1// Copyright (c) 2012 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 BASE_SYNCHRONIZATION_WAITABLE_EVENT_WATCHER_H_
6#define BASE_SYNCHRONIZATION_WAITABLE_EVENT_WATCHER_H_
7
8#include "base/base_export.h"
9#include "build/build_config.h"
10
11#if defined(OS_WIN)
12#include "base/win/object_watcher.h"
13#else
14#include "base/callback.h"
15#include "base/message_loop/message_loop.h"
16#include "base/synchronization/waitable_event.h"
17#endif
18
19namespace base {
20
21class Flag;
22class AsyncWaiter;
23class AsyncCallbackTask;
24class WaitableEvent;
25
26// This class provides a way to wait on a WaitableEvent asynchronously.
27//
28// Each instance of this object can be waiting on a single WaitableEvent. When
29// the waitable event is signaled, a callback is made in the thread of a given
30// MessageLoop. This callback can be deleted by deleting the waiter.
31//
32// Typical usage:
33//
34//   class MyClass {
35//    public:
36//     void DoStuffWhenSignaled(WaitableEvent *waitable_event) {
37//       watcher_.StartWatching(waitable_event,
38//           base::Bind(&MyClass::OnWaitableEventSignaled, this);
39//     }
40//    private:
41//     void OnWaitableEventSignaled(WaitableEvent* waitable_event) {
42//       // OK, time to do stuff!
43//     }
44//     base::WaitableEventWatcher watcher_;
45//   };
46//
47// In the above example, MyClass wants to "do stuff" when waitable_event
48// becomes signaled. WaitableEventWatcher makes this task easy. When MyClass
49// goes out of scope, the watcher_ will be destroyed, and there is no need to
50// worry about OnWaitableEventSignaled being called on a deleted MyClass
51// pointer.
52//
53// BEWARE: With automatically reset WaitableEvents, a signal may be lost if it
54// occurs just before a WaitableEventWatcher is deleted. There is currently no
55// safe way to stop watching an automatic reset WaitableEvent without possibly
56// missing a signal.
57//
58// NOTE: you /are/ allowed to delete the WaitableEvent while still waiting on
59// it with a Watcher. It will act as if the event was never signaled.
60
61class BASE_EXPORT WaitableEventWatcher
62#if defined(OS_WIN)
63    : public win::ObjectWatcher::Delegate {
64#else
65    : public MessageLoop::DestructionObserver {
66#endif
67 public:
68  typedef Callback<void(WaitableEvent*)> EventCallback;
69  WaitableEventWatcher();
70  ~WaitableEventWatcher() override;
71
72  // When @event is signaled, the given callback is called on the thread of the
73  // current message loop when StartWatching is called.
74  bool StartWatching(WaitableEvent* event, const EventCallback& callback);
75
76  // Cancel the current watch. Must be called from the same thread which
77  // started the watch.
78  //
79  // Does nothing if no event is being watched, nor if the watch has completed.
80  // The callback will *not* be called for the current watch after this
81  // function returns. Since the callback runs on the same thread as this
82  // function, it cannot be called during this function either.
83  void StopWatching();
84
85  // Return the currently watched event, or NULL if no object is currently being
86  // watched.
87  WaitableEvent* GetWatchedEvent();
88
89  // Return the callback that will be invoked when the event is
90  // signaled.
91  const EventCallback& callback() const { return callback_; }
92
93 private:
94#if defined(OS_WIN)
95  void OnObjectSignaled(HANDLE h) override;
96  win::ObjectWatcher watcher_;
97#else
98  // Implementation of MessageLoop::DestructionObserver
99  void WillDestroyCurrentMessageLoop() override;
100
101  MessageLoop* message_loop_;
102  scoped_refptr<Flag> cancel_flag_;
103  AsyncWaiter* waiter_;
104  base::Closure internal_callback_;
105  scoped_refptr<WaitableEvent::WaitableEventKernel> kernel_;
106#endif
107
108  WaitableEvent* event_;
109  EventCallback callback_;
110};
111
112}  // namespace base
113
114#endif  // BASE_SYNCHRONIZATION_WAITABLE_EVENT_WATCHER_H_
115