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docs/mojo_in_chromium.md

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-# Mojo in Chromium
-
-This document is intended to serve as a Mojo primer for Chromium developers. No
-prior knowledge of Mojo is assumed.
-
-[TOC]
-
-## Should I Bother Reading This?
-
-If you're planning to build a Chromium feature that needs IPC and you aren't
-already using Mojo, YES! **Legacy IPC is deprecated.**
-
-## Why Mojo?
-
-TL;DR: The long-term intent is to refactor Chromium into a large set of smaller
-services.
-
-We can be smarter about:
-
-  * Which services we bring up or don't
-  * How we isolate these services to improve security and stability
-  * Which binary features we ship to one user or another
-
-A more robust messaging layer opens the door for a number of interesting
-possibilities; in particular it allows us to integrate a large number of
-components without link-time interdependencies, and it breaks down the growing
-number of interesting cross-language boundaries across the codebase.
-
-Much has been learned from using Chromium IPC and maintaining Chromium
-dependencies in anger over the past several years and we feel there's now a
-significant opportunity to make life easier for developers, and to help them
-build more and better features, faster, and with much less cost to users.
-
-## Mojo Overview
-
-The Mojo system API provides a small suite of low-level IPC primitives:
-**message pipes**, **data pipes**, and **shared buffers**. On top of this API
-we've built higher-level bindings APIs to simplify messaging for consumers
-writing C++, Java, or JavaScript code.
-
-This document focuses primarily on using C++ bindings with message pipes, which
-is likely to be the most common usage encountered by Chromium developers.
-
-### Message Pipes
-
-A message pipe is a lightweight primitive for reliable bidirectional transfer of
-relatively small packets of data. Unsurprisingly a pipe has two endpoints, and
-either endpoint may be transferred over another message pipe.
-
-Because we bootstrap a primordial message pipe between the browser process and
-each child process, this in turn means that you can create a new pipe and
-ultimately send either end to any process, and the two ends will still be
-able to talk to each other seamlessly and exclusively. Goodbye, routing IDs!
-
-While message pipes can carry arbitrary packets of unstructured data we
-generally use them in conjunction with generated bindings to ensure a
-consistent, well-defined, versioned message structure on all endpoints.
-
-### Mojom
-
-Mojom is the IDL for Mojo interfaces. Given a `.mojom` file, the bindings
-generator outputs bindings for all three of the currently supported languages.
-
-For example:
-
-```
-// src/components/frob/public/interfaces/frobinator.mojom
-module frob.mojom;
-
-interface Frobinator {
-  Frobinate();
-};
-```
-
-would generate the following outputs:
-
-```
-out/Debug/gen/components/frob/public/interfaces/frobinator.mojom.cc
-out/Debug/gen/components/frob/public/interfaces/frobinator.mojom.h
-out/Debug/gen/components/frob/public/interfaces/frobinator.mojom.js
-out/Debug/gen/components/frob/public/interfaces/frobinator.mojom.srcjar
-...
-```
-
-The generated code hides away all the details of serializing and deserializing
-messages on either end of a pipe.
-
-The C++ header (`frobinator.mojom.h`) defines an abstract class for each
-mojom interface specified. Namespaces are derived from the `module` name.
-
-**NOTE:** Chromium convention for component `foo`'s module name is `foo.mojom`.
-This means all mojom-generated C++ typenames for component `foo` will live in
-the `foo::mojom` namespace to avoid collisions with non-generated typenames.
-
-In this example the generated `frob::mojom::Frobinator` has a single
-pure virtual function:
-
-```cpp
-namespace frob {
-
-class Frobinator {
- public:
-  virtual void Frobinate() = 0;
-};
-
-}  // namespace frob
-```
-
-To create a `Frobinator` service, one simply implements `foo::Frobinator` and
-provides a means of binding pipes to it.
-
-### Binding to Pipes
-
-Let's look at some sample code:
-
-```cpp
-// src/components/frob/frobinator_impl.cc
-
-#include "components/frob/public/interfaces/frobinator.mojom.h"
-#include "mojo/public/cpp/bindings/binding.h"
-#include "mojo/public/cpp/bindings/interface_request.h"
-
-namespace frob {
-
-class FrobinatorImpl : public mojom::Frobinator {
- public:
-  FrobinatorImpl(mojom::FrobinatorRequest request)
-      : binding_(this, std::move(request)) {}
-  ~FrobinatorImpl() override {}
-
-  // mojom::Frobinator:
-  void Frobinate() override { DLOG(INFO) << "I can't stop frobinating!"; }
-
- private:
-  mojo::Binding<mojom::Frobinator> binding_;
-};
-
-}  // namespace frob
-```
-
-The first thing to note is that `mojo::Binding<T>` *binds* one end of a message
-pipe to an implementation of a service. This means it watches that end of the
-pipe for incoming messages; it knows how to decode messages for interface `T`,
-and it dispatches them to methods on the bound `T` implementation.
-
-`mojom::FrobinatorRequest` is a generated type alias for
-`mojo::InterfaceRequest<mojom::Frobinator>` and is essentially semantic sugar
-for a strongly-typed message pipe endpoint. A common way to create new message
-pipes is via the `GetProxy` call defined in `interface_request.h`:
-
-```cpp
-mojom::FrobinatorPtr proxy;
-mojom::FrobinatorRequest request = mojo::GetProxy(&proxy);
-```
-
-This creates a new message pipe with one end owned by `proxy` and the other end
-owned by `request`. It has the nice property of attaching common type
-information to each end of the pipe.
-
-Note that `InterfaceRequest<T>` doesn't actually **do** anything. It just scopes
-a pipe endpoint and associates it with an interface type at compile time. As
-such, other typed service binding primitives such as `mojo::Binding<T>` take
-these objects as input when they need an endpoint to bind to.
-
-`mojom::FrobinatorPtr` is a generated type alias for
-`mojo::InterfacePtr<mojom::Frobinator>`. An `InterfacePtr<T>` scopes a message
-pipe endpoint as well, but it also internally implements every method on `T` by
-serializing a corresponding message and writing it to the pipe.
-
-Hence we can put this together to talk to a `FrobinatorImpl` over a pipe:
-
-```cpp
-frob:mojom::FrobinatorPtr frobinator;
-frob::FrobinatorImpl impl(GetProxy(&frobinator));
-
-// Tada!
-frobinator->Frobinate();
-```
-
-Behind the scenes this serializes a message corresponding to the `Frobinate`
-request and writes it to one end of the pipe. Eventually (and incidentally,
-very soon after), `impl`'s internal `mojo::Binding` will decode this message and
-dispatch a call to `impl.Frobinate()`.
-
-**NOTE:** In this example the service and client are in the same process, and
-this works just fine. If they were in different processes (see the example below
-in [Exposing Services in Chromium](#Exposing-Services-in-Chromium)), the call
-to `Frobinate()` would look exactly the same!
-
-### Responding to Requests
-
-A common idiom in Chromium IPC is to keep track of IPC requests with some kind
-of opaque identifier (i.e. an integer *request ID*) so that you can later
-respond to a specific request using some nominally related message in the other
-direction.
-
-This is baked into mojom interface definitions. We can extend our `Frobinator`
-service like so:
-
-```
-module frob.mojom;
-
-interface Frobinator {
-  Frobinate();
-  GetFrobinationLevels() => (int min, int max);
-};
-```
-
-and update our implementation:
-
-```cpp
-class FrobinatorImpl : public mojom::Frobinator {
- public:
-  // ...
-
-  // mojom::Frobinator:
-  void Frobinate() override { /* ... */ }
-  void GetFrobinationLevels(const GetFrobinationLevelsCallback& callback) {
-    callback.Run(1, 42);
-  }
-};
-```
-
-When the service implementation runs `callback`, the response arguments are
-serialized and sent back over the pipe. The proxy on the other end knows how to
-read this response and will in turn dispatch it to a callback on that end:
-
-```cpp
-void ShowLevels(int min, int max) {
-  DLOG(INFO) << "Frobinator min=" << min << " max=" << max;
-}
-
-// ...
-
-  mojom::FrobinatorPtr frobinator;
-  FrobinatorImpl impl(GetProxy(&frobinator));
-
-  frobinator->GetFrobinatorLevels(base::Bind(&ShowLevels));
-```
-
-This does what you'd expect.
-
-## Exposing Services in Chromium
-
-There are a number of ways one might expose services across various surfaces of
-the browser. One common approach now is to use a
-[`content::ServiceRegistry` (link)](https://goo.gl/uEhx06). These come in
-pairs generally spanning a process boundary, and they provide primitive service
-registration and connection interfaces. For one example, [every
-`RenderFrameHost` has a `ServiceRegistry`](https://goo.gl/4YR3j5), as does
-[every corresponding `RenderFrame`](https://goo.gl/YhrgXa). These registries are
-intertwined.
-
-The gist is that you can add a service to the local side of the registry -- it's
-just a mapping from interface name to factory function -- or you can connect by
-name to services registered on the remote side.
-
-**NOTE:** In this context the "factory function" is simply a callback which
-takes a pipe endpoint and does something with it. It's expected that you'll
-either bind it to a service implementation of some kind or you will close it, effectively rejecting the connection request.
-
-We can build a simple browser-side `FrobinatorImpl` service that has access to a
-`BrowserContext` for any frame which connects to it:
-
-```cpp
-#include "base/macros.h"
-#include "components/frob/public/interfaces/frobinator.mojom.h"
-#include "content/public/browser/browser_context.h"
-#include "mojo/public/cpp/system/interface_request.h"
-#include "mojo/public/cpp/system/strong_binding.h"
-
-namespace frob {
-
-class FrobinatorImpl : public mojom::Frobinator {
- public:
-  FrobinatorImpl(content::BrowserContext* context,
-                 mojom::FrobinatorRequest request)
-      : context_(context), binding_(this, std::move(request)) {}
-  ~FrobinatorImpl() override {}
-
-  // A factory function to use in conjunction with ServiceRegistry.
-  static void Create(content::BrowserContext* context,
-                     mojom::FrobinatorRequest request) {
-    // See comment below for why this doesn't leak.
-    new FrobinatorImpl(context, std::move(request));
-  }
-
- private:
-  // mojom::Frobinator:
-  void Frobinate() override { /* ... */ }
-
-  content::BrowserContext* context_;
-
-  // A StrongBinding is just like a Binding, except that it takes ownership of
-  // its bound implementation and deletes itself (and the impl) if and when the
-  // bound pipe encounters an error or is closed on the other end.
-  mojo::StrongBinding<mojom::Frobinator> binding_;
-
-  DISALLOW_COPY_AND_ASSIGN(FrobinatorImpl);
-};
-
-}  // namespace frob
-```
-
-Now somewhere in the browser we register the Frobinator service with each
-`RenderFrameHost` ([this](https://goo.gl/HEFn63) is a popular spot):
-
-```cpp
-frame_host->GetServiceRegistry()->AddService<frob::mojom::Frobinator>(
-    base::Bind(
-        &frob::FrobinatorImpl::Create,
-        base::Unretained(frame_host->GetProcess()->GetBrowserContext())));
-```
-
-And in the render process we can now do something like:
-
-```cpp
-mojom::FrobinatorPtr frobinator;
-render_frame->GetServiceRegistry()->ConnectToRemoteService(
-    mojo::GetProxy(&frobinator));
-
-// It's IPC!
-frobinator->Frobinate();
-```
-
-There are now plenty of concrete examples of Mojo usage in the Chromium tree.
-Poke around at existing mojom files and see how their implementions are built
-and connected.
-
-## Mojo in Blink
-
-*TODO*
-
-This is a work in progress. TL;DR: We'll also soon begin using Mojo services
-from Blink so that the platform layer can consume browser services
-directly via Mojo. The long-term goal there is to eliminate `content/renderer`.
-
-## Questions, Discussion, etc.
-
-A good place to find highly concentrated doses of people who know and care
-about Mojo in Chromium would be the [chromium-mojo](https://goo.gl/A4ebWB)
-mailing list[.](https://goo.gl/L70ihQ)