Android Canvas.clipRect裁剪矩形

更新时间：2022-01-21

文章目录

1. 先来看个效果

1) 效果图2) MainActivity.java3) MyClipRectView4) activity_main.xml 2. 方法分析

1) Rect的构造方法2) canvas.clipRect(Rect rect)3) canvas.clipRect(Rect rect, Region.Op op)4) Op枚举变量说明 3. 效果演示

1) Op为 INTERSECT2) Op为 DIFFERENCE3) 全部显示4) 源码地址 4. 参考文档

1. 先来看个效果 1) 效果图

2) MainActivity.java

import androidx.appcompat.app.AppCompatActivity;
import android.os.Bundle;
import android.view.Window;

public class MainActivity extends AppCompatActivity {

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        supportRequestWindowFeature(Window.FEATURE_NO_TITLE);
        setContentView(R.layout.activity_main);
    }
}

3) MyClipRectView

import android.content.Context;
import android.graphics.Canvas;
import android.graphics.Color;
import android.graphics.Rect;
import android.graphics.Region;
import android.util.AttributeSet;
import android.view.View;
import androidx.annotation.Nullable;

public class MyClipRectView extends View {
    public MyClipRectView(Context context) {
        super(context);
    }
    public MyClipRectView(Context context, @Nullable AttributeSet attrs) {
        super(context, attrs);
    }
    public MyClipRectView(Context context, @Nullable AttributeSet attrs, 
    	int defStyleAttr) {
        super(context, attrs, defStyleAttr);
    }

    @Override
    protected void onDraw(Canvas canvas) {
        super.onDraw(canvas);

        // ****** 先给当前的屏幕设置一个矩形画面 **********
        int measuredHeight = getMeasuredHeight();
        int measuredWidth = getMeasuredWidth();
        Paint paint = new Paint();
        paint.setColor(Color.GRAY);
        canvas.drawRect(0, 0, measuredWidth, measuredHeight, paint);

        // ***************************** 演示1 *****************************
        Rect rect = new Rect(0, 0, measuredWidth/2, measuredHeight/3);
        canvas.clipRect(rect);
        canvas.drawColor(Color.BLUE);
    }
}

4) activity_main.xml

2. 方法分析 1) Rect的构造方法

根据4个参数，来决定矩形区域的位置。

public Rect(int left, int top, int right, int bottom) {
    this.left = left;
    this.top = top;
    this.right = right;
    this.bottom = bottom;
}

2) canvas.clipRect(Rect rect)

裁剪矩形区域，这个矩形的区域，由 Rect来指定。
从下方代码可以看出，会调用nClipRect方法，其中最后一个参数，传入的INTERSECT，默认传入的，咱们没有手动去指定，也可以调用另外一个clipRect方法，来进行手动指定它。

public boolean clipRect(@NonNull Rect rect) {
    return nClipRect(mNativeCanvasWrapper, rect.left, rect.top, 
    	rect.right, rect.bottom, Region.Op.INTERSECT.nativeInt);
}

3) canvas.clipRect(Rect rect, Region.Op op)

public boolean clipRect(@NonNull Rect rect, @NonNull Region.Op op) {
    checkValidClipOp(op);
    return nClipRect(mNativeCanvasWrapper, rect.left, rect.top, 
    	rect.right, rect.bottom, op.nativeInt);
}

private static void checkValidClipOp(@NonNull Region.Op op) {
    if (sCompatiblityVersion >= Build.VERSION_CODES.P
            && op != Region.Op.INTERSECT && op != Region.Op.DIFFERENCE) {
        throw new IllegalArgumentException(
                "Invalid Region.Op - only INTERSECT and DIFFERENCE are allowed");
    }
}

public enum Op {
    DIFFERENCE(0),
    INTERSECT(1),
    UNIOn(2),
    XOR(3),
    REVERSE_DIFFERENCE(4),
    REPLACE(5);

    Op(int nativeInt) {
        this.nativeInt = nativeInt;
    }

    
    @UnsupportedAppUsage
    public final int nativeInt;
}

4) Op枚举变量说明

下方截图，AB为第一个canvas，BC为第二个canvas：

变量	描述
DIFFERENCE	第一次不同于第二次的部分显示，即显示A
INTERSECT	交集显示，即显示B
UNIOn	全部显示，即显示ABC
XOR	补集显示，即显示AC
REVERSE_DIFFERENCE	第二次不同于第一次的部分显示，即显示C
REPLACE	显示第二次的，即显示BC

注意：28以上，只能使用INTERSECT 、DIFFERENCE ，否则会抛出异常！

3. 效果演示 1) Op为 INTERSECT

import android.content.Context;
import android.graphics.Canvas;
import android.graphics.Color;
import android.graphics.Paint;
import android.graphics.Rect;
import android.graphics.Region;
import android.util.AttributeSet;
import android.view.View;
import androidx.annotation.Nullable;

public class MyClipRectView extends View {
    public MyClipRectView(Context context) {
        super(context);
    }
    public MyClipRectView(Context context, @Nullable AttributeSet attrs) {
        super(context, attrs);
    }
    public MyClipRectView(Context context, @Nullable AttributeSet attrs, 
    	int defStyleAttr) {
        super(context, attrs, defStyleAttr);
    }

    @Override
    protected void onDraw(Canvas canvas) {
        super.onDraw(canvas);

        // ****** 先给当前的屏幕设置一个矩形画面 **********
        int measuredHeight = getMeasuredHeight();
        int measuredWidth = getMeasuredWidth();
        Paint paint = new Paint();
        paint.setColor(Color.GRAY);
        canvas.drawRect(0, 0, measuredWidth, measuredHeight, paint);

        // ***************************** 演示2 *****************************
        // 第一次
        Rect rect = new Rect(0, 0, 500, 500);
        canvas.clipRect(rect);
        // 第二次
        rect = new Rect(300, 300, 800, 800);
        canvas.clipRect(rect, Region.Op.INTERSECT);
        canvas.drawColor(Color.RED);
    }
}

2) Op为 DIFFERENCE

import android.content.Context;
import android.graphics.Canvas;
import android.graphics.Color;
import android.graphics.Paint;
import android.graphics.Rect;
import android.graphics.Region;
import android.util.AttributeSet;
import android.view.View;
import androidx.annotation.Nullable;

public class MyClipRectView extends View {
    public MyClipRectView(Context context) {
        super(context);
    }
    public MyClipRectView(Context context, @Nullable AttributeSet attrs) {
        super(context, attrs);
    }
    public MyClipRectView(Context context, @Nullable AttributeSet attrs, 
    	int defStyleAttr) {
        super(context, attrs, defStyleAttr);
    }

    @Override
    protected void onDraw(Canvas canvas) {
        super.onDraw(canvas);

        // ****** 先给当前的屏幕设置一个矩形画面 **********
        int measuredHeight = getMeasuredHeight();
        int measuredWidth = getMeasuredWidth();
        Paint paint = new Paint();
        paint.setColor(Color.GRAY);
        canvas.drawRect(0, 0, measuredWidth, measuredHeight, paint);

        // ***************************** 演示2 *****************************
        // 第一次
        Rect rect = new Rect(0, 0, 500, 500);
        canvas.clipRect(rect);
        // 第二次
        rect = new Rect(300, 300, 800, 800);
        canvas.clipRect(rect, Region.Op.DIFFERENCE);
        canvas.drawColor(Color.RED);
    }
}

3) 全部显示

import android.content.Context;
import android.graphics.Canvas;
import android.graphics.Color;
import android.graphics.Paint;
import android.graphics.Rect;
import android.graphics.Region;
import android.util.AttributeSet;
import android.view.View;
import androidx.annotation.Nullable;

public class MyClipRectView extends View {
    public MyClipRectView(Context context) {
        super(context);
    }
    public MyClipRectView(Context context, @Nullable AttributeSet attrs) {
        super(context, attrs);
    }
    public MyClipRectView(Context context, @Nullable AttributeSet attrs,
                          int defStyleAttr) {
        super(context, attrs, defStyleAttr);
    }

    @Override
    protected void onDraw(Canvas canvas) {
        super.onDraw(canvas);

        // ****** 先给当前的屏幕设置一个矩形画面 **********
        int measuredHeight = getMeasuredHeight();
        int measuredWidth = getMeasuredWidth();
        Paint paint = new Paint();
        paint.setColor(Color.GRAY);
        canvas.drawRect(0, 0, measuredWidth, measuredHeight, paint);

        // ***************************** 演示3 *****************************
        canvas.save();

        // 第一次
        Rect rect = new Rect(0, 0, 500, 500);
        canvas.clipRect(rect);
        canvas.drawColor(Color.BLUE);

        canvas.restore();

        // 第二次
        rect = new Rect(300, 300, 800, 800);
        canvas.clipRect(rect, Region.Op.INTERSECT);
        canvas.drawColor(Color.RED);
    }
}

4) 源码地址

地址：ClipRect

4. 参考文档

Android Developers 中描述的 Canvas.clipRect方法，参考文档: 地址

Android Canvas.clipRect裁剪矩形

Java相关栏目本月热门文章