正確的姿勢：
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<?xml version="1.0" encoding="utf-8"?>
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<selector xmlns:android="http://schemas.android.com/apk/res/android">
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<!--
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android:state_enabled="true" 不能省，否則disable狀態會無效
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-->
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<item android:state_enabled="true" android:state_pressed="true" android:drawable="@drawable/btn_chat_pressed" />
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<item android:state_enabled="true" android:drawable="@drawable/btn_chat_normal" />
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<item android:state_enabled="false" android:drawable="@drawable/btn_chat_disable"/>
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</selector>
----EOF----