以编程方式使用Androids加载旋转器

Question

我目前想要使用Android的系统Spinners，它位于..\sdk\platforms\android-19\data\res\

我目前有.png文件示例spinner_white_48.png

和动画xml progress_medium_holo.xml：

<layer-list xmlns:android="http://schemas.android.com/apk/res/android">
    <item>
        <rotate
             android:drawable="@drawable/spinner_48_outer_holo"
             android:pivotX="50%"
             android:pivotY="50%"
             android:fromDegrees="0"
             android:toDegrees="1080" />
    </item>
    <item>
        <rotate
             android:drawable="@drawable/spinner_48_inner_holo"
             android:pivotX="50%"
             android:pivotY="50%"
             android:fromDegrees="720"
             android:toDegrees="0" />
    </item>
</layer-list>

以下不起作用：

         // Load the ImageView that will host the animation and
         // set its background to our AnimationDrawable XML resource.
         ImageView img = new ImageView(context);
         img.setBackgroundResource(my.package.R.drawable.progress_medium_holo);

         // Get the background, which has been compiled to an AnimationDrawable object.
         AnimationDrawable frameAnimation = (AnimationDrawable) img.getBackground();

         // Start the animation (looped playback by default).
         frameAnimation.start();

我现在正在接收错误：

12-05 23:31:15.731: E/AndroidRuntime(30685): java.lang.ClassCastException: android.graphics.drawable.LayerDrawable cannot be cast to android.graphics.drawable.AnimationDrawable

我找不到有关如何以编程方式实现这些内容的任何信息。