base/win/object_watcher.h

   1 // Copyright (c) 2011 The Chromium Authors. All rights reserved.
   2 // Use of this source code is governed by a BSD-style license that can be
   3 // found in the LICENSE file.
   4
   5 #ifndef BASE_WIN_OBJECT_WATCHER_H_
   6 #define BASE_WIN_OBJECT_WATCHER_H_
   7
   8 #include <windows.h>
   9
  10 #include "base/base_export.h"
  11 #include "base/callback.h"
  12 #include "base/memory/weak_ptr.h"
  13 #include "base/message_loop/message_loop.h"
  14
  15 namespace base {
  16 namespace win {
  17
  18 // A class that provides a means to asynchronously wait for a Windows object to
  19 // become signaled.  It is an abstraction around RegisterWaitForSingleObject
  20 // that provides a notification callback, OnObjectSignaled, that runs back on
  21 // the origin thread (i.e., the thread that called StartWatching).
  22 //
  23 // This class acts like a smart pointer such that when it goes out-of-scope,
  24 // UnregisterWaitEx is automatically called, and any in-flight notification is
  25 // suppressed.
  26 //
  27 // Typical usage:
  28 //
  29 //   class MyClass : public base::ObjectWatcher::Delegate {
  30 //    public:
  31 //     void DoStuffWhenSignaled(HANDLE object) {
  32 //       watcher_.StartWatching(object, this);
  33 //     }
  34 //     virtual void OnObjectSignaled(HANDLE object) {
  35 //       // OK, time to do stuff!
  36 //     }
  37 //    private:
  38 //     base::ObjectWatcher watcher_;
  39 //   };
  40 //
  41 // In the above example, MyClass wants to "do stuff" when object becomes
  42 // signaled.  ObjectWatcher makes this task easy.  When MyClass goes out of
  43 // scope, the watcher_ will be destroyed, and there is no need to worry about
  44 // OnObjectSignaled being called on a deleted MyClass pointer.  Easy!
  45 // If the object is already signaled before being watched, OnObjectSignaled is
  46 // still called after (but not necessarily immediately after) watch is started.
  47 //
  48 class BASE_EXPORT ObjectWatcher : public MessageLoop::DestructionObserver {
  49  public:
  50   class BASE_EXPORT Delegate {
  51    public:
  52     virtual ~Delegate() {}
  53     // Called from the MessageLoop when a signaled object is detected.  To
  54     // continue watching the object, StartWatching must be called again.
  55     virtual void OnObjectSignaled(HANDLE object) = 0;
  56   };
  57
  58   ObjectWatcher();
  59   ~ObjectWatcher() override;
  60
  61   // When the object is signaled, the given delegate is notified on the thread
  62   // where StartWatching is called.  The ObjectWatcher is not responsible for
  63   // deleting the delegate.
  64   //
  65   // Returns true if the watch was started.  Otherwise, false is returned.
  66   //
  67   bool StartWatching(HANDLE object, Delegate* delegate);
  68
  69   // Stops watching.  Does nothing if the watch has already completed.  If the
  70   // watch is still active, then it is canceled, and the associated delegate is
  71   // not notified.
  72   //
  73   // Returns true if the watch was canceled.  Otherwise, false is returned.
  74   //
  75   bool StopWatching();
  76
  77   // Returns true if currently watching an object.
  78   bool IsWatching() const;
  79
  80   // Returns the handle of the object being watched.
  81   HANDLE GetWatchedObject() const;
  82
  83  private:
  84   // Called on a background thread when done waiting.
  85   static void CALLBACK DoneWaiting(void* param, BOOLEAN timed_out);
  86
  87   void Signal(Delegate* delegate);
  88
  89   // MessageLoop::DestructionObserver implementation:
  90   void WillDestroyCurrentMessageLoop() override;
  91
  92   // Internal state.
  93   Closure callback_;
  94   HANDLE object_;             // The object being watched
  95   HANDLE wait_object_;        // Returned by RegisterWaitForSingleObject
  96   MessageLoop* origin_loop_;  // Used to get back to the origin thread
  97
  98   WeakPtrFactory<ObjectWatcher> weak_factory_;
  99
 100   DISALLOW_COPY_AND_ASSIGN(ObjectWatcher);
 101 };
 102
 103 }  // namespace win
 104 }  // namespace base
 105
 106 #endif  // BASE_WIN_OBJECT_WATCHER_H_