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// 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.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();
virtual ~WaitableEventWatcher();
// 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)
virtual void OnObjectSignaled(HANDLE h) OVERRIDE;
win::ObjectWatcher watcher_;
#else
// Implementation of MessageLoop::DestructionObserver
virtual 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_
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