An Observable emits item only if _current == timeToLucky. Useful for testing.

NthTimeLucky.swift

//
//  NthTimeLucky.swift
//
//  Created by はるふ on 2018/05/24.
//  Copyright © 2018年 ha1f. All rights reserved.
//

import Foundation
import RxSwift

extension ObservableType {
    static func nthTimeLucky(_ item: E, timeToLucky: Int, errorItem: Error = NSError()) -> Observable<E> {
        return NthTimeLucky(item, timeToLucky: timeToLucky, errorItem: errorItem).asObservable()
    }
}

/// An Observable emits item only if _current == timeToLucky.
/// TBH, I want to inherit from Observable<E>
private class NthTimeLucky<E>: ObservableType {
    /// item to emit on lucky time
    let item: E

    /// item to emit except lucky time
    let errorItem: Error

    let timeToLucky: Int

    private var _current: Int = 1

    init(_ item: E, timeToLucky: Int, errorItem: Error) {
        self.item = item
        self.timeToLucky = timeToLucky
        self.errorItem = errorItem
    }

    func subscribe<O>(_ observer: O) -> Disposable where O: ObserverType, E == O.E {
        let disposable = Disposables.create()
        defer {
            _current += 1
        }
        guard _current == timeToLucky else {
            observer.onError(errorItem)
            return disposable
        }
        observer.onNext(item)
        return disposable
    }
}