How to make restartable countdown using Rxswift - ios

Question

I want to create a countdown feature that restart every time I press a button.
However, the code I wrote terminate the subscription when the countdown is completed.
What can I do to ensure that my subscription is not terminated and the countdown is restarted?
fileprivate let counter = 10
fileprivate let startCountDown = PublishRelay<Void>()
startCountDown
.flatMapLatest { _ -> Observable<Int> in
return Observable<Int>.timer(.seconds(0), period: .seconds(1), scheduler: MainScheduler.instance)
}
.take(counter + 1)
.subscribe(onNext: { time in
print(time)
}, onCompleted: {
print("Completed")
})
.disposed(by: rx.disposeBag)

Answer 1

When take(_:) completes, the Observable chain is disposed. However, if the Observable inside the flatMapLatest closure completes, the chain is not disposed because startCountDown hasn't completed yet. The flatMapLatest observable won't complete until all the observables it subscribes to completes. So the solution is to put the take(_:) inside the flatMapLatest.
The View Model should look like this:
func startCountDown(counter: Int, trigger: Observable<Void>, scheduler: SchedulerType) -> Observable<Int> {
trigger
.flatMapLatest {
Observable<Int>.timer(.seconds(0), period: .seconds(1), scheduler: scheduler)
.take(counter + 1)
}
}
You use the above view model in your view controller like this:
startCountdown(counter: 10, trigger: startButton.rx.tap.asObservable(), scheduler: MainScheduler.instance)
.subscribe(onNext: { time in
print(time)
})
.disposed(by: rx.disposeBag)
Strictly speaking, this is a count-up timer. It will go from 0 to counter and then wait until the button is tapped again. If the button is tapped while it is counting, it will restart. If you want it to ignore taps until it's done counting, then use flatMapFirst instead.
Learn more in by reading this article: RxSwift's Many Faces of FlatMap
As a bonus, here's how you can test the view model:
final class CountdownTests: XCTestCase {
func test() {
let scheduler = TestScheduler(initialClock: 0)
let trigger = scheduler.createObservable(timeline: "--V---V-|", values: ["V": ()])
let expected = parseEventsAndTimes(timeline: "---012-0123456789|", values: { Int(String($0))! })
let result = scheduler.start(created: 0, subscribed: 0, disposed: 100) {
startCountDown(counter: 9, trigger: trigger, scheduler: scheduler)
}
XCTAssertEqual(result.events, expected[0])
}
}
The above uses my TestScheduler