自定义组件生命周期回调函数

@Component
struct CountDownTimerComponent {
    @State countDownFrom: number = 10
    private timerId: number = -1

    private aboutToAppear(): void  {
        this.timerId = setInterval(() => {
            if (this.countDownFrom <= 1) {
                clearTimeout(this.timerId)
            }
            this.countDownFrom -= 1
        }, 1000) // decr counter by 1 every second
    }

    private aboutToDisappear(): void {
        if (this.timerId > 0) {
            clearTimeout(this.timerId)
            this.timerId = -1
        }
    }

    build() {
        Text(`${this.countDownFrom} sec left`)
    }
}