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 
19 namespace base {
20 
21 class Flag;
22 class AsyncWaiter;
23 class AsyncCallbackTask;
24 class 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 
61 class 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.
callback()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