The file containing the source code shown below is located in the corresponding directory in <sdk>/samples/android-<version>/...
<!--
Copyright (C) 2011 The Android开源工程
Licensed under the Apache License, Version 2.0 (the "License");
you may not use this file except in compliance with the License.
You may obtain a copy of the License at
http://www.apache.org/licenses/LICENSE-2.0
Unless required by applicable law or agreed to in writing, software
distributed under the License is distributed on an "AS IS" BASIS,
WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
See the License for the specific language governing permissions and
limitations under the License.
-->
<selector xmlns:android="http://schemas.android.com/apk/res/android">
<item android:state_pressed="true" android:drawable="@drawable/stop_pressed" />
<item android:state_focused="true" android:drawable="@drawable/stop_pressed" />
<item android:drawable="@drawable/stop" />
</selector>