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| 1 // Copyright 2017 The Chromium Authors. All rights reserved. |
| 2 // Use of this source code is governed by a BSD-style license that can be |
| 3 // found in the LICENSE file. |
| 4 |
| 5 #ifndef MOJO_PUBLIC_CPP_SYSTEM_SIMPLE_WATCHER_H_ |
| 6 #define MOJO_PUBLIC_CPP_SYSTEM_SIMPLE_WATCHER_H_ |
| 7 |
| 8 #include "base/callback.h" |
| 9 #include "base/location.h" |
| 10 #include "base/macros.h" |
| 11 #include "base/memory/ref_counted.h" |
| 12 #include "base/memory/weak_ptr.h" |
| 13 #include "base/threading/thread_checker.h" |
| 14 #include "base/threading/thread_task_runner_handle.h" |
| 15 #include "mojo/public/c/system/types.h" |
| 16 #include "mojo/public/cpp/system/system_export.h" |
| 17 #include "mojo/public/cpp/system/watcher.h" |
| 18 |
| 19 namespace base { |
| 20 class SingleThreadTaskRunner; |
| 21 } |
| 22 |
| 23 namespace mojo { |
| 24 |
| 25 // This provides a convenient thread-bound watcher implementation to safely |
| 26 // watch a single handle, dispatching state change notifications to an arbitrary |
| 27 // SingleThreadTaskRunner running on the same thread as the SimpleWatcher. |
| 28 // |
| 29 // SimpleWatcher exposes the concept of "arming" from the low-level Watcher API. |
| 30 // In general, a SimpleWatcher must be "armed" in order to dispatch a single |
| 31 // notification, and must then be rearmed before it will dispatch another. For |
| 32 // more details, see the documentation for ArmingPolicy and the Arm() and |
| 33 // ArmOrNotify() methods below. |
| 34 class MOJO_CPP_SYSTEM_EXPORT SimpleWatcher { |
| 35 public: |
| 36 // A callback to be called any time a watched handle changes state in some |
| 37 // interesting way. The |result| argument indicates one of the following |
| 38 // conditions depending on its value: |
| 39 // |
| 40 // |MOJO_RESULT_OK|: One or more of the signals being watched is satisfied. |
| 41 // |
| 42 // |MOJO_RESULT_FAILED_PRECONDITION|: None of the signals being watched can |
| 43 // ever be satisfied again. |
| 44 // |
| 45 // |MOJO_RESULT_CANCELLED|: The watched handle has been closed. No further |
| 46 // notifications will be fired, as this equivalent to an implicit |
| 47 // CancelWatch(). |
| 48 // |
| 49 // Note that unlike the first two conditions, this callback may be invoked |
| 50 // with |MOJO_RESULT_CANCELLED| even while the SimpleWatcher is disarmed. |
| 51 using ReadyCallback = base::Callback<void(MojoResult result)>; |
| 52 |
| 53 // Selects how this SimpleWatcher is to be armed. |
| 54 enum class ArmingPolicy { |
| 55 // The SimpleWatcher is armed automatically on Watch() and rearmed again |
| 56 // after every invocation of the ReadyCallback. There is no need to manually |
| 57 // call Arm() on a SimpleWatcher using this policy. This mode is equivalent |
| 58 // to calling ArmOrNotify() once after Watch() and once again after every |
| 59 // dispatched notification in MANUAL mode. |
| 60 // |
| 61 // This provides a reasonable approximation of edge-triggered behavior, |
| 62 // mitigating (but not completely eliminating) the potential for redundant |
| 63 // notifications. |
| 64 // |
| 65 // NOTE: It is important when using AUTOMATIC policy that your ReadyCallback |
| 66 // always attempt to change the state of the handle (e.g. read available |
| 67 // messages on a message pipe.) Otherwise this will result in a potentially |
| 68 // large number of avoidable redundant tasks. |
| 69 // |
| 70 // For perfect edge-triggered behavior, use MANUAL policy and manually Arm() |
| 71 // the SimpleWatcher as soon as it becomes possible to do so again. |
| 72 AUTOMATIC, |
| 73 |
| 74 // The SimpleWatcher is never armed automatically. Arm() or ArmOrNotify() |
| 75 // must be called manually before any non-cancellation notification can be |
| 76 // dispatched to the ReadyCallback. See the documentation for Arm() and |
| 77 // ArmNotify() methods below for more details. |
| 78 MANUAL, |
| 79 }; |
| 80 |
| 81 SimpleWatcher(const tracked_objects::Location& from_here, |
| 82 ArmingPolicy arming_policy, |
| 83 scoped_refptr<base::SingleThreadTaskRunner> runner = |
| 84 base::ThreadTaskRunnerHandle::Get()); |
| 85 ~SimpleWatcher(); |
| 86 |
| 87 // Indicates if the SimpleWatcher is currently watching a handle. |
| 88 bool IsWatching() const; |
| 89 |
| 90 // Starts watching |handle|. A SimpleWatcher may only watch one handle at a |
| 91 // time, but it is safe to call this more than once as long as the previous |
| 92 // watch has been cancelled (i.e. |IsWatching()| returns |false|.) |
| 93 // |
| 94 // If |handle| is not a valid watchable (message or data pipe) handle or |
| 95 // |signals| is not a valid set of signals to watch, this returns |
| 96 // |MOJO_RESULT_INVALID_ARGUMENT|. |
| 97 // |
| 98 // Otherwise |MOJO_RESULT_OK| is returned and the handle will be watched until |
| 99 // either |handle| is closed, the SimpleWatcher is destroyed, or Cancel() is |
| 100 // explicitly called. |
| 101 // |
| 102 // Once the watch is started, |callback| may be called at any time on the |
| 103 // current thread until |Cancel()| is called or the handle is closed. Note |
| 104 // that |callback| can be called for results other than |
| 105 // |MOJO_RESULT_CANCELLED| only if the SimpleWatcher is currently armed. Use |
| 106 // ArmingPolicy to configure how a SimpleWatcher is armed. |
| 107 // |
| 108 // |MOJO_RESULT_CANCELLED| may be dispatched even while the SimpleWatcher |
| 109 // is disarmed, and no further notifications will be dispatched after that. |
| 110 // |
| 111 // Destroying the SimpleWatcher implicitly calls |Cancel()|. |
| 112 MojoResult Watch(Handle handle, |
| 113 MojoHandleSignals signals, |
| 114 const ReadyCallback& callback); |
| 115 |
| 116 // Cancels the current watch. Once this returns, the ReadyCallback previously |
| 117 // passed to |Watch()| will never be called again for this SimpleWatcher. |
| 118 // |
| 119 // Note that when cancelled with an explicit call to |Cancel()| the |
| 120 // ReadyCallback will not be invoked with a |MOJO_RESULT_CANCELLED| result. |
| 121 void Cancel(); |
| 122 |
| 123 // Manually arms the SimpleWatcher. |
| 124 // |
| 125 // Arming the SimpleWatcher allows it to fire a single notification regarding |
| 126 // some future relevant change in the watched handle's state. It's only valid |
| 127 // to call Arm() while a handle is being watched (see Watch() above.) |
| 128 // |
| 129 // SimpleWatcher is always disarmed immediately before invoking its |
| 130 // ReadyCallback and must be rearmed again before another notification can |
| 131 // fire. |
| 132 // |
| 133 // If the watched handle already meets the watched signaling conditions - |
| 134 // i.e., if it would have notified immediately once armed - the SimpleWatcher |
| 135 // is NOT armed, and this call fails with a return value of |
| 136 // |MOJO_RESULT_FAILED_PRECONDITION|. In that case, what would have been the |
| 137 // result code for that immediate notification is instead placed in |
| 138 // |*ready_result| if |ready_result| is non-null. |
| 139 // |
| 140 // If the watcher is successfully armed, this returns |MOJO_RESULT_OK| and |
| 141 // |ready_result| is ignored. |
| 142 MojoResult Arm(MojoResult* ready_result = nullptr); |
| 143 |
| 144 // Manually arms the SimpleWatcher OR posts a task to invoke the ReadyCallback |
| 145 // with the ready result of the failed arming attempt. |
| 146 // |
| 147 // This is meant as a convenient helper for a common usage of Arm(), and it |
| 148 // ensures that the ReadyCallback will be invoked asynchronously again as soon |
| 149 // as the watch's conditions are satisfied, assuming the SimpleWatcher isn't |
| 150 // cancelled first. |
| 151 // |
| 152 // Unlike Arm() above, this can never fail. |
| 153 void ArmOrNotify(); |
| 154 |
| 155 Handle handle() const { return handle_; } |
| 156 ReadyCallback ready_callback() const { return callback_; } |
| 157 |
| 158 // Sets the tag used by the heap profiler. |
| 159 // |tag| must be a const string literal. |
| 160 void set_heap_profiler_tag(const char* heap_profiler_tag) { |
| 161 heap_profiler_tag_ = heap_profiler_tag; |
| 162 } |
| 163 |
| 164 private: |
| 165 class Context; |
| 166 |
| 167 void OnHandleReady(scoped_refptr<const Context> context, MojoResult result); |
| 168 |
| 169 base::ThreadChecker thread_checker_; |
| 170 |
| 171 // The policy used to determine how this SimpleWatcher is armed. |
| 172 const ArmingPolicy arming_policy_; |
| 173 |
| 174 // The TaskRunner of this SimpleWatcher's owning thread. This field is safe to |
| 175 // access from any thread. |
| 176 const scoped_refptr<base::SingleThreadTaskRunner> task_runner_; |
| 177 |
| 178 // Whether |task_runner_| is the same as base::ThreadTaskRunnerHandle::Get() |
| 179 // for the thread. |
| 180 const bool is_default_task_runner_; |
| 181 |
| 182 ScopedWatcherHandle watcher_handle_; |
| 183 |
| 184 // A thread-safe context object corresponding to the currently active watch, |
| 185 // if any. |
| 186 scoped_refptr<Context> context_; |
| 187 |
| 188 // Fields below must only be accessed on the SimpleWatcher's owning thread. |
| 189 |
| 190 // The handle currently under watch. Not owned. |
| 191 Handle handle_; |
| 192 |
| 193 // The callback to call when the handle is signaled. |
| 194 ReadyCallback callback_; |
| 195 |
| 196 // Tracks if the SimpleWatcher has already notified of unsatisfiability. This |
| 197 // is used to prevent redundant notifications in AUTOMATIC mode. |
| 198 bool unsatisfiable_ = false; |
| 199 |
| 200 // Tag used to ID memory allocations that originated from notifications in |
| 201 // this watcher. |
| 202 const char* heap_profiler_tag_ = nullptr; |
| 203 |
| 204 base::WeakPtrFactory<SimpleWatcher> weak_factory_; |
| 205 |
| 206 DISALLOW_COPY_AND_ASSIGN(SimpleWatcher); |
| 207 }; |
| 208 |
| 209 } // namespace mojo |
| 210 |
| 211 #endif // MOJO_PUBLIC_CPP_SYSTEM_SIMPLE_WATCHER_H_ |
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