它有个属性石drawable, 设置这个属性指向一个xml文件, 这个xml设置一个selector. 
<?xml version="1.0" encoding="utf-8"?>
<selector xmlns:android="http://schemas.android.com/apk/res/android">
    
    <item android:state_focused="true" android:state_enabled="true"
        android:drawable="@drawable/p" />        
    <item android:state_pressed="true" android:state_enabled="true"
        android:drawable="@drawable/p" />
    <item android:state_enabled="true"
        android:drawable="@drawable/n" />
    <item android:state_enabled="false"
        android:drawable="@drawable/d" />
    <item
         android:drawable="@drawable/n" />
</selector>
p和n和d是具体图片, 里面的属性就不用我翻译了吧...