1// Copyright (c) 2011 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#pragma once
8
9#include "build/build_config.h"
10
11#if defined(OS_WIN)
12#include "base/win/object_watcher.h"
13#else
14#include "base/message_loop.h"
15#include "base/synchronization/waitable_event.h"
16#endif
17
18#include "base/base_api.h"
19
20namespace base {
21
22class Flag;
23class AsyncWaiter;
24class AsyncCallbackTask;
25class WaitableEvent;
26
27// -----------------------------------------------------------------------------
28// This class provides a way to wait on a WaitableEvent asynchronously.
29//
30// Each instance of this object can be waiting on a single WaitableEvent. When
31// the waitable event is signaled, a callback is made in the thread of a given
32// MessageLoop. This callback can be deleted by deleting the waiter.
33//
34// Typical usage:
35//
36//   class MyClass : public base::WaitableEventWatcher::Delegate {
37//    public:
38//     void DoStuffWhenSignaled(WaitableEvent *waitable_event) {
39//       watcher_.StartWatching(waitable_event, this);
40//     }
41//     virtual void OnWaitableEventSignaled(WaitableEvent* waitable_event) {
42//       // OK, time to do stuff!
43//     }
44//    private:
45//     base::WaitableEventWatcher watcher_;
46//   };
47//
48// In the above example, MyClass wants to "do stuff" when waitable_event
49// becomes signaled. WaitableEventWatcher makes this task easy. When MyClass
50// goes out of scope, the watcher_ will be destroyed, and there is no need to
51// worry about OnWaitableEventSignaled being called on a deleted MyClass
52// pointer.
53//
54// BEWARE: With automatically reset WaitableEvents, a signal may be lost if it
55// occurs just before a WaitableEventWatcher is deleted. There is currently no
56// safe way to stop watching an automatic reset WaitableEvent without possibly
57// missing a signal.
58//
59// NOTE: you /are/ allowed to delete the WaitableEvent while still waiting on
60// it with a Watcher. It will act as if the event was never signaled.
61// -----------------------------------------------------------------------------
62
63class BASE_API WaitableEventWatcher
64#if defined(OS_POSIX)
65    : public MessageLoop::DestructionObserver
66#endif
67{
68 public:
69
70  WaitableEventWatcher();
71  ~WaitableEventWatcher();
72
73  class Delegate {
74   public:
75    virtual ~Delegate() { }
76
77    // -------------------------------------------------------------------------
78    // This is called on the MessageLoop thread when WaitableEvent has been
79    // signaled.
80    //
81    // Note: the event may not be signaled by the time that this function is
82    // called. This indicates only that it has been signaled at some point in
83    // the past.
84    // -------------------------------------------------------------------------
85    virtual void OnWaitableEventSignaled(WaitableEvent* waitable_event) = 0;
86  };
87
88  // ---------------------------------------------------------------------------
89  // When @event is signaled, the given delegate is called on the thread of the
90  // current message loop when StartWatching is called. The delegate is not
91  // deleted.
92  // ---------------------------------------------------------------------------
93  bool StartWatching(WaitableEvent* event, Delegate* delegate);
94
95  // ---------------------------------------------------------------------------
96  // Cancel the current watch. Must be called from the same thread which
97  // started the watch.
98  //
99  // Does nothing if no event is being watched, nor if the watch has completed.
100  // The delegate will *not* be called for the current watch after this
101  // function returns. Since the delegate runs on the same thread as this
102  // function, it cannot be called during this function either.
103  // ---------------------------------------------------------------------------
104  void StopWatching();
105
106  // ---------------------------------------------------------------------------
107  // Return the currently watched event, or NULL if no object is currently being
108  // watched.
109  // ---------------------------------------------------------------------------
110  WaitableEvent* GetWatchedEvent();
111
112  // ---------------------------------------------------------------------------
113  // Return the delegate, or NULL if there is no delegate.
114  // ---------------------------------------------------------------------------
115  Delegate* delegate() {
116    return delegate_;
117  }
118
119 private:
120#if defined(OS_WIN)
121  // ---------------------------------------------------------------------------
122  // The helper class exists because, if WaitableEventWatcher were to inherit
123  // from ObjectWatcher::Delegate, then it couldn't also have an inner class
124  // called Delegate (at least on Windows). Thus this object exists to proxy
125  // the callback function
126  // ---------------------------------------------------------------------------
127  class ObjectWatcherHelper : public win::ObjectWatcher::Delegate {
128   public:
129    ObjectWatcherHelper(WaitableEventWatcher* watcher);
130
131    // -------------------------------------------------------------------------
132    // Implementation of ObjectWatcher::Delegate
133    // -------------------------------------------------------------------------
134    void OnObjectSignaled(HANDLE h);
135
136   private:
137    WaitableEventWatcher *const watcher_;
138  };
139
140  void OnObjectSignaled();
141
142  ObjectWatcherHelper helper_;
143  win::ObjectWatcher watcher_;
144#else
145  // ---------------------------------------------------------------------------
146  // Implementation of MessageLoop::DestructionObserver
147  // ---------------------------------------------------------------------------
148  virtual void WillDestroyCurrentMessageLoop();
149
150  MessageLoop* message_loop_;
151  scoped_refptr<Flag> cancel_flag_;
152  AsyncWaiter* waiter_;
153  AsyncCallbackTask* callback_task_;
154  scoped_refptr<WaitableEvent::WaitableEventKernel> kernel_;
155#endif
156
157  WaitableEvent* event_;
158
159  Delegate* delegate_;
160};
161
162}  // namespace base
163
164#endif  // BASE_SYNCHRONIZATION_WAITABLE_EVENT_WATCHER_H_
165