demo比较简单，背景颜色是黄色，然后在上面覆盖上自定义View。实际需求是在相机的View上盖上一层遮罩，遮罩中心镂空一个透明显示的圆形部分。这个需求一般的布局比较难以实现，如果要ui给一张素材图片又存在不同分辨率手机对图片的拉伸问题，中间的圆会变形，适配不好。所以想要用一个自定义View来实现，这样不仅不存在适配问题而且也比较简单。

效果图：

简单粗暴直接上代码：

package com.xingyun.scandemo;


import android.content.Context;
import android.graphics.Bitmap;
import android.graphics.Canvas;
import android.graphics.Color;
import android.graphics.LinearGradient;
import android.graphics.Paint;
import android.graphics.PorterDuff;
import android.graphics.PorterDuffXfermode;
import android.graphics.RectF;
import android.graphics.Shader;
import android.support.annotation.Nullable;
import android.util.AttributeSet;
import android.view.View;


/**
 * Created by SY on 2018/5/11.
 */


public class ScanView extends View {


    private int measuredHeight;
    private int measuredWidth;
    //圆心x坐标
    private int centerX;
    //圆心y坐标
    private int centerY;
    //圆半径
    private int radius;
    //起始弧度、扫过弧度
    private float startAngle=-90, sweepAngle=180;


    public ScanView(Context context) {
        super(context);
    }


    public ScanView(Context context, @Nullable AttributeSet attrs) {
        super(context, attrs);
    }


    public ScanView(Context context, @Nullable AttributeSet attrs, int defStyleAttr) {
        super(context, attrs, defStyleAttr);
    }




    @Override
    protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
        super.onMeasure(widthMeasureSpec, heightMeasureSpec);
        measuredHeight = getMeasuredHeight();
        measuredWidth = getMeasuredWidth();
    }


    @Override
    protected void onDraw(Canvas canvas) {
        //创建镂空圆和遮罩bitmap
        Bitmap rectBitmap = createRectBitmap();
        Bitmap circleBitmap = createCircleBitmap();


        Paint paint = new Paint();
        paint.setFilterBitmap(false);
        //保存所有的标识
        canvas.saveLayer(0, 0, measuredWidth, measuredHeight, null,
                Canvas.MATRIX_SAVE_FLAG);
        //画圆
        canvas.drawBitmap(circleBitmap, 0, 0, paint);
        //setXfermode   为SRC_OUT   只在源图像和目标图像不相交的地方绘制
        paint.setXfermode(new PorterDuffXfermode(PorterDuff.Mode.SRC_OUT));
        //画遮罩矩形
        canvas.drawBitmap(rectBitmap, 0, 0, paint);


        //画外部圆环
        Paint loopPaint = new Paint();
        int strokeWidth = 20;
        loopPaint.setStrokeWidth(strokeWidth);
        loopPaint.setAntiAlias(true);
        loopPaint.setStyle(Paint.Style.STROKE);
        loopPaint.setStrokeCap(Paint.Cap.ROUND);


        float distance = 40;
        RectF oval = new RectF((int) (measuredWidth / 2f) - radius - distance, (int) (measuredHeight / 2f) - radius - distance,
                (int) (measuredWidth / 2f) - radius - distance + 2 * (radius + distance), (int) (measuredHeight / 2f) - radius - distance + 2 * (radius + distance));
        loopPaint.setColor(Color.GRAY);
        //底层灰色圆
        canvas.drawCircle(centerX, centerY, radius + distance, loopPaint);
        //渐变色
        LinearGradient lg = new LinearGradient(0, 0, 1000, 1000, Color.parseColor("#a5d2fe"), Color.parseColor("#519aff"), Shader.TileMode.MIRROR);
        loopPaint.setShader(lg);
        //蓝色圆弧
        canvas.drawArc(oval, startAngle, sweepAngle, false, loopPaint);
        startAngle = (startAngle + 1) % 360;
        //回收
        rectBitmap .recycle();
        circleBitmap .recycle();
        rectBitmap =null;
        circleBitmap =null;
        //重绘
        invalidate();
    }




    /**
     * 创建镂空层圆形形状
     */
    private Bitmap createCircleBitmap() {
        Bitmap bm = Bitmap.createBitmap(measuredWidth, measuredHeight, Bitmap.Config.ARGB_8888);
        Canvas canvas = new Canvas(bm);
        Paint paint = new Paint(Paint.ANTI_ALIAS_FLAG);
        paint.setColor(Color.WHITE);
        centerX = (int) (measuredWidth / 2f);
        centerY = (int) (measuredHeight / 2f);
        radius = measuredWidth / 3;
        canvas.drawCircle(centerX, centerY, radius, paint);
        return bm;
    }


    /**
     * 创建遮罩层形状
     */
    private Bitmap createRectBitmap() {
        Bitmap bm = Bitmap.createBitmap(measuredWidth, measuredHeight, Bitmap.Config.ARGB_8888);
        Canvas canvas = new Canvas(bm);
        Paint paint = new Paint(Paint.ANTI_ALIAS_FLAG);
        paint.setColor(Color.WHITE);
        canvas.drawRect(new RectF(0, 0, measuredWidth, measuredHeight), paint);
        return bm;
    }
}

Activity里添加：

<?xml version="1.0" encoding="utf-8"?>
<FrameLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:background="@color/yellow"
    tools:context="com.xingyun.scandemo.MainActivity">

    <com.xingyun.scandemo.ScanView
        android:layout_width="match_parent"
        android:layout_height="match_parent" />

</FrameLayout>

直接用FrameLayout将自定义的View覆盖在其他View上面即可。

代码很简单，就是在onMeasure方法里获取了宽高，然后onDraw方法里通过createCircleBitmap和createRectBitmap方法获得遮罩矩形和中间圆形的bitmap，再分别绘制这两个bitmap，将画笔Xfermode设置为PorterDuff.Mode.SRC_OUT，表示只在源图像和目标图像不相交的地方绘制。最后在圆的外面绘制了一个圆弧，并且不断修改起始弧度再次绘制，从而实现旋转的动画。关于Paint的Xfermode的属性可以看下图：

具体可以参考文章https://www.cnblogs.com/libertycode/p/6290497.html。