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// 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/weak_ptr.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,
// UnregisterWaitEx is automatically called, and any in-flight notification is
// suppressed.
//
// Typical usage:
//
// class MyClass : public base::win::ObjectWatcher::Delegate {
// public:
// void DoStuffWhenSignaled(HANDLE object) {
// watcher_.StartWatchingOnce(object, this);
// }
// void OnObjectSignaled(HANDLE object) override {
// // OK, time to do stuff!
// }
// private:
// 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: 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 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() 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
// the event or else the delegate won't be called.
// Returns whether watching was successfully initiated.
bool StartWatchingMultipleTimes(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 true if currently watching an object.
bool IsWatching() const;
// Returns the handle of the object being watched.
HANDLE GetWatchedObject() const;
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);
// MessageLoop::DestructionObserver implementation:
void WillDestroyCurrentMessageLoop() override;
// Internal state.
Closure callback_;
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_
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