实现一个背景透明,中间矩形镂空的界面

import display from '@ohos.display'; 
import { Logger } from '../common/Logger'; 
 
const log: Logger = new Logger('CustomPage'); 
 
@Entry 
@Component 
struct CustomPage { 
  //用来配置CanvasRenderingContext2D对象的参数，包括是否开启抗锯齿，true表明开启抗锯齿。 
  private settings: RenderingContextSettings = new RenderingContextSettings(true) 
  //用来创建CanvasRenderingContext2D对象，通过在canvas中调用CanvasRenderingContext2D对象来绘制。 
  private context: CanvasRenderingContext2D = new CanvasRenderingContext2D(this.settings) 
  private pointX: number = 30; 
  private pointY: number = 150; 
  private mScanWidth: number = 300; 
  private mScreenWidth: number = 1080; 
  private mScreenHeight: number = 1920; 
  @State mIsInit: boolean = false; 
 
  onPageShow(): void { 
    log.info('onPageShow'); 
    // 获取屏幕参数 
    let defaultDisplay = display.getDefaultDisplaySync(); 
    this.mScreenWidth = px2vp(defaultDisplay.width); 
    this.mScreenHeight = px2vp(defaultDisplay.height); 
    // 计算扫描框边长 
    this.mScanWidth = this.mScreenWidth * 0.8; 
    // 计算扫描框左上角位置 
    this.pointX = (this.mScreenWidth - this.mScanWidth) / 2; 
    this.pointY = (this.mScreenHeight - this.mScanWidth) / 2; 
    log.info(`init, pointX: ${this.pointX}, pointY: ${this.pointY}, mScanWidth: ${this.mScanWidth}`); 
    this.mIsInit = true; 
  } 
 
  build() { 
    Stack() { 
      Text('这里是底色').fontSize(180).width('100%').height('100%') 
      if (this.mIsInit) { // 数据初始化完后才需要显示 
        Canvas(this.context) 
          .width('100%') 
          .height('100%') 
          .onReady(() => { 
            this.context.fillStyle = '#33000000'; 
            this.context.fillRect(0, 0, this.mScreenWidth, this.mScreenHeight); 
            this.context.clearRect(this.pointX, this.pointY, this.mScanWidth, this.mScanWidth); 
          }) 
      } 
    }.width('100%') 
    .height('100%') 
  } 
}