如何实现局部刷新？我用了invalidate(Rect dirty)还是全局刷新。。。

public void onDraw(Canvas canvas) {

		

	    super.onDraw(canvas);



		int images[][] = MainActivity.images;

		int number = images.length;

		int canvasWidth = canvas.getWidth();

		int imageWidth = canvasWidth / number;

		Bitmap bmp = BitmapFactory.decodeResource(getResources(), images[0][0]);

		int srcWidth = bmp.getWidth();

		float scale = (float) imageWidth / srcWidth;

		

		if (!isStarted) {

			

			canvas.drawColor(Color.WHITE);



			for (int i = 0; i < number; i++) {

				for (int j = 0; j < number; j++) {

					Random rand = new Random();

					Bitmap srcBmp = BitmapFactory.decodeResource(

							getResources(), images[rand.nextInt(number)][0]);



					Matrix matrix = new Matrix();

					matrix.postScale(scale, scale);

					Bitmap dstBmp = Bitmap

							.createBitmap(srcBmp, 0, 0, srcBmp.getWidth(),

									srcBmp.getHeight(), matrix, true);



					canvas.drawBitmap(dstBmp, i * imageWidth, j * imageWidth,

							null);

				}

			}

			

			//isStarted = true;  //加上的话就只执行一次，下次刷新图就都没了；不加的话每次都是全局刷新，相当于重新加载了

			//我想要的效果是第一次完全加载，以后就是局部刷新。

		}

		else

		{

			if(isTouched)

			{

				

			}

		}

	}

Rect DrawRect = new Rect(srcPosition.x*imageWidth, srcPosition.y*imageWidth,

					tarPosition.x*imageWidth, tarPosition.y*imageWidth);

			

			touchView.invalidate(DrawRect);