content\_utility\ _client.cc](https://code.google.com/p/chromium/codesearch#chromium/src/content/s hell/utility/shell_content_utility_client.cc). This code registers a common OOP test app. 889 works. In fact you might want to take a peek at
890 `MojoShellTest.TestBrowserConnection` (in
891 `/content/browser/mojo_shell_browsertest.cc`) which registers and tests an
892 in-process Mojo app.
893
894 Finally, note that `MojoAppConnection::Create` takes two URLs. The first is the
895 target app URL, and the second is the source URL. Since we're not really a Mojo
896 app, but we are still trusted browser code, the shell will gladly use this URL
897 as the `requestor_url` when establishing an incoming connection to the target
898 app. This allows browser code to masquerade as a Mojo app at the given URL.
899 `content::kBrowserMojoAppUrl` (which is presently `"system:content_browser"`) is
900 a reasonable default choice when a more specific app identity isn't required.
901
902 ### Out-of-Process Applications
903
904 If an app URL isn't registered for in-process loading, the shell assumes it must
905 be an out-of-process application. If the shell doesn't already have a known
906 instance of the app running, a new utility process is launched and the
907 application request is passed onto it. Then if the app URL is registered in the
908 utility process, the app will be loaded there.
909
910 Similar to in-process registration, a URL mapping needs to be registered in
911 `ContentUtilityClient::RegisterMojoApplications`.
912
913 Once again you can take a peek at `/content/browser/mojo_shell_browsertest.cc`
914 for an end-to-end example of testing an out-of-process Mojo app from browser
915 code. Note that `content_browsertests` runs on `content_shell`, which uses
916 `ShellContentUtilityClient` as defined
917 `/content/shell/utility/shell_content_utility_client.cc`. This code registers a
918 common OOP test app.
679 919
680 ## Unsandboxed Out-of-Process Applications 920 ## Unsandboxed Out-of-Process Applications
681 921
682 By default new utility processes run in a sandbox. If you want your Mojo app to run out-of-process and unsandboxed (which you **probably do not**), you can regi ster its URL via `ContentBrowserClient::RegisterUnsandboxedOutOfProcessMojoAppli cations`. 922 By default new utility processes run in a sandbox. If you want your Mojo app to
923 run out-of-process and unsandboxed (which you **probably do not**), you can
924 register its URL via
925 `ContentBrowserClient::RegisterUnsandboxedOutOfProcessMojoApplications`.
683 926
684 ## Connecting From `RenderFrame` 927 ## Connecting From `RenderFrame`
685 928
686 We can also connect to Mojo apps from a `RenderFrame`. This is made possible by `RenderFrame`'s `GetServiceRegistry()` interface. The `ServiceRegistry` can be u sed to acquire a shell proxy and in turn connect to an app like so: 929 We can also connect to Mojo apps from a `RenderFrame`. This is made possible by
930 `RenderFrame`'s `GetServiceRegistry()` interface. The `ServiceRegistry` can be
931 used to acquire a shell proxy and in turn connect to an app like so:
687 932
688 ``` 933 ```
689 void GreetWorld(content::RenderFrame* frame) { 934 void GreetWorld(content::RenderFrame* frame) {
690 mojo::ShellPtr shell; 935 mojo::ShellPtr shell;
691 frame->GetServiceRegistry()->ConnectToRemoteService( 936 frame->GetServiceRegistry()->ConnectToRemoteService(
692 mojo::GetProxy(&shell)); 937 mojo::GetProxy(&shell));
693 938
694 mojo::URLRequestPtr request = mojo::URLRequest::New(); 939 mojo::URLRequestPtr request = mojo::URLRequest::New();
695 request->url = "mojo:hello"; 940 request->url = "mojo:hello";
696 941
697 mojo::ServiceProviderPtr hello_services; 942 mojo::ServiceProviderPtr hello_services;
698 shell->ConnectToApplication( 943 shell->ConnectToApplication(
699 request.Pass(), mojo::GetProxy(&hello_services), nullptr); 944 request.Pass(), mojo::GetProxy(&hello_services), nullptr);
700 945
701 hello::GreeterPtr greeter; 946 hello::GreeterPtr greeter;
702 hello_services->ConnectToService( 947 hello_services->ConnectToService(
703 hello::Greeter::Name_, mojo::GetProxy(&greeter).PassMessagePipe()); 948 hello::Greeter::Name_, mojo::GetProxy(&greeter).PassMessagePipe());
704 } 949 }
705 ``` 950 ```
706 951
707 It's important to note that connections made through the frame's shell proxy wil l appear to come from the frame's `SiteInstance` URL. For example, if the frame has loaded `https://example.com/`, `HelloApp`'s incoming `mojo::ApplicationConne ction` in this case will have a remote application URL of `"https://example.com/ "`. This allows apps to expose their services to web frames on a per-origin basi s if needed. 952 It's important to note that connections made through the frame's shell proxy
953 will appear to come from the frame's `SiteInstance` URL. For example, if the
954 frame has loaded `https://example.com/`, `HelloApp`'s incoming
955 `mojo::ApplicationConnection` in this case will have a remote application URL of
956 `"https://example.com/"`. This allows apps to expose their services to web
957 frames on a per-origin basis if needed.
708 958
709 ## Connecting From Java 959 ### Connecting From Java
710 960
711 TODO 961 TODO
712 962
713 ## Connecting From `JavaScript` 963 ### Connecting From `JavaScript`
714 964
715 This is still a work in progress and might not really take shape until the Blink +Chromium merge. In the meantime there are some end-to-end WebUI examples in [// content/browser/webui/web\_ui\_mojo\_browsertest.cc](https://code.google.com/p/c hromium/codesearch#chromium/src/content/browser/webui/web_ui_mojo_browsertest.cc ). In particular, `WebUIMojoTest.ConnectToApplication` connects from a WebUI fra me to a test app running in a new utility process. 965 This is still a work in progress and might not really take shape until the
966 Blink+Chromium merge. In the meantime there are some end-to-end WebUI examples
967 in `/content/browser/webui/web_ui_mojo_browsertest.cc`. In particular,
968 `WebUIMojoTest.ConnectToApplication` connects from a WebUI frame to a test app
969 running in a new utility process.
716 970
717 # FAQ 971 ## FAQ
718 972
719 Nothing here yet! 973 Nothing here yet!
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