求一个关于ImageView控件的invalidateDrawable方法的例子，多谢～

风情123

看到API文档：
public void invalidateDrawable (Drawable dr)  
Since: API Level 1 Invalidates the specified Drawable.

Parameters
dr the drawable to invalidate   


看到这个API文档，我的理解是它试图刷新图片的；但是我尝试写改个例子但是没效果的；
请大家指正我下面的例子哪里错了啊？
package com.android.Invalidate;

import android.app.Activity;
import android.content.Context;
import android.graphics.Canvas;
import android.graphics.Paint;
import android.graphics.RectF;
import android.graphics.drawable.Drawable;
import android.os.Bundle;
import android.util.Log;
import android.view.View;
import android.widget.ImageView;

public class InvalidateActivity extends Activity {
  /** Called when the activity is first created. */
static int times = 1;
  @Override
  public void onCreate(Bundle savedInstanceState) {
  super.onCreate(savedInstanceState);
  setContentView(R.layout.main);
  ImageView imageView = (ImageView)findViewById(R.id.ImageView);  
  Drawable drawable = getResources().getDrawable(R.drawable.ic_launcher);
  imageView.setImageDrawable(drawable);
  imageView.invalidateDrawable(drawable);
  imageView.setImageDrawable(drawable);
  }
}