android如何设置Activity背景色为透明色

 <!--透明activity的style-->  <style name="transparent_activity">    <item name="android:windowBackground">@color/transparent_background</item>    <item name="android:windowNoTitle">true</item>    <item name="android:windowIsTranslucent">true</item>    <item name="android:windowAnimationStyle">@android:style/Animation.Translucent</item>  </style>

<color name="transparent_background">#50000000</color>

 <activity      android:name=".TransparentActivity"      android:theme="@style/transparent_activity" />

android:theme="@android:style/Theme.Translucent"

//改成activity继承自Activity：public class MainActivity extends Activity