ios - RxSwift-启动和更新倒数计时器

Question

我正在尝试构建一个计时器，该计时器从15秒开始，递减计数到0。
事情是，我想根据事件将计时器更新2秒。
到目前为止，这是我尝试做的事情:

struct ViewModel {
    struct Input {
        let add_time: Observable<Void>
    }
    struct Output {
        let current_time: Observable<String>
        let timer_over: Observable<Void>
    }

    private let current_time = BehaviorRelay(value: 15)
    private let timer_over = PublishSubject<Void>()
    
    func transform(input: Input) -> Output {
        let current_time = self.current_time
            .flatMapLatest { time in
                Observable<Int>
                    .timer(.seconds(0), period: .seconds(1), scheduler: MainScheduler.instance)
                    .take(self.current_time.value + 1)
                    .map { "\(self.current_time.value - $0)" }
                    .do(onCompleted: { self.timer_over.onNext(()) })
            }
        return Output(
            current_time: current_time,
            timer_over: timer_over
        )
    }
}

let disposeBag = DisposeBag()

let add_time_subject = PublishSubject<Void>()
let input = ViewModel.Input(
    add_time: add_time_subject.asObservable()
)
let output = ViewModel().transform(input: input)
output.current_time
    .subscribe(onNext: { (time) in
        print(time)
    })
    .disposed(by: disposeBag)
output.timer_over
    .subscribe(onNext: {
        print("timer_over")
    })
    .disposed(by: disposeBag)

问题是，当我运行 add_time_subject.onNext(()) 时，我只想在计时器未达到0秒时再将计时器更新2秒。
我该怎么办？

Answer 1

您想要实现的目标听起来像是状态机。
您可以通过将计时器操作拆分为实际的“操作”并根据触发器进行合并来实现(一个是手动的“加两秒”，另一个是自动的“减少一秒”)。
我尚未对其进行全面测试，但这可能是一个很好的起点:

enum TimerAction {
    case tick
    case addTwoSeconds
}

let trigger = PublishRelay<Void>()

let timer = Observable<Int>
    .interval(.seconds(1), scheduler: MainScheduler.instance)
    .map { _ in TimerAction.tick }

let addSeconds = trigger.map { TimerAction.addTwoSeconds }

Observable
    .merge(timer, addSeconds)
    .scan(into: 15) { totalSeconds, action in
        totalSeconds += action == .addTwoSeconds ? 2 : -1
    }
    .takeUntil(.inclusive) { $0 == 0 }
    .subscribe()
    .disposed(by: disposeBag)

DispatchQueue.main.asyncAfter(deadline: .now() + 5) {
    trigger.accept(()) // increase the timer by two seconds after 5 seconds
}