blob: 44ef5047edfebcb0160b7cbc397750045676d0d9 [file] [log] [blame]
// 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.
#include "base/base_export.h"
#include "base/macros.h"
#include "base/sequence_checker.h"
#include "build/build_config.h"
#if defined(OS_WIN)
#include "base/win/object_watcher.h"
#include "base/callback.h"
#include "base/synchronization/waitable_event.h"
namespace base {
class Flag;
class AsyncWaiter;
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 invoked on the sequence that
// called StartWatching(). 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
typedef Callback<void(WaitableEvent*)> EventCallback;
#if defined(OS_WIN)
~WaitableEventWatcher() override;
// When |event| is signaled, |callback| is called on the sequence that called
// StartWatching().
bool StartWatching(WaitableEvent* event, const EventCallback& callback);
// Cancel the current watch. Must be called from the same sequence 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 sequence as this
// function, it cannot be called during this function either.
void StopWatching();
#if defined(OS_WIN)
void OnObjectSignaled(HANDLE h) override;
win::ObjectWatcher watcher_;
EventCallback callback_;
WaitableEvent* event_ = nullptr;
// Instantiated in StartWatching(). Set before the callback runs. Reset in
// StopWatching() or StartWatching().
scoped_refptr<Flag> cancel_flag_;
// Enqueued in the wait list of the watched WaitableEvent.
AsyncWaiter* waiter_ = nullptr;
// Kernel of the watched WaitableEvent.
scoped_refptr<WaitableEvent::WaitableEventKernel> kernel_;
// Ensures that StartWatching() and StopWatching() are called on the same
// sequence.
SequenceChecker sequence_checker_;
} // namespace base