[Aura] Implement views-based applist.

ui/base/models/list_model_observer.h

+// Copyright (c) 2011 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_BASE_MODELS_LIST_MODEL_OBSERVER_H_
+#define UI_BASE_MODELS_LIST_MODEL_OBSERVER_H_
+#pragma once
+
+#include "ui/base/ui_export.h"
+
+namespace ui {
+
+class UI_EXPORT ListModelObserver {
+ public:
+  // Invoked after items has been added to the model.
+  virtual void ListItemsAdded(int start, int count) = 0;
+
+  // Invoked after items has been removed. |start| is the index before the
+  // removal.
+  virtual void ListItemsRemoved(int start, int count) = 0;
+
+  // Invoked after items has been changed.
+  virtual void ListItemsChanged(int start, int count) = 0;
+
+ protected:
+  virtual ~ListModelObserver() {}
+};
+
+}  // namespace ui
+
+#endif  // UI_BASE_MODELS_LIST_MODEL_OBSERVER_H_