ViewController.swift

//
//  ViewController.swift
//  VideoChatMessage
//
//  Created by LateRain on 1/4/19.
//  Copyright © 2019 LateRain. All rights reserved.
//

import UIKit

class ViewController: UIViewController {

    override func viewDidLoad() {
        super.viewDidLoad()
        // Do any additional setup after loading the view, typically from a nib.

        setUpChatMessageScheduler()
    }

    private func setUpChatMessageScheduler() {
        let chatMessageScheduler = ChatMessageScheduler()
        chatMessageScheduler.startPrintingMessage(1.0)

        DispatchQueue.global().asyncAfter(deadline: .now()) {
            chatMessageScheduler.chatMessageDidArrive(message: "Message @ time 0.0")
            chatMessageScheduler.chatMessageDidArrive(message: "Message @ time 0.0")
            chatMessageScheduler.chatMessageDidArrive(message: "Message @ time 0.0")
        }

        DispatchQueue.global().asyncAfter(deadline: .now() + .milliseconds(2500)) {
            chatMessageScheduler.chatMessageDidArrive(message: "Message @ time 2.5")
        }
    }
}

protocol ChatMessage {
    func chatMessageDidArrive(message: String)
    func startPrintingMessage(_ timeIntervalInSecond: TimeInterval)
}

class ChatMessageScheduler: ChatMessage {
    var queue: Queue<String>

    init() {
        queue = Queue()
    }

    func chatMessageDidArrive(message: String) {
        queue.enqueue(element: message)
    }

    func startPrintingMessage(_ timeIntervalInSecond: TimeInterval) {
        let timer = Timer(timeInterval: timeIntervalInSecond, repeats: true) { timer in
            print(self.queue.dequeue() ?? "No message")
        }
        RunLoop.current.add(timer, forMode: .common)
    }
}

struct Queue<T> {
    var reversed: [T]
    var array: [T]

    init() {
        reversed = [T]()
        array = [T]()
    }

    mutating func enqueue(element: T) {
        array.append(element)
    }

    mutating func dequeue() -> T? {
        guard reversed.count > 0 || array.count > 0 else {
            return nil
        }
        if reversed.count == 0 {
            reversed = array.reversed()
            array.removeAll()
        }
        return reversed.removeLast()
    }
}