How to draw radius background to Button?

在 drawable 資料夾裡，新增一個 xml，定義 shapre 物件和 radius 屬性，範例如下:

// bg_selector_radius_button.xml
<layer-list xmlns:android="http://schemas.android.com/apk/res/android">
    <item>
     <selector>
      <item android:state_pressed="true">
        <shape 
          android:shape="rectangle">
          <corners android:radius="2dp" />
          <solid android:color="#ffff00" />
        </shape>
      </item>
      <item  android:state_enabled="false">
        <shape 
          android:shape="rectangle">
          <corners android:radius="2dp" />
          <solid android:color="#999999" />
        </shape>
      </item>
      <item>
        <shape 
          android:shape="rectangle">
          <corners android:radius="2dp" />
          <solid android:color="#ff0000" />
        </shape>
      </item>
     </selector>
    </item>
</layer-list>