base/synchronization/waitable_event_watcher.h

   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.
  91   const EventCallback& callback() const { return callback_; }
  92
  93  private:
  94 #if defined(OS_WIN)
  95   virtual 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_