如何实现图片按下有遮罩层效果，抬起遮罩层消失的效果？

@Entry@Componentstruct Index {  @State mask:boolean=false  build() {    Column() {      Image('/comment/bg.jpg')        .mask(this.mask?new Rect({ width: '500px', height: '280px' }).fill(Color.Gray):null)        .width('500px').height('280px')        .onTouch((event: TouchEvent) => {          switch(event.type){            case TouchType.Down:              this.mask=true              break;            case TouchType.Up:              this.mask=false              break;          }        })    }.width('100%').margin({ top: 5 })  }}