我已经将SurfaceView子类化,并在Activity的onCreate中实例化了它.预览已生成,但控件从未输入onDraw(),该值在SurfaceView的子类中被覆盖.这是为什么？

class ActivityClass extends Activity{

    onCreate(){

        mPreview = new Preview(this);
        setContentView(mPreview);
    }
    public void startPreview(){

        rec = new MediaRecorder();
        rec.setVideoSource();.......
        rec.setPreviewDisplay(mPreview.getSurfaceHolder.getSurface());
    } 
}

class Preview extends SurfaceView implements SurfaceHolder.Callback{

    SurfaceHolder mHolder;

    public Preview(Context context){
        super(context);
        mHolder = getHolder();     
        mHolder.addCallback(this);      
        mHolder.setType(SurfaceHolder.SURFACE_TYPE_PUSH_BUFFERS); 
        bringToFront();//this is not
        invalidate();//making a difference
    }

    SurfaceHolder getSurfaceHolder(){

        return mHolder;
    }
    //Surface callback methods implemented here
}

在Surface上绘制预览之前,是否应将控件提供给onDraw回调(如果已实现)？

因为onDraw回调对Android框架说’您不绘制视图.自实施以来,我将对其进行绘制.我对吗？

那么,为什么控件无法输入onDraw()？请帮忙.