events: Introduce EventRewriter.

ui/events/event_rewriter.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_EVENT_REWRITER_H_
+#define UI_EVENTS_EVENT_REWRITER_H_
+
+#include "ui/events/events_export.h"
+
+namespace ui {
+
+class Event;
+
+enum EventRewriteStatus {
+  EVENT_REWRITE_CONTINUE,
+  EVENT_REWRITE_REWRITTEN,
+  EVENT_REWRITE_DISCARD,
+  EVENT_REWRITE_DISPATCH_ANOTHER,

[sadrul, 2014/03/24 21:15:38]

Comments for each to explain what they mean.

+};
+
+class EVENTS_EXPORT EventRewriter {

[sadrul, 2014/03/24 21:15:38]

A brief class-level comment to give an overview of the responsibilities of
EventRewriter.

+ public:
+  virtual ~EventRewriter() {}
+
+  // If the rewriter wants to rewrite an event, and dispatch
+  // another event once the rewritten event is dispatched, it
+  // should return EVENT_REWRITE_DISPATCH_ANOTHER, and return
+  // the next event to dispatch from |NextDispatchEvent()|.
+  virtual EventRewriteStatus RewriteEvent(const Event& event,
+                                          Event** rewritten_event) = 0;
+
+  virtual EventRewriteStatus NextDispatchEvent(const Event& last_event,
+                                               Event** new_event) = 0;
+};
+
+}  // namespace ui
+
+#endif  // UI_EVENTS_EVENT_REWRITER_H_