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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_
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