object_watcher.h - mozsearch

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/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */

/* vim: set ts=8 sts=2 et sw=2 tw=80: */

// Copyright (c) 2006-2008 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_OBJECT_WATCHER_H_

#define BASE_OBJECT_WATCHER_H_

#include <windows.h>

#ifdef GetClassName

#  undef GetClassName

#endif

#include "base/message_loop.h"

namespace base {

// 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,

// UnregisterWaitEx is automatically called, and any in-flight notification is

// suppressed.

//

// Typical usage:

//

// ```

//   class MyClass : public base::ObjectWatcher::Delegate {

//    public:

//     void DoStuffWhenSignaled(HANDLE object) {

//       watcher_.StartWatching(object, this);

//     }

//     virtual void OnObjectSignaled(HANDLE object) {

//       // OK, time to do stuff!

//     }

//    private:

//     base::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!

//

//////

//

// Mozilla/Gecko addendum:

//

// An undocumented (but runtime-asserted) requirement for the above is that

// `MyClass` must be strictly thread-affine. In particular, `StartWatching()`

// and `StopWatching()` -- including the implicit `StopWatching()` call in

// `~ObjectWatcher()` -- must always be called on the same thread and from

// within the same `MessageLoop`.

//

// (If this did not hold, `OnObjectSignaled()` might be called on one thread

// while `MyClass` is in the middle of being destroyed on another.)

//

// This condition cannot be guaranteed for potentially-asynchronously-destroyed

// classes, nor for their owned or shared subobjects, nor for anything which

// might be sent to a non-thread-affine task queue.

//

class ObjectWatcher : public MessageLoop::DestructionObserver {

 public:

  class Delegate {

   public:

    virtual ~Delegate() {}

    // Called from the MessageLoop when a signaled object is detected.  To

    // continue watching the object, AddWatch must be called again.

    virtual void OnObjectSignaled(HANDLE object) = 0;

};

  ObjectWatcher();

  ~ObjectWatcher();

  // When the object is signaled, the given delegate is notified on the thread

  // where StartWatching is called.  The ObjectWatcher is not responsible for

  // deleting the delegate.

//

  // Returns true if the watch was started.  Otherwise, false is returned.

//

  bool StartWatching(HANDLE object, Delegate* delegate);

  // Stops watching.  Does nothing if the watch has already completed.  If the

  // watch is still active, then it is canceled, and the associated delegate is

  // not notified.

//

  // Returns true if the watch was canceled.  Otherwise, false is returned.

//

  bool StopWatching();

  // Returns the handle of the object being watched, or NULL if the object

  // watcher is stopped.

  HANDLE GetWatchedObject();

 private:

  // Called on a background thread when done waiting.

  static void CALLBACK DoneWaiting(void* param, BOOLEAN timed_out);

  // MessageLoop::DestructionObserver implementation:

  virtual void WillDestroyCurrentMessageLoop();

  // Internal state.

  class Watch;

  RefPtr<Watch> watch_;

  DISALLOW_COPY_AND_ASSIGN(ObjectWatcher);

};

}  // namespace base

#endif  // BASE_OBJECT_WATCHER_H_