// Copyright (c) 2012 The Chromium Authors. All rights reserved. // Use of this source code is governed by a BSD-style license that can be // found in the LICENSE file. #ifndef BASE_SYNCHRONIZATION_WAITABLE_EVENT_WATCHER_H_ #define BASE_SYNCHRONIZATION_WAITABLE_EVENT_WATCHER_H_ #include "base/base_export.h" #include "build/build_config.h" #if defined(OS_WIN) #include "base/win/object_watcher.h" #else #include "base/callback.h" #include "base/message_loop/message_loop.h" #include "base/synchronization/waitable_event.h" #endif namespace base { class Flag; class AsyncWaiter; class AsyncCallbackTask; class WaitableEvent; // This class provides a way to wait on a WaitableEvent asynchronously. // // Each instance of this object can be waiting on a single WaitableEvent. When // the waitable event is signaled, a callback is made in the thread of a given // MessageLoop. This callback can be deleted by deleting the waiter. // // Typical usage: // // class MyClass { // public: // void DoStuffWhenSignaled(WaitableEvent *waitable_event) { // watcher_.StartWatching(waitable_event, // base::Bind(&MyClass::OnWaitableEventSignaled, this); // } // private: // void OnWaitableEventSignaled(WaitableEvent* waitable_event) { // // OK, time to do stuff! // } // base::WaitableEventWatcher watcher_; // }; // // In the above example, MyClass wants to "do stuff" when waitable_event // becomes signaled. WaitableEventWatcher makes this task easy. When MyClass // goes out of scope, the watcher_ will be destroyed, and there is no need to // worry about OnWaitableEventSignaled being called on a deleted MyClass // pointer. // // BEWARE: With automatically reset WaitableEvents, a signal may be lost if it // occurs just before a WaitableEventWatcher is deleted. There is currently no // safe way to stop watching an automatic reset WaitableEvent without possibly // missing a signal. // // NOTE: you /are/ allowed to delete the WaitableEvent while still waiting on // it with a Watcher. It will act as if the event was never signaled. class BASE_EXPORT WaitableEventWatcher #if defined(OS_WIN) : public win::ObjectWatcher::Delegate { #else : public MessageLoop::DestructionObserver { #endif public: typedef Callback<void(WaitableEvent*)> EventCallback; WaitableEventWatcher(); ~WaitableEventWatcher() override; // When @event is signaled, the given callback is called on the thread of the // current message loop when StartWatching is called. bool StartWatching(WaitableEvent* event, const EventCallback& callback); // Cancel the current watch. Must be called from the same thread which // started the watch. // // Does nothing if no event is being watched, nor if the watch has completed. // The callback will *not* be called for the current watch after this // function returns. Since the callback runs on the same thread as this // function, it cannot be called during this function either. void StopWatching(); // Return the currently watched event, or NULL if no object is currently being // watched. WaitableEvent* GetWatchedEvent(); // Return the callback that will be invoked when the event is // signaled. const EventCallback& callback() const { return callback_; } private: #if defined(OS_WIN) void OnObjectSignaled(HANDLE h) override; win::ObjectWatcher watcher_; #else // Implementation of MessageLoop::DestructionObserver void WillDestroyCurrentMessageLoop() override; MessageLoop* message_loop_; scoped_refptr<Flag> cancel_flag_; AsyncWaiter* waiter_; base::Closure internal_callback_; scoped_refptr<WaitableEvent::WaitableEventKernel> kernel_; #endif WaitableEvent* event_; EventCallback callback_; }; } // namespace base #endif // BASE_SYNCHRONIZATION_WAITABLE_EVENT_WATCHER_H_