点击按钮切换背景

解释：按钮的时候按钮变色，移开手指，恢复原样

<?xml version="1.0" encoding="utf-8"?>

<selector xmlns:android="http://schemas.android.com/apk/res/android" >

<!-- 获得焦点但未按下时的背景图片 -->

<item android:state_focused="true" android:state_enabled="true" android:state_pressed="false" android:drawable="@drawable/head_nitify_selecta" />

<!-- 按下时的背景图片 -->

<item android:state_enabled="true" android:state_pressed="true" android:drawable="@drawable/head_nitify_selectaa" />

<!-- 按下时的背景图片 -->

<item android:state_enabled="true" android:state_checked="true" android:drawable="@drawable/head_nitify_selectaa" />

<!-- 默认时的背景图片 -->

<item android:drawable="@drawable/head_nitify_selecta" />

</selector>