Updates shadow when active window changes

components/mus/public/cpp/window_tree_connection_observer.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 COMPONENTS_MUS_PUBLIC_CPP_WINDOW_TREE_CONNECTION_OBSERVER_H_
+#define COMPONENTS_MUS_PUBLIC_CPP_WINDOW_TREE_CONNECTION_OBSERVER_H_
+
+namespace mus {
+
+class Window;
+
+class WindowTreeConnectionObserver {
+ public:
+  virtual void OnWindowTreeFocusChanged(Window* gained_focus,
+                                        Window* lost_focus) {}
+
+ protected:
+  virtual ~WindowTreeConnectionObserver() {}
+};
+
+}  // namespace mus
+
+#endif  // COMPONENTS_MUS_PUBLIC_CPP_WINDOW_TREE_CONNECTION_OBSERVER_H_