怎么让Canvas刷新呀？

@component  
struct MyCanvas {  
  @State context: CanvasRenderingContext2D = new CanvasRenderingContext2D();  
  @State 起始位置  
  
  build( ) {  
    Canvas(this.context)  
      .onReady(() => {  
        在“起始位置”处画圆  
      })  
      .onTouch(event: TouchEvent) => {  
      更改“起始位置”  
    })  
  }  
}
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