正确的姿势：

1. <?xml version="1.0" encoding="utf-8"?>
2. <selector xmlns:android="http://schemas.android.com/apk/res/android">
3.     <!--
4.         android:state_enabled="true" 不能省，否则disable状态会无效
5.     -->
6.     <item android:state_enabled="true" android:state_pressed="true" android:drawable="@drawable/btn_chat_pressed" />
7.     <item android:state_enabled="true" android:drawable="@drawable/btn_chat_normal" />
8.     <item android:state_enabled="false" android:drawable="@drawable/btn_chat_disable"/>
9. </selector>

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