Android selector shape 无效问题

<?xml version="1.0" encoding="utf-8"?>
<selector xmlns:android="http://schemas.android.com/apk/res/android">
    <item>
        <shape android:shape="rectangle">
            <corners android:bottomLeftRadius="16dp" android:bottomRightRadius="16dp" android:topLeftRadius="16dp" android:topRightRadius="16dp"></corners>
            <stroke android:width="1dp" android:color="#999999"></stroke>
            <solid android:color="#fefefe"></solid>
        </shape>
    </item>

  
    <item android:state_selected="true">

        <shape android:shape="rectangle">
            <corners android:bottomLeftRadius="17dp" android:bottomRightRadius="17dp" android:topLeftRadius="17dp" android:topRightRadius="17dp"></corners>
            <stroke android:width="1dp" android:color="#999999"></stroke>
            <solid android:color="#cccccc"></solid>
        </shape>

    </item>

    <item android:state_pressed="true">
        <shape android:shape="rectangle">
            <corners android:bottomLeftRadius="17dp" android:bottomRightRadius="17dp" android:topLeftRadius="17dp" android:topRightRadius="17dp"></corners>
            <stroke android:width="1dp" android:color="#999999"></stroke>
            <solid android:color="#cccccc"></solid>
        </shape>

    </item>


</selector>

//上面的selector是不能生效的！！！！

//正确的案例如下：【原因是顺序问题】

<?xml version="1.0" encoding="utf-8"?>
<selector xmlns:android="http://schemas.android.com/apk/res/android">
    
    <item android:state_selected="true">

        <shape android:shape="rectangle">
            <corners android:bottomLeftRadius="17dp" android:bottomRightRadius="17dp" android:topLeftRadius="17dp" android:topRightRadius="17dp"></corners>
            <stroke android:width="1dp" android:color="#999999"></stroke>
            <solid android:color="#cccccc"></solid>
        </shape>

    </item>

    <item android:state_pressed="true">
        <shape android:shape="rectangle">
            <corners android:bottomLeftRadius="17dp" android:bottomRightRadius="17dp" android:topLeftRadius="17dp" android:topRightRadius="17dp"></corners>
            <stroke android:width="1dp" android:color="#999999"></stroke>
            <solid android:color="#cccccc"></solid>
        </shape>

    </item>
    //位置应该放在最底部，保证上面的效果不被覆盖
    <item>
        <shape android:shape="rectangle">
            <corners android:bottomLeftRadius="16dp" android:bottomRightRadius="16dp" android:topLeftRadius="16dp" android:topRightRadius="16dp"></corners>
            <stroke android:width="1dp" android:color="#999999"></stroke>
            <solid android:color="#fefefe"></solid>
        </shape>
    </item>
</selector>