Refactoring to make adding ink drop animations easier

ui/events/scoped_target_handler.h

+// Copyright 2015 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_SCOPED_TARGET_HANDLER_H_
+#define UI_EVENTS_SCOPED_TARGET_HANDLER_H_
+
+#include "base/macros.h"
+#include "ui/events/event_handler.h"
+#include "ui/events/events_export.h"
+
+namespace ui {
+
+class EventTarget;
+
+// An EventHandler that replaces an EventTarget's target handler with itself
+// to pass events to both the original handlers and an additional new
+// EventHandler.

[sadrul, 2015/11/25 22:38:58]

Document the ordering (i.e. whether the new handler receives the event or the
old one). Also document whether SetHandled()/StopPropagation()'ed on the event
affects whether the event reaches both handlers or not.

[varkha, 2015/11/26 00:05:46]

Done.

+class EVENTS_EXPORT ScopedTargetHandler : public EventHandler {
+ public:
+  ScopedTargetHandler(EventTarget* target, EventHandler* new_handler);
+  ~ScopedTargetHandler() override;
+
+  // ui::EventHandler:
+  void OnEvent(ui::Event* event) override;
+  void OnKeyEvent(ui::KeyEvent* event) override;
+  void OnMouseEvent(ui::MouseEvent* event) override;
+  void OnScrollEvent(ui::ScrollEvent* event) override;
+  void OnTouchEvent(ui::TouchEvent* event) override;
+  void OnGestureEvent(ui::GestureEvent* event) override;
+  void OnCancelMode(ui::CancelModeEvent* event) override;
+
+ private:
+  // If non-null the destructor sets this to true. This is set while handling
+  // an event and used to detect if |this| has been deleted.
+  bool* destroyed_flag_;
+
+  // An EventTarget that has its target handler replaced with |this| for a life
+  // time of |this|.
+  EventTarget* target_;
+
+  // An EventHandler that gets restored on |target_| when |this| is destroyed.
+  EventHandler* original_handler_;
+
+  // A new handler that gets events in addition to the |original_handler_| or
+  // |target_|.
+  EventHandler* new_handler_;
+
+  DISALLOW_COPY_AND_ASSIGN(ScopedTargetHandler);
+};
+
+}  // namespace ui
+
+#endif  // UI_EVENTS_SCOPED_TARGET_HANDLER_H_