Make WaitableEventWatcher TaskScheduler-friendly.

base/synchronization/waitable_event_watcher.h

 // 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.
 
 #ifndef BASE_SYNCHRONIZATION_WAITABLE_EVENT_WATCHER_H_
 #define BASE_SYNCHRONIZATION_WAITABLE_EVENT_WATCHER_H_
 
 #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"
 #else
 #include "base/callback.h"
-#include "base/message_loop/message_loop.h"
 #include "base/synchronization/waitable_event.h"
 #endif
 
 namespace base {
 
 class Flag;
 class AsyncWaiter;
 class AsyncCallbackTask;
 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 made in the thread of a given
-// MessageLoop. This callback can be deleted by deleting the waiter.
+// 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:
@@ skipping 12 matching lines @@
 // 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 {
-#else
-    : public MessageLoop::DestructionObserver {
+    : public win::ObjectWatcher::Delegate
 #endif
+{
  public:
   typedef Callback<void(WaitableEvent*)> EventCallback;
   WaitableEventWatcher();
+
+#if defined(OS_WIN)
   ~WaitableEventWatcher() override;
+#else
+  ~WaitableEventWatcher();
+#endif
 
-  // When @event is signaled, the given callback is called on the thread of the
-  // current message loop when StartWatching is called.
+  // 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 thread which
+  // 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 thread as this
+  // function returns. Since the callback runs on the same sequence as this
   // function, it cannot be called during this function either.
   void StopWatching();
 
-  // Return the currently watched event, or NULL if no object is currently being
-  // watched.
-  WaitableEvent* GetWatchedEvent();
-
-  // Return the callback that will be invoked when the event is
-  // signaled.
-  const EventCallback& callback() const { return callback_; }
-
  private:
 #if defined(OS_WIN)
   void OnObjectSignaled(HANDLE h) override;
+
   win::ObjectWatcher watcher_;
+  EventCallback callback_;
+  WaitableEvent* event_ = nullptr;
 #else
-  // Implementation of MessageLoop::DestructionObserver
-  void WillDestroyCurrentMessageLoop() override;
+  // Instantiated in StartWatching(). Set before the callback runs. Reset in
+  // StopWatching() or StartWatching().
+  scoped_refptr<Flag> cancel_flag_;
 
-  MessageLoop* message_loop_;
-  scoped_refptr<Flag> cancel_flag_;
-  AsyncWaiter* waiter_;
-  base::Closure internal_callback_;
+  // 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_;
 #endif
 
-  WaitableEvent* event_;
-  EventCallback callback_;
+  DISALLOW_COPY_AND_ASSIGN(WaitableEventWatcher);
 };
 
 }  // namespace base
 
 #endif  // BASE_SYNCHRONIZATION_WAITABLE_EVENT_WATCHER_H_