android学习7——canvas.concat(Matrix matrix)作用

canvas.concat的作用可以理解成对matrix的变换应用到canvas上的所有对象.
看下面的代码.

public class ConcatMatrixActivity extends Activity { @Override public void onCreate(Bundle savedInstanceState) { super.onCreate(savedInstanceState); requestWindowFeature(Window.FEATURE_NO_TITLE); getWindow().setFlags( WindowManager.LayoutParams.FLAG_FULLSCREEN, WindowManager.LayoutParams.FLAG_FULLSCREEN); setContentView(new ConcatMatrixView(this)); } } public class ConcatMatrixView extends View { private Matrix matrix = new Matrix(); private Paint bgPaint = new Paint(); public ConcatMatrixView(Context context) { super(context); matrix.setScale(2f, 2f); } @Override protected void onDraw(Canvas canvas) { super.onDraw(canvas); bgPaint.setColor(Color.RED); canvas.drawRect(0, 0, 100, 100, bgPaint); canvas.save(); canvas.concat(matrix); canvas.drawRect(100, 100, 200, 200, bgPaint); canvas.restore(); canvas.drawRect(400, 400, 500, 500, bgPaint); } }

以上代码对matrix的x和y坐标都拉伸了2倍.变化的效果如下所示:

在没有concat的情况下canvas.drawRect(100, 100, 200, 200, bgPaint);是在(100, 100), (200, 200)的区域下画一个矩形.因为执行了matrix.setScale(2f, 2f)所以matrix的x和y坐标都拉伸了2倍.这时候在concat的作用下canvas.drawRect(100, 100, 200, 200, bgPaint)变成了在(200, 200), (400, 400)的区域上画矩形.
同理，如果把对matrix的变换由matrix.setScale(2f, 2f)改成matrix.setTranslate(100f, 100f),效果如下所示:

x轴和y轴坐标都移动了100个像素.

posted on