viewflipper add view not showing

Question

this code works in another project.setcontentView(a viewgroup).this view group addView(mainView),i think thats the problem,but i dont know how to slove it

mainView = mLInflater.inflate(R.layout.playsong, null);

mflipper = (MyFlipper) mainView.findViewById(R.id.viewflipper);    
 protected void onPostExecute(Void result)
{
    super.onPostExecute(result);

  mflipper.removeAllViews();

    for(int i=0;i<listBitmap.size();i++)
    {
        mflipper.addView(addImageView(listBitmap.get(i)));
    }

    mflipper.invalidate();  
    mflipper.showNext();
}
private View addImageView(Bitmap bitmap) {

    ImageView imageView = new ImageView(PlayingSongActivity.this);
    imageView.setImageBitmap(bitmap);
    imageView.setScaleType(ImageView.ScaleType.CENTER);
    return imageView;
}
 <cn.duole.util.MyFlipper 
  android:layout_width="fill_parent"
   android:layout_height="200dip"
   android:background="@android:color/white"
  android:id="@+id/viewflipper"
  android:layout_marginTop="40dip"
  android:padding="20dip"
  android:layout_gravity="center_horizontal"

  />

after the code, the viewflipper is not showing at all.please help me,i dont know whats the problem.addImageView(listBitmap.get(i)) is right.i wonder if the problem is that the viewflipper is not refreshed