Revert of Remove MessageLoop::current() from base::win::ObjectWatcher.

base/win/object_watcher.h

 // Copyright (c) 2011 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_WIN_OBJECT_WATCHER_H_
 #define BASE_WIN_OBJECT_WATCHER_H_
 
 #include <windows.h>
 
 #include "base/base_export.h"
 #include "base/callback.h"
 #include "base/macros.h"
-#include "base/memory/ref_counted.h"
 #include "base/memory/weak_ptr.h"
-#include "base/single_thread_task_runner.h"
+#include "base/message_loop/message_loop.h"
 
 namespace base {
 namespace win {
 
 // A class that provides a means to asynchronously wait for a Windows object to
 // become signaled.  It is an abstraction around RegisterWaitForSingleObject
 // that provides a notification callback, OnObjectSignaled, that runs back on
 // the origin thread (i.e., the thread that called StartWatching).
 //
 // This class acts like a smart pointer such that when it goes out-of-scope,
@@ skipping 14 matching lines @@
 //     base::win::ObjectWatcher watcher_;
 //   };
 //
 // In the above example, MyClass wants to "do stuff" when object becomes
 // signaled.  ObjectWatcher makes this task easy.  When MyClass goes out of
 // scope, the watcher_ will be destroyed, and there is no need to worry about
 // OnObjectSignaled being called on a deleted MyClass pointer.  Easy!
 // If the object is already signaled before being watched, OnObjectSignaled is
 // still called after (but not necessarily immediately after) watch is started.
 //
-// NOTE: Except for the constructor, all public methods of this class must be
-// called on the same thread. A ThreadTaskRunnerHandle must be set on that
-// thread.
-class BASE_EXPORT ObjectWatcher {
+// NOTE: Use of this class requires that there be a current message loop;
+// otherwise, when the object is signaled, there would be no loop to post the
+// callback task to.  This means that you cannot use ObjectWatcher in test code
+// that doesn't create a message loop (unless you add such a loop).
+class BASE_EXPORT ObjectWatcher : public MessageLoop::DestructionObserver {
  public:
   class BASE_EXPORT Delegate {
    public:
     virtual ~Delegate() {}
-    // Called from the thread that started the watch when a signaled object is
-    // detected. To continue watching the object, StartWatching must be called
-    // again.
+    // Called from the MessageLoop when a signaled object is detected.  To
+    // continue watching the object, StartWatching must be called again.
     virtual void OnObjectSignaled(HANDLE object) = 0;
   };
 
   ObjectWatcher();
-  ~ObjectWatcher();
+  ~ObjectWatcher() override;
 
   // When the object is signaled, the given delegate is notified on the thread
   // where StartWatchingOnce is called. The ObjectWatcher is not responsible for
   // deleting the delegate.
   // Returns whether watching was successfully initiated.
   bool StartWatchingOnce(HANDLE object, Delegate* delegate);
 
   // Notifies the delegate, on the thread where this method is called, each time
   // the object is set. By definition, the handle must be an auto-reset object.
   // The caller must ensure that it (or any Windows system code) doesn't reset
@@ skipping 17 matching lines @@
  private:
   // Called on a background thread when done waiting.
   static void CALLBACK DoneWaiting(void* param, BOOLEAN timed_out);
 
   // Helper used by StartWatchingOnce and StartWatchingMultipleTimes.
   bool StartWatchingInternal(HANDLE object, Delegate* delegate,
                              bool execute_only_once);
 
   void Signal(Delegate* delegate);
 
-  void Reset();
+  // MessageLoop::DestructionObserver implementation:
+  void WillDestroyCurrentMessageLoop() override;
 
-  // A callback pre-bound to Signal() that is posted to the caller's task runner
-  // when the wait completes.
+  // Internal state.
   Closure callback_;
-
-  // The object being watched.
-  HANDLE object_ = nullptr;
-
-  // The wait handle returned by RegisterWaitForSingleObject.
-  HANDLE wait_object_ = nullptr;
-
-  // The task runner of the thread on which the watch was started.
-  scoped_refptr<SingleThreadTaskRunner> task_runner_;
-
-  bool run_once_ = true;
-
+  HANDLE object_;             // The object being watched
+  HANDLE wait_object_;        // Returned by RegisterWaitForSingleObject
+  MessageLoop* origin_loop_;  // Used to get back to the origin thread
+  bool run_once_;
   WeakPtrFactory<ObjectWatcher> weak_factory_;
 
   DISALLOW_COPY_AND_ASSIGN(ObjectWatcher);
 };
 
 }  // namespace win
 }  // namespace base
 
 #endif  // BASE_WIN_OBJECT_WATCHER_H_