Canvas剪裁

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剪裁是Canvas的重要功能之一。可以剪切画裁的指定区域。并且可以对多次剪切的结果进行组合，实现组合效果。android画布类提供了如下函数实现剪切功能。
boolean clipPath(Path path)。用指定路径剪切画布。
boolean clipPath(Path path, Region.Op op)。用指定路径剪裁画布，并根据Region.Op标记对多次剪裁结果进行组合。
boolean clipRect(Rect rect, Region.Op op)。用指定矩形剪裁画布，并根据Region.Op标记对多次剪裁结果进行组合。
boolean clipRect(RectF rect, Region.Op op)。用指定矩形剪裁画布，并根据Region.Op标记对多次剪裁结果进行组合。
boolean clipRect(int left, int top, int right, int bottom)。用指定矩形剪裁画布。
boolean clipRect(float left, float top, float right, float bottom) 。用指定矩形剪裁画布。
boolean clipRect(RectF rect)。用指定矩形剪裁画布。
boolean clipRect(float left, float top, float right, float bottom, Region.Op op)。用指定举行剪裁画布。并根据Region.Op标记对多次剪裁结果进行组合。
boolean clipRect(Rect rect)。用指定矩形剪裁画布。
boolean clipRegion(Region region)。用指定区域剪裁画布。
boolean clipRegion(Region region, Region.Op op)。用指定区域剪裁画布。并根据Region.Op标记对多次剪裁结果进行组合。

Region.Op 表示多次裁剪的组合方式,有如下选择：

假设有两次clip操作，结果分别为集合A 和 B,那么下面的操作方式分别表示：
Region.Op DIFFERENCE A - B
Region.Op INTERSECT A∩B
Region.Op REPLACE B
Region.Op REVERSE_DIFFERENCE B - A
Region.Op UNION A ∪ B
Region.Op XOR (A∪B) - (A∩B)

效果图如下：
图片说明文字

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代码如下：package com.example.androidgraphicsprogramm;import android.app.Activity;import android.content.Context;import android.graphics.*;import android.os.Bundle;import android.util.Log;import android.view.View;public class Clipping extends Activity {    private static final String TAG = "Clipping";    @Override    protected void onCreate(Bundle savedInstanceState) {        super.onCreate(savedInstanceState);        setContentView(new SampleView(this));    }    private static class SampleView extends View {        private Paint mPaint;        private Path mPath;        public SampleView(Context context) {            super(context);            setFocusable(true);            mPaint = new Paint();            mPaint.setAntiAlias(true);            mPaint.setStrokeWidth(6);            mPaint.setTextSize(16);            mPaint.setTextAlign(Paint.Align.RIGHT);            mPath = new Path();        }        private void drawScene(Canvas canvas) {            canvas.clipRect(0, 0, 160, 160);            canvas.drawColor(Color.WHITE);            mPaint.setColor(Color.RED);            canvas.drawLine(0, 0, 160, 160, mPaint);            mPaint.setColor(Color.GREEN);            canvas.drawCircle(47, 113, 47, mPaint);            mPaint.setColor(Color.BLUE);            canvas.drawText("Clipping", 140, 30, mPaint);        }        @Override protected void onDraw(Canvas canvas) {            boolean clipresult = false;            canvas.drawColor(Color.GRAY);                        canvas.save();            canvas.translate(10, 10);            drawScene(canvas);            canvas.restore();            canvas.save();            canvas.translate(180, 10);            clipresult = canvas.clipRect(20, 20, 140, 140);            Log.d(TAG,"clip result is: " + clipresult);            clipresult = canvas.clipRect(50, 50, 110, 110, Region.Op.DIFFERENCE);            Log.d(TAG,"clip result with op is: " + clipresult);            drawScene(canvas);            canvas.restore();            canvas.save();            canvas.translate(10, 200);            canvas.clipRect(20, 20, 140, 140);            mPath.reset();            canvas.clipPath(mPath); // makes the clip empty            mPath.addCircle(80, 80, 80, Path.Direction.CCW);            canvas.clipPath(mPath, Region.Op.REPLACE);            drawScene(canvas);            canvas.restore();            canvas.save();            canvas.translate(180, 200);            canvas.clipRect(10, 10, 100, 100);            canvas.clipRect(80, 80, 170, 170, Region.Op.UNION);            drawScene(canvas);            canvas.restore();            canvas.save();            canvas.translate(10, 390);            canvas.clipRect(0, 0, 80, 80);            canvas.clipRect(40, 40, 120, 120, Region.Op.XOR);            drawScene(canvas);            canvas.restore();            canvas.save();            canvas.translate(180, 390);            canvas.clipRect(0, 0, 80, 80);            canvas.clipRect(40, 40, 120, 120, Region.Op.REVERSE_DIFFERENCE);            drawScene(canvas);            canvas.restore();            canvas.save();            canvas.translate(10, 580);            canvas.clipRect(0, 0, 80, 80);            canvas.clipRect(40, 40, 120, 120, Region.Op.INTERSECT);            drawScene(canvas);            canvas.restore();        }    }}

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