Drawable State List

http://developer.android.com/guide/topics/resources/drawable-resource.html#StateList

"You can describe the state list in an XML file. Each graphic is represented by an <item> element inside a single <selector>

element. Each <item> uses various attributes to describe the state in which it should be used as the graphic for the drawable."

Here is an example:

1) Save this file in res/drawable as button_selector.xml

<?xml version="1.0" encoding="utf-8"?>

<selector xmlns:android="http://schemas.android.com/apk/res/android">

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

<item android:drawable="@drawable/rounded_rect_w_gradient"/>

</selector>

2) Save this file in res/drawable as rounded_rect_no_gradient.xml

<?xml version="1.0" encoding="utf-8"?>

<shape xmlns:android="http://schemas.android.com/apk/res/android"

android:shape="rectangle" >

<solid android:color="#BD33ff00" />

<corners android:radius="3dp" />

<stroke android:width="5px" />

</shape>

3) Save this file in res/drawable as rounded_rect_w_gradient.xml

<?xml version="1.0" encoding="utf-8"?>

<shape xmlns:android="http://schemas.android.com/apk/res/android"

android:shape="rectangle" >

<gradient

android:angle="270"

android:endColor="#BD33ff00"

android:startColor="#BDffffff" />

<corners android:radius="3dp" />

</shape>

4) In your layout.xml add a reference to the button_selector.xml file.

<Button

android:layout_height="wrap_content"

android:layout_width="wrap_content"

android:background="@drawable/button_selector" />

This is the official Android example:

http://developer.android.com/guide/topics/resources/drawable-resource.html#StateList

XML file saved at res/drawable/button.xml:

<?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 -->

<item android:state_hovered="true"

android:drawable="@drawable/button_focused" /> <!-- hovered -->

<item android:drawable="@drawable/button_normal" /> <!-- default -->

</selector>

This layout XML applies the state list drawable to a Button:

<Button

android:layout_height="wrap_content"

android:layout_width="wrap_content"

android:background="@drawable/button" />