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:
+  enum EventStatus {
+    EVENT_STATUS_CONTINUE,
+    EVENT_STATUS_HANDLED
+  };
+
+  // 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 EVENT_STATUS_HANDLED from this function. The event
+  // dispatch continues as usual if this returns EVENT_STATUS_CONTINUE.
+  virtual EventStatus 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;
+
+ protected:
+  virtual ~PlatformEventObserver() {}
+};
+
+}  // namespace ui
+
+#endif  // UI_EVENTS_PLATFORM_PLATFORM_EVENT_OBSERVER_H_