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