selector放置的路径res/drawable/button.xml:
	<?xml version="1.0" encoding="utf-8"?>
	<selector xmlns:android="http://schemas.android.com/apk/res/android">
	    <item android:state_pressed="true"
		  android:drawable="@drawable/button_pressed" /> <!-- 选中按钮图片 -->
	    <item android:state_focused="true"
		  android:drawable="@drawable/button_focused" /> <!-- 按钮获取焦点图片 -->
	    <item android:state_hovered="true"
		  android:drawable="@drawable/button_focused" /> <!-- 平板电视,悬浮选中某个应用图片 -->
	    <item android:drawable="@drawable/button_normal" /> <!-- 默认图片-->
	</selector>

	<Button
	    android:layout_height="wrap_content"
	    android:layout_width="wrap_content"
	    android:background="@drawable/button" />