Create a custom Observable.

createObservable.swift

enum MyError: Error {
    case anError
}

let disposeBag = DisposeBag()

let observable: Observable<String> = Observable<String>.create { observer in
    observer.onNext("A")
    observer.onNext("B")

    // If we comment both of these, we will create an Observable that NEVER terminates!
    observer.onError(MyError.anError)
    observer.onCompleted()

    observer.onNext("C")

    return Disposables.create()
}
let subscription: Disposable = observable.subscribe(onNext: { element in
                                                        print(element)
                                                    },
                                                    onError: { error in
                                                        print(error)
                                                    },
                                                    onCompleted: {
                                                        print("Completed")
                                                    },
                                                    onDisposed: {
                                                        print("Disposed")
                                                    })
subscription.disposed(by: disposeBag)