WaitableEvent

base/waitable_event_watcher.h

+// Copyright (c) 2006-2008 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_WAITABLE_EVENT_WATCHER_H_
+#define BASE_WAITABLE_EVENT_WATCHER_H_
+
+#include "build/build_config.h"
+
+#if defined(OS_WIN)
+#include "base/object_watcher.h"
+#else
+#include "base/message_loop.h"
+#endif
+
+namespace base {
+
+class WaitableEvent;
+class Flag;
+class AsyncWaiter;
+class AsyncCallbackTask;
+
+// -----------------------------------------------------------------------------
+// This class provides a way to wait on a WaitableEvent asynchronously.
+//
+// Each instance of this object can be waiting on a single WaitableEvent. When
+// the waitable event is signaled, a callback is made in the thread of a given
+// MessageLoop. This callback can be deleted by deleting the waiter.
+//
+// Typical usage:
+//
+//   class MyClass : public base::WaitableEventWatcher::Delegate {
+//    public:
+//     void DoStuffWhenSignaled(WaitableEvent *waitable_event) {
+//       watcher_.StartWatching(waitable_event, this);
+//     }
+//     virtual void OnWaitableEventSignaled(WaitableEvent* waitable_event) {
+//       // OK, time to do stuff!
+//     }
+//    private:
+//     base::WaitableEventWatcher watcher_;
+//   };
+//
+// In the above example, MyClass wants to "do stuff" when waitable_event
+// becomes signaled. WaitableEventWatcher makes this task easy. When MyClass
+// goes out of scope, the watcher_ will be destroyed, and there is no need to
+// worry about OnWaitableEventSignaled being called on a deleted MyClass
+// pointer.
+//
+// BEWARE: With automatically reset WaitableEvents, a signal may be lost if it
+// occurs just before a WaitableEventWatcher is deleted. There is currently no
+// safe way to stop watching an automatic reset WaitableEvent without possibly
+// missing a signal.
+// -----------------------------------------------------------------------------
+
+class WaitableEventWatcher
+#if defined(OS_POSIX)
+        : public MessageLoop::DestructionObserver
+#endif
+{
+ public:
+
+  WaitableEventWatcher();
+  ~WaitableEventWatcher();
+
+  class Delegate {
+   public:
+    virtual ~Delegate() { }
+
+    // -------------------------------------------------------------------------
+    // This is called on the MessageLoop thread when WaitableEvent has been
+    // signaled.
+    //
+    // Note: the event may not be signaled by the time that this function is
+    // called. This indicates only that it has been signaled at some point in
+    // the past.
+    // -------------------------------------------------------------------------
+    virtual void OnWaitableEventSignaled(WaitableEvent* waitable_event) = 0;
+  };
+
+  // ---------------------------------------------------------------------------
+  // When @event is signaled, the given delegate is called on the thread of the
+  // current message loop when StartWatching is called. The delegate is not
+  // deleted.
+  // ---------------------------------------------------------------------------
+  bool StartWatching(WaitableEvent* event, Delegate* delegate);
+
+  // ---------------------------------------------------------------------------
+  // Cancel the current watch. Must be called from the same thread which
+  // started the watch.
+  //
+  // Does nothing if no event is being watched, nor if the watch has completed.
+  // The delegate will *not* be called for the current watch after this
+  // function returns. Since the delegate runs on the same thread as this
+  // function, it cannot be called during this function either.
+  // ---------------------------------------------------------------------------
+  void StopWatching();
+
+  // ---------------------------------------------------------------------------
+  // Return the currently watched event, or NULL if no object is currently being
+  // watched.
+  // ---------------------------------------------------------------------------
+  WaitableEvent* GetWatchedEvent();
+
+ private:
+  WaitableEvent* event_;
+
+#if defined(OS_WIN)
+  // ---------------------------------------------------------------------------
+  // The helper class exists because, if WaitableEventWatcher were to inherit
+  // from ObjectWatcher::Delegate, then it couldn't also have an inner class
+  // called Delegate (at least on Windows). Thus this object exists to proxy
+  // the callback function
+  // ---------------------------------------------------------------------------
+  class ObjectWatcherHelper : public ObjectWatcher::Delegate {
+   public:
+    ObjectWatcherHelper(WaitableEventWatcher* watcher);
+
+    // -------------------------------------------------------------------------
+    // Implementation of ObjectWatcher::Delegate
+    // -------------------------------------------------------------------------
+    void OnObjectSignaled(HANDLE h);
+
+   private:
+    WaitableEventWatcher *const watcher_;
+  };
+
+  void OnObjectSignaled();
+
+  Delegate* delegate_;
+  ObjectWatcherHelper helper_;
+  ObjectWatcher watcher_;
+#else
+  // ---------------------------------------------------------------------------
+  // Implementation of MessageLoop::DestructionObserver
+  // ---------------------------------------------------------------------------
+  void WillDestroyCurrentMessageLoop();
+
+  MessageLoop* message_loop_;
+  scoped_refptr<Flag> cancel_flag_;
+  AsyncWaiter* waiter_;
+  AsyncCallbackTask* callback_task_;
+#endif
+};
+
+}  // namespace base
+
+#endif  // BASE_WAITABLE_EVENT_WATCHER_H_