Button選擇器（使按鈕具有點擊效果）

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

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

            <item android:state_pressed="true" android:drawable="@drawable/button_pressed" /><!-- pressed -->

            <item android:state_focused="true" android:drawable="@drawable/button_focused" /><!-- focused -->

            <item android:drawable="@drawable/button_focused" /> <!-- default -->

     </selector>