mus: Add watcher for all touch events.

ui/views/touch_event_watcher.h

+// Copyright 2016 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_VIEWS_TOUCH_EVENT_WATCHER_H_
+#define UI_VIEWS_TOUCH_EVENT_WATCHER_H_
+
+#include "base/macros.h"
+#include "ui/views/views_export.h"
+
+namespace gfx {
+class Point;
+}
+
+namespace ui {
+class LocatedEvent;
+}
+
+namespace views {
+class Widget;
+
+// An interface for read-only observation of touch events (in particular, the
+// events cannot be marked as handled). Only touch pointer kind are supported.
+// The |target| is the top-level widget that will receive the event, if any.
+// NOTE: On mus this allows observation of events outside of windows owned
+// by the current process, in which case the |target| will be null. On mus
+// event.target() is always null.
+class VIEWS_EXPORT TouchEventWatcher {
+ public:
+  TouchEventWatcher() {}
+  virtual void OnTouchEventObserved(const ui::LocatedEvent& event,
+                                    Widget* target) = 0;
+
+ protected:
+  virtual ~TouchEventWatcher() {}
+
+ private:
+  DISALLOW_COPY_AND_ASSIGN(TouchEventWatcher);
+};
+
+}  // namespace views
+
+#endif  // UI_VIEWS_TOUCH_EVENT_WATCHER_H_