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.