在res/drawable文件夹新增一个文件，此文件设置了图片的触发状态，你可以设置 state_pressed，state_checked，state_pressed，state_selected，state_focused，state_enabled 等几个状态：
android:state_pressed
Boolean. "true" if this item should be used when the object is pressed (such as when a button is touched/clicked); "false" if this item should be used in the default, non-pressed state.
如果是true，当被点击时显示该图片，如果是false没被按下时显示默认。
android:state_focused
Boolean. "true" if this item should be used when the object is focused (such as when a button is highlighted using the trackball/d-pad); "false" if this item should be used in the default, non-focused state.
true，获得焦点时显示；false，没获得焦点显示默认。
android:state_selected
Boolean. "true" if this item should be used when the object is selected (such as when a tab is opened); "false" if this item should be used when the object is not selected.
true，当被选择时显示该图片；false，当未被选择时显示该图片。
android:state_checkable
Boolean. "true" if this item should be used when the object is checkable; "false" if this item should be used when the object is not checkable. (Only useful if the object can transition between a checkable and non-checkable widget.)
true，当CheckBox能使用时显示该图片；false，当CheckBox不能使用时显示该图片。
android:state_checked
Boolean. "true" if this item should be used when the object is checked; "false" if it should be used when the object is un-checked.
true，当CheckBox选中时显示该图片；false，当CheckBox为未选中时显示该图片。
android:state_enabled
Boolean. "true" if this item should be used when the object is enabled (capable of receiving touch/click events); "false" if it should be used when the object is disabled.
true，当该组件能使用时显示该图片；false，当该组件不能使用时显示该图片。
android:state_window_focused
Boolean. "true" if this item should be used when the application window has focus (the application is in the foreground), "false" if this item should be used when the application window does not have focus (for example, if the notification shade is pulled down or a dialog appears).
true，当此activity获得焦点在最前面时显示该图片；false，当没在最前面时显示该图片。
<?xml version="1.0" encoding="utf-8"?>
<selector xmlns:android="http://schemas.android.com/apk/res/android">
<item android:state_pressed="true"
android:drawable="@drawable/button_pressed"/><!-- pressed -->
<item android:state_focused="true"
android:drawable="@drawable/button_focused"/><!-- focused -->
<itemandroid:drawable="@drawable/button_normal"/><!-- default -->
</selector>