HarmonyOS 如何让Canvas重绘

canvas不会根据数据变化自行重绘，需要调用canvas的clearRect方法清空画布后再重新绘制。同时，需要使用@Watch监听更新的那个变量，在更新后触发方法重新绘制canvas画布。

可以参考这个demo：

@Entry
@Component
export struct AudioRecorderDialog {
  controller: CustomDialogController
  @State  @Watch('draw') volumeList:number[] = [1,1,1,1,1,1,1,1,1,1];
  private settings: RenderingContextSettings = new RenderingContextSettings(true)
  private context: CanvasRenderingContext2D = new CanvasRenderingContext2D(this.settings)
  private timer: number = 0;

  aboutToAppear() {
    this.volumeList.fill(Math.floor(Math.random() * 100) + 1);
  }

  aboutToDisappear() {
    clearInterval(this.timer);
  }

  build() {
    Column() {
      Canvas(this.context)
        .width('100%')
        .height('50%')
        .backgroundColor('#ffff00')
        .onReady(() => {
          this.draw()
        })

      Text(this.volumeList.toString())
        .onClick(() => {
          this.timer = setInterval(() => {
            let tem = Math.floor(Math.random() * 100) + 1;
            this.volumeList.shift();
            this.volumeList.push(tem)
            console.error('licheeng', '点击执行 ' + this.volumeList);
          }, 1000)
        })
        .height(50)
    }
  }

  draw(){
    console.log('demoTest :' + 's')
    // this.context.clearRect()
    for (let index = 1; index < this.volumeList.length; index++) {
      let element = this.volumeList[index];
      this.context.fillRect(index * 5, (100 - element) / 2, 1, element);
    }
  }
}
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