按钮按下渐变效果

<?xml version="1.0" encoding="utf-8"?>
<selector xmlns:android="http://schemas.android.com/apk/res/android">
<item android:state_window_focused="false" android:state_pressed="false" android:drawable="@drawable/left_switcher_collapsed" />
<item android:state_window_focused="true" android:state_pressed="true" android:drawable="@drawable/left_switcher_collapsed_selected" />
<item android:state_focused="true" android:drawable="@drawable/left_switcher_collapsed_focused" />
</selector>