RxSwift: An example of causing one observable to rely on the output of another

RxSwiftChainObservablesExample.swift

import RxSwift

func submit(number: Int) {
    let validateAndPerformServiceCallIfSuccessful = performValidation(numberThatShouldBeEven: number)
        .flatMap { () -> Observable<Void> in
            print("Validated successfully")
            return performServiceCall() // For any .next events from performValidation, perform the service call
        }
    
    // By subscribing, validation is performed and the service call is executed for any .next events from performValidation
    validateAndPerformServiceCallIfSuccessful.subscribe(onNext: { _ in
        print("Service call was a success!")
    }, onError: { _ in
        print("Something went wrong.")
    })
}

private func performValidation(numberThatShouldBeEven: Int) -> Observable<Void> {
    print("Performing validation")
    return numberThatShouldBeEven % 2 == 0 ?
        Observable.just(()) : // Validation was successful
        Observable.error(NSError(domain: "com.thecodedself.rxswift", code: 0, userInfo: nil)) // Validation failed
}

private func performServiceCall() -> Observable<Void> {
    print("Performing service call")
    return Observable.just(()) // Return the results from the service
}

submit(number: 2)
print("--------")
submit(number: 1)