设置Button的点击效果为40%透明

my_layout.xml

<?xml version="1.0" encoding="utf-8"?>
<Button xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:background="@drawable/selector"
    android:padding="10dp"
    android:text="@string/hello" />

selector.xml

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

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

</selector>

shape_drawable.xml

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

    <solid android:color="#66ffffff" />

</shape>