VideoView回调中的表面无效

Question

我有一个TabbedActivity，它有2个标签。每个选项卡都有自己的活动，活动会进行视频播放。我想在这些活动中访问VideoViews的底层表面，因此我实现了SurfaceHolder.Callback，如下所示：

public void surfaceChanged(SurfaceHolder holder, int format, int width,
        int height) {
    // TODO Auto-generated method stub
    Log.e(TAG,"surfaceChanged "+ holder+" format "+format+" w "+width+" h "+height);
    if (holder.getSurface().isValid())
        Log.e(TAG,"holder has a valid surface");
    else
        Log.e(TAG,"holder has an invalid surface");     
}

public void surfaceCreated(SurfaceHolder holder) {
    // TODO Auto-generated method stub
    if (holder.getSurface().isValid())
        Log.e(TAG,"holder has a valid surface");
    else
        Log.e(TAG,"holder has an invalid surface");
    mHolder = holder;

}

当我运行此代码时，回调始终显示表面所有者具有无效表面。所以，无论我在表面上做什么，mHolder.getSurface（）都会进入NULL表面。我在这里缺少什么？

提前致谢