assertStartWithEvent.swift

import RxSwift
import RxCocoa
import UIKit

extension SharedSequence {
    func assertStartWithEvent(file: StaticString = #file, line: UInt = #line) -> Self {
        let deadline = Observable<Element>.create { observer -> Disposable in
            fatalError("Event didn't come immediately after subscribe.", file: file, line: line)
        }
        .delaySubscription(.nanoseconds(0), scheduler: SharingStrategy.scheduler)

        return asObservable()
            .amb(deadline)
            .asSharedSequence(sharingStrategy: SharingStrategy.self, onErrorRecover: { _ in fatalError("Never come here") })
    }
}

class ViewController: UIViewController {
    override func viewDidLoad() {
        super.viewDidLoad()


        let driverWithInitialValue = Driver.just(1)
        _ = driverWithInitialValue
            .assertStartWithEvent()
            .drive()

        let relay = PublishRelay<Int>()
        let driverWithNoReplay = relay.asDriver(onErrorJustReturn: -1)
        _ = driverWithNoReplay
            .assertStartWithEvent()
            .drive()

        relay.accept(1)
        DispatchQueue.main.async {
            relay.accept(1)
        }
        print("hi")
    }
}