Android实现扫一扫功能之绘制指定区域透明区域

package per.juan.scandome;

import android.graphics.Canvas;
import android.graphics.ColorFilter;
import android.graphics.Paint;
import android.graphics.Path;
import android.graphics.PorterDuff;
import android.graphics.PorterDuffXfermode;
import android.graphics.drawable.Drawable;
import android.support.annotation.NonNull;
import android.support.annotation.Nullable;

/**
 * 支持中间出现透明区域的drawable
 * 通过{@link #setSrcPath(Path)}设定透明区域的形状
 * Created by juan on 2018/07/20.
 */
public class CustomDrawable extends Drawable {
 private Paint srcPaint;
 private Path srcPath = new Path();

 private Drawable innerDrawable;


 public CustomDrawable(Drawable innerDrawable) {
  this.innerDrawable = innerDrawable;
  srcPath.addRect(100, 100, 200, 200, Path.Direction.CW);
  srcPaint = new Paint(Paint.ANTI_ALIAS_FLAG);
  srcPaint.setColor(0xffffffff);
 }

 /**
  * 设置内部透明的部分
  *
  * @param srcPath
  */
 public void setSrcPath(Path srcPath) {
  this.srcPath = srcPath;
 }

 @Override
 public void draw(@NonNull Canvas canvas) {
  innerDrawable.setBounds(getBounds());
  if (srcPath == null || srcPath.isEmpty()) {
   innerDrawable.draw(canvas);
  } else {
   //将绘制操作保存到新的图层，因为图像合成是很昂贵的操作，将用到硬件加速，这里将图像合成的处理放到离屏缓存中进行
   int saveCount = canvas.saveLayer(0, 0, canvas.getWidth(), canvas.getHeight(), srcPaint, Canvas.ALL_SAVE_FLAG);

   //dst 绘制目标图
   innerDrawable.draw(canvas);

   //设置混合模式
   srcPaint.setXfermode(new PorterDuffXfermode(PorterDuff.Mode.CLEAR));
   //src 绘制源图
   canvas.drawPath(srcPath, srcPaint);
   //清除混合模式
   srcPaint.setXfermode(null);
   //还原画布
   canvas.restoreToCount(saveCount);
  }
 }

 @Override
 public void setAlpha(int alpha) {
  innerDrawable.setAlpha(alpha);
 }

 @Override
 public void setColorFilter(@Nullable ColorFilter colorFilter) {
  innerDrawable.setColorFilter(colorFilter);
 }

 @Override
 public int getOpacity() {
  return innerDrawable.getOpacity();
 }
}

package per.juan.scandome;

import android.annotation.SuppressLint;
import android.content.Context;
import android.graphics.Path;
import android.support.annotation.NonNull;
import android.support.annotation.Nullable;
import android.util.AttributeSet;
import android.view.View;
import android.widget.FrameLayout;

/**
 * 根据layout中子View的位置，确定局部透明区域
 * Created by juan on 2018/07/20.
 */
public class CustomLayout extends FrameLayout {

 private Context mContext;
 private CustomDrawable background;

 public CustomLayout(@NonNull Context context) {
  super(context);
  initView(context, null, 0);
 }

 public CustomLayout(@NonNull Context context, @Nullable AttributeSet attrs) {
  super(context, attrs);
  this.mContext=context;
  initView(context, attrs, 0);
 }

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

 @SuppressLint("NewApi")
 private void initView(@NonNull Context context, @Nullable AttributeSet attrs, int defStyleAttr) {
  background = new CustomDrawable(getBackground());
  setBackground(background);
 }

 @Override
 protected void onLayout(boolean changed, int left, int top, int right, int bottom) {
  super.onLayout(changed, left, top, right, bottom);
  resetBackgroundHoleArea();
 }

 @SuppressLint("NewApi")
 private void resetBackgroundHoleArea() {
  Path path = null;
  // 以子View为范围构造需要透明显示的区域
  View view = findViewById(R.id.iv_scan);
  if (view != null) {
   path = new Path();
   // 矩形透明区域
   path.addRoundRect(view.getLeft(), view.getTop(), view.getRight(), view.getBottom(), dp2Px(mContext,10), dp2Px(mContext,10),Path.Direction.CW);
  }
  if (path != null) {
   background.setSrcPath(path);
  }
 }

 public int dp2Px(Context context, float dp) {
  final float scale = context.getResources().getDisplayMetrics().density;
  return (int) (dp * scale + 0.5f);
 }
}

<?xml version="1.0" encoding="utf-8"? 
<FrameLayout xmlns:android="http://schemas.android.com/apk/res/android"
 android:layout_width="match_parent"
 android:id="@+id/frame_layout"
 android:layout_height="match_parent" 

 <ImageView
  android:layout_width="match_parent"
  android:layout_height="match_parent"
  android:background="@mipmap/bg_image" / 

 <per.juan.scandome.CustomLayout
  android:layout_width="match_parent"
  android:id="@+id/layout"
  android:background="#8c565658"
  android:layout_height="match_parent" 

  <!-- 根据这个子View所在的位置，计算出透明矩形的位置 -- 
  <FrameLayout
   android:id="@+id/iv_scan"
   android:layout_width="200dp"
   android引用块内容center" / 

 </per.juan.scandome.CustomLayout 
</FrameLayout