<?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:drawable="@drawable/ic_launcher" android:state_enabled="true" android:state_focused="true" android:state_pressed="false"/>

    <!-- 按下时的背景图片 -->

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

    <!-- 按下时的背景图片 -->

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

    <!-- 默认时的背景图片 -->

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

</selector>

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

    xmlns:tools="http://schemas.android.com/tools"

    android:layout_width="match_parent"

    android:layout_height="match_parent"

    tools:context=".MainActivity" >

    <Button

        android:id="@+id/btn_user_selected"

        android:layout_width="wrap_content"

        android:layout_height="wrap_content"

        android:background="@drawable/btn_selected" >

    </Button>

</RelativeLayout>