SurfaceHolder.lockCanvas() 报 java.lang.NullPointerException

在onCreate里面直接调用 Canvas mCanvas = mSurfaceHolder.lockCanvas();报了 java.lang.NullPointerException
mCanvas 始终是 null。  google下都无法解决。最后在api文档里面看到
Start editing the pixels in the surface. The returned Canvas can be used to draw into the surface's bitmap. A null is returned if the surface has not been created or otherwise cannot be edited. You will usually need to implement Callback.surfaceCreated to find out when the Surface is available for use.

返回null有可能是还没有创建好或是不能编辑。解决方法：
实现Callback

	@Override
	protected void onCreate(Bundle savedInstanceState) {
		// TODO Auto-generated method stub
		super.onCreate(savedInstanceState);
		setContentView(R.layout.scale);
		mSurfaceView = (SurfaceView) this.findViewById(R.id.surfaceView1);
		mSurfaceHolder = mSurfaceView.getHolder();
		mSurfaceHolder.addCallback(this);
	}

	@Override
	public void surfaceCreated(SurfaceHolder holder) {
		// TODO Auto-generated method stub
		mBitmap = BitmapFactory.decodeResource(getResources(),
				R.drawable.ironman);
		Canvas mCanvas = mSurfaceHolder.lockCanvas();
		mCanvas.drawBitmap(mBitmap, 0f, 0f, null);
		mSurfaceHolder.unlockCanvasAndPost(mCanvas);
		// 重新锁一次
		mSurfaceHolder.lockCanvas(new Rect(0, 0, 0, 0));
		mSurfaceHolder.unlockCanvasAndPost(mCanvas);
	}