android中SurfaceView的基本使用方法和带来的好处

##为什么要使用SurfaceView来实现动画? ###因为View的绘图存在以下缺陷:

  1. View缺乏双缓冲机制
  2. 当程序需要更新View上的图像时,程序必须重绘View上显示的整张图片
  3. 新线程无法直接更新View组件

##SurfaceView的绘图机制

  • 一般会与SurfaceView结合使用
  • 调用SurfaceView的getHolder()方法即可获得SurfaceView关联的SurfaceHolder

##SurfaceHolder提供了如下方法来获取Canvas对象

  1. Canvas lockCanvas():锁定整个SurfaceView对象,获取该Surface上的Canvas
  2. Canvas lockCanvas(Rect dirty):锁定SurfaceView上Rect划分的区域,获取该Surface上的Canvas
  3. unlockCanvasAndPost(canvas):释放绘图、提交所绘制的图形,需要注意,当调用SurfaceHolder上的unlockCanvasAndPost方法之后,该方法之前所绘制的图形还处于缓冲之中,下一次lockCanvas()方法锁定的区域可能会“遮挡”它

	public class SurfaceViewTest extends Activity
{
	// SurfaceHolder负责维护SurfaceView上绘制的内容
	private SurfaceHolder holder;
	private Paint paint;

	@Override
	public void onCreate(Bundle savedInstanceState)
	{
		super.onCreate(savedInstanceState);
		setContentView(R.layout.main);
		paint = new Paint();
		SurfaceView surface = (SurfaceView) findViewById(R.id.show);
		// 初始化SurfaceHolder对象
		holder = surface.getHolder();
		holder.addCallback(new Callback()
		{
			@Override
			public void surfaceChanged(SurfaceHolder arg0, int arg1, int arg2,
					int arg3)
			{
			}

			@Override
			public void surfaceCreated(SurfaceHolder holder)
			{
				// 锁定整个SurfaceView
				Canvas canvas = holder.lockCanvas();
				// 绘制背景
				Bitmap back = BitmapFactory.decodeResource(
					SurfaceViewTest.this.getResources()
					, R.drawable.sun);
				// 绘制背景
				canvas.drawBitmap(back, 0, 0, null);
				// 绘制完成,释放画布,提交修改
				holder.unlockCanvasAndPost(canvas);
				// 重新锁一次,"持久化"上次所绘制的内容
				holder.lockCanvas(new Rect(0, 0, 0, 0));
				holder.unlockCanvasAndPost(canvas);
			}

			@Override
			public void surfaceDestroyed(SurfaceHolder holder)
			{
			}
		});
		// 为surface的触摸事件绑定监听器
		surface.setOnTouchListener(new OnTouchListener()
		{
			@Override
			public boolean onTouch(View source, MotionEvent event)
			{
				// 只处理按下事件
				if (event.getAction() == MotionEvent.ACTION_DOWN)
				{
					int cx = (int) event.getX();
					int cy = (int) event.getY();
					// 锁定SurfaceView的局部区域,只更新局部内容
					Canvas canvas = holder.lockCanvas(new Rect(cx - 50,
							cy - 50, cx + 50, cy + 50));
					// 保存canvas的当前状态
					canvas.save();
					// 旋转画布
					canvas.rotate(30, cx, cy);
					paint.setColor(Color.RED);
					// 绘制红色方块
					canvas.drawRect(cx - 40, cy - 40, cx, cy, paint);
					// 恢复Canvas之前的保存状态
					canvas.restore();
					paint.setColor(Color.GREEN);
					// 绘制绿色方块
					canvas.drawRect(cx, cy, cx + 40, cy + 40, paint);
					// 绘制完成,释放画布,提交修改
					holder.unlockCanvasAndPost(canvas);
				}
				return false;
			}
		});
	}
	}
	

上面的程序为SurfaceHolder添加了一个CallBack实例,该Callback中定义了如下三个方法:

  • void surfaceChanged(SurfaceHolder holder, int format, int width, int height):当一个surface的格式或大小发生改变时回调该方法。
  • void surfaceCreated(SurfaceHolder holder):当surface被创建时回调该方法
  • void surfaceDestroyed(SurfaceHolder holder):当surface将要被销毁时回调该方法