android按钮按下的效果

xml文件名为 ic_tab_artists
<?xml version="1.0" encoding="utf-8"?>   
    <selector xmlns:android="http://schemas.android.com/apk/res/android">   
        <item android:drawable="@drawable/android_pressed"   
              android:state_pressed="true" />   
        <item android:drawable="@drawable/android_focused"   
              android:state_focused="true" />   
        <item android:drawable="@drawable/android_normal" />   
    </selector>  


<ImageButton    
        android:layout_width="fill_parent"    
        android:layout_height="wrap_content"   
        android:background="@drawable/ic_tab_artists"   
    />