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/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_