events: Introduce PlatformEventDispatcher and PlatformEventSource.

ui/events/platform/scoped_event_dispatcher.h

+// Copyright 2014 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 UI_EVENTS_PLATFORM_SCOPED_EVENT_DISPATCHER_H_
+#define UI_EVENTS_PLATFORM_SCOPED_EVENT_DISPATCHER_H_
+
+#include "base/auto_reset.h"
+#include "base/basictypes.h"
+#include "ui/events/events_export.h"
+
+namespace ui {
+
+class PlatformEventDispatcher;
+
+// A temporary PlatformEventDispatcher can be installed on a
+// PlatformEventSource that overrides all installed event dispatchers, and
+// always gets a chance to dispatch the event first. The PlatformEventSource
+// returns a ScopedEventDispatcher object in such cases. This
+// ScopedEventDispatcher object can be used to dispatch the event to any
+// previous overridden dispatcher. When this object is destroyed, it removes the
+// override-dispatcher, and restores the previous override-dispatcher.
+class EVENTS_EXPORT ScopedEventDispatcher {
+ public:
+  ScopedEventDispatcher(PlatformEventDispatcher** scoped_dispatcher,
+                        PlatformEventDispatcher* new_dispatcher);
+  ~ScopedEventDispatcher();
+
+  operator PlatformEventDispatcher*() const { return original_; }
+
+ private:
+  PlatformEventDispatcher* original_;
+  base::AutoReset<PlatformEventDispatcher*> restore_;
+
+  DISALLOW_COPY_AND_ASSIGN(ScopedEventDispatcher);
+};
+
+}  // namespace ui
+
+#endif  // UI_EVENTS_PLATFORM_SCOPED_EVENT_DISPATCHER_H_