mus: Add PointerWatcher for passively observing mouse and touch events

ui/views/pointer_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_POINTER_WATCHER_H_
+#define UI_VIEWS_POINTER_WATCHER_H_
+
+#include "ui/views/views_export.h"
+
+namespace ui {
+class MouseEvent;
+class TouchEvent;
+}
+
+namespace views {
+
+// An interface for read-only observation of pointer events (in particular, the
+// events cannot be marked as handled). Only certain event types are supported.
+class VIEWS_EXPORT PointerWatcher {
+ public:
+  virtual ~PointerWatcher() {}
+
+  virtual void OnMousePressed(const ui::MouseEvent& event) = 0;
+  virtual void OnTouchPressed(const ui::TouchEvent& event) = 0;
+};
+
+}  // namespace views
+
+#endif  // UI_VIEWS_POINTER_WATCHER_H_