ipc/chromium/src/base/object_watcher.h

   1 /* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
   2 /* vim: set ts=8 sts=2 et sw=2 tw=80: */
   3 // Copyright (c) 2006-2008 The Chromium Authors. All rights reserved.
   4 // Use of this source code is governed by a BSD-style license that can be
   5 // found in the LICENSE file.
   6
   7 #ifndef BASE_OBJECT_WATCHER_H_
   8 #define BASE_OBJECT_WATCHER_H_
   9
  10 #include <windows.h>
  11 #ifdef GetClassName
  12 #  undef GetClassName
  13 #endif
  14
  15 #include "base/message_loop.h"
  16
  17 namespace base {
  18
  19 // A class that provides a means to asynchronously wait for a Windows object to
  20 // become signaled.  It is an abstraction around RegisterWaitForSingleObject
  21 // that provides a notification callback, OnObjectSignaled, that runs back on
  22 // the origin thread (i.e., the thread that called StartWatching).
  23 //
  24 // This class acts like a smart pointer such that when it goes out-of-scope,
  25 // UnregisterWaitEx is automatically called, and any in-flight notification is
  26 // suppressed.
  27 //
  28 // Typical usage:
  29 //
  30 // ```
  31 //   class MyClass : public base::ObjectWatcher::Delegate {
  32 //    public:
  33 //     void DoStuffWhenSignaled(HANDLE object) {
  34 //       watcher_.StartWatching(object, this);
  35 //     }
  36 //     virtual void OnObjectSignaled(HANDLE object) {
  37 //       // OK, time to do stuff!
  38 //     }
  39 //    private:
  40 //     base::ObjectWatcher watcher_;
  41 //   };
  42 // ```
  43 //
  44 // In the above example, MyClass wants to "do stuff" when object becomes
  45 // signaled.  ObjectWatcher makes this task easy.  When MyClass goes out of
  46 // scope, the watcher_ will be destroyed, and there is no need to worry about
  47 // OnObjectSignaled being called on a deleted MyClass pointer.  Easy!
  48 //
  49 //////
  50 //
  51 // Mozilla/Gecko addendum:
  52 //
  53 // An undocumented (but runtime-asserted) requirement for the above is that
  54 // `MyClass` must be strictly thread-affine. In particular, `StartWatching()`
  55 // and `StopWatching()` -- including the implicit `StopWatching()` call in
  56 // `~ObjectWatcher()` -- must always be called on the same thread and from
  57 // within the same `MessageLoop`.
  58 //
  59 // (If this did not hold, `OnObjectSignaled()` might be called on one thread
  60 // while `MyClass` is in the middle of being destroyed on another.)
  61 //
  62 // This condition cannot be guaranteed for potentially-asynchronously-destroyed
  63 // classes, nor for their owned or shared subobjects, nor for anything which
  64 // might be sent to a non-thread-affine task queue.
  65 //
  66 class ObjectWatcher : public MessageLoop::DestructionObserver {
  67  public:
  68   class Delegate {
  69    public:
  70     virtual ~Delegate() {}
  71     // Called from the MessageLoop when a signaled object is detected.  To
  72     // continue watching the object, AddWatch must be called again.
  73     virtual void OnObjectSignaled(HANDLE object) = 0;
  74   };
  75
  76   ObjectWatcher();
  77   ~ObjectWatcher();
  78
  79   // When the object is signaled, the given delegate is notified on the thread
  80   // where StartWatching is called.  The ObjectWatcher is not responsible for
  81   // deleting the delegate.
  82   //
  83   // Returns true if the watch was started.  Otherwise, false is returned.
  84   //
  85   bool StartWatching(HANDLE object, Delegate* delegate);
  86
  87   // Stops watching.  Does nothing if the watch has already completed.  If the
  88   // watch is still active, then it is canceled, and the associated delegate is
  89   // not notified.
  90   //
  91   // Returns true if the watch was canceled.  Otherwise, false is returned.
  92   //
  93   bool StopWatching();
  94
  95   // Returns the handle of the object being watched, or NULL if the object
  96   // watcher is stopped.
  97   HANDLE GetWatchedObject();
  98
  99  private:
 100   // Called on a background thread when done waiting.
 101   static void CALLBACK DoneWaiting(void* param, BOOLEAN timed_out);
 102
 103   // MessageLoop::DestructionObserver implementation:
 104   virtual void WillDestroyCurrentMessageLoop();
 105
 106   // Internal state.
 107   class Watch;
 108   RefPtr<Watch> watch_;
 109
 110   DISALLOW_COPY_AND_ASSIGN(ObjectWatcher);
 111 };
 112
 113 }  // namespace base
 114
 115 #endif  // BASE_OBJECT_WATCHER_H_