text: // 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 "base/win/windows_types.h"
  
  #include "base/base_export.h"
  #include "base/callback.h"
  #include "base/macros.h"
  #include "base/memory/ref_counted.h"
  #include "base/memory/weak_ptr.h"
  #include "base/sequenced_task_runner.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 sequence (i.e., the sequence 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.
  //
  // The waiting handle MUST NOT be closed while watching is in progress. If this
  // handle is closed while the wait is still pending, the behavior is undefined
  // (see MSDN:RegisterWaitForSingleObject).
  //
  // 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: Except for the constructor, all public methods of this class must be
  // called in sequence, in a scope where SequencedTaskRunnerHandle::IsSet().
  class BASE_EXPORT ObjectWatcher {
   public:
    class BASE_EXPORT Delegate {
     public:
      virtual ~Delegate() {}
      // Called from the sequence that started the watch when a signaled object is
      // detected. To continue watching the object, StartWatching must be called
      // again.
      virtual void OnObjectSignaled(HANDLE object) = 0;
    };
  
    ObjectWatcher();
    ~ObjectWatcher();
  
    // When the object is signaled, the given delegate is notified on the sequence
    // 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 sequence 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);
  
    void Reset();
  
    // A callback pre-bound to Signal() that is posted to the caller's task runner
    // when the wait completes.
    Closure callback_;
  
    // The object being watched.
    HANDLE object_ = nullptr;
  
    // The wait handle returned by RegisterWaitForSingleObject.
    HANDLE wait_object_ = nullptr;
  
    // The task runner of the sequence on which the watch was started.
    scoped_refptr<SequencedTaskRunner> task_runner_;
  
    bool run_once_ = true;
  
    WeakPtrFactory<ObjectWatcher> weak_factory_;
  
    DISALLOW_COPY_AND_ASSIGN(ObjectWatcher);
  };
  
  }  // namespace win
  }  // namespace base
  
  #endif  // BASE_WIN_OBJECT_WATCHER_H_
  