android 帧动画不显示大家帮我看看怎么个情况啊

 package com.example.animation;

import android.os.Bundle;
import android.app.Activity;
import android.graphics.drawable.AnimationDrawable;
import android.view.Menu;
import android.view.MotionEvent;
import android.view.View;
import android.widget.Button;
import android.widget.ImageView;

public class MainActivity extends Activity {
    AnimationDrawable an;
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        ImageView  iv = (ImageView) findViewById(R.id.imageView1);
        iv.setBackgroundResource(R.anim.my_animation);
        an = (AnimationDrawable) iv.getBackground();
        Button btn = (Button) findViewById(R.id.button1);
        btn.setOnClickListener(new  View.OnClickListener() {

            @Override
            public void onClick(View arg0) {
                // TODO 自动生成的方法存根
                an.start();
            }
        });

    }
}


<?xml version="1.0" encoding="utf-8"?>
<animation-list xmlns:android="http://schemas.android.com/apk/res/android" 
    android:oneshot="false">
    <Item android:drawable="@drawable/flash201" android:duration = "400"/>
    <Item android:drawable="@drawable/flash202" android:duration = "400"/>
    <Item android:drawable="@drawable/flash203" android:duration = "400"/>
    <Item android:drawable="@drawable/flash204" android:duration = "400"/>
    <Item android:drawable="@drawable/flash205" android:duration = "400"/>


</animation-list>




<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"

    tools:context=".MainActivity" >

    <ImageView
        android:id="@+id/imageView1"
        android:layout_width="100dp"
        android:layout_height="100dp"
         />

    <Button
        android:id="@+id/button1"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_alignParentLeft="true"
        android:layout_below="@+id/imageView1"
        android:layout_marginLeft="43dp"
        android:layout_marginTop="50dp"
        android:text="Button" />

</RelativeLayout>

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