events: Introduce PlatformEventDispatcher and PlatformEventSource.

ui/events/platform/platform_event_observer.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_PLATFORM_EVENT_OBSERVER_H_
+#define UI_EVENTS_PLATFORM_PLATFORM_EVENT_OBSERVER_H_
+
+#include "ui/events/events_export.h"
+#include "ui/events/platform/platform_event_types.h"
+
+namespace ui {
+
+// PlatformEventObserver can be installed on a PlatformEventSource, and it
+// receives all events that are dispatched to the dispatchers.
+class EVENTS_EXPORT PlatformEventObserver {
+ public:
+  virtual ~PlatformEventObserver() {}

[sky, 2014/03/25 15:51:15]

protected?

[sadrul, 2014/03/25 18:33:15]

Done.

+
+  // This is called before the dispatcher receives the event. The observer can
+  // consume the event and stop the event from reaching the dispatcher or other
+  // observers by returning true from this function. The event dispatch

[sky, 2014/03/25 15:51:15]

nit: I think an enum would be cleaner here.

[sadrul, 2014/03/25 18:33:15]

Done.

+  // continues as usual if this returns false.
+  virtual bool WillProcessEvent(const PlatformEvent& event) = 0;
+
+  // This is called after the event has been dispatched to the dispatcher(s).
+  virtual void DidProcessEvent(const PlatformEvent& event) = 0;
+};
+
+}  // namespace ui
+
+#endif  // UI_EVENTS_PLATFORM_PLATFORM_EVENT_OBSERVER_H_