NSNotification Scheduling Service in Swift. (the only required file is `SchedulingService.swift`)

Scheduling-AppDelegate.swift

//
//  AppDelegate.swift
//  Scheduling
//
//  Created by Shaps Benkau on 19/02/2018.
//  Copyright © 2018 152percent Ltd. All rights reserved.
//

import UIKit

extension Notification.Name {
    public static let CountdownDidUpdate = Notification.Name(rawValue: "CountdownDidUpate")
}

@UIApplicationMain
class AppDelegate: UIResponder, UIApplicationDelegate {
    var window: UIWindow?
    
    private var store: SchedulingServiceStore!
    
    func application(_ application: UIApplication, didFinishLaunchingWithOptions launchOptions: [UIApplicationLaunchOptionsKey: Any]?) -> Bool {
        store = SchedulingServiceStore()
        store.service.delegate = self
        
        if store.service.scheduledRequests.isEmpty == true {
            scheduleRequests(service: store.service)
        }
        
        store.service.resume()
        print("\(store.service)")
        
        return true
    }
    
    private func scheduleRequests(service: SchedulingService) {
        /**
         Closure based example
         */
        let request = service.schedule(date: Date().addingTimeInterval(5))
        service.cancel(request: request)
        
        /**
         Notification based example
         */
        service.schedule(date: Date().addingTimeInterval(20), notification: .CountdownDidUpdate)
    }
    
}

extension AppDelegate: SchedulingServiceDelegate {
    
    func schedulingServiceDidChange(_ service: SchedulingService) {
        print("Changed: \(service)")
    }
    
    func schedulingService(_ service: SchedulingService, didAdd request: SchedulingRequest) {
        print("Added: \(request)")
    }
    
    func schedulingService(_ service: SchedulingService, didComplete request: SchedulingRequest) {
        print("Completed: \(request)")
    }
    
    func schedulingService(_ service: SchedulingService, didCancel request: SchedulingRequest) {
        print("Cancelled: \(request)")
    }
    
}

SchedulingService.swift

//
//  SchedulingService.swift
//  Scheduling
//
//  Created by Shaps Benkau on 19/02/2018.
//  Copyright © 2018 152percent Ltd. All rights reserved.
//

import Foundation

public protocol SchedulingServiceDelegate: class {
    func schedulingService(_ service: SchedulingService, didAdd request: SchedulingRequest)
    func schedulingService(_ service: SchedulingService, didComplete request: SchedulingRequest)
    func schedulingService(_ service: SchedulingService, didCancel request: SchedulingRequest)
    func schedulingServiceDidChange(_ service: SchedulingService)
}

public final class SchedulingService {
    
    deinit {
        cancelAllRequests()
    }
    
    public weak var delegate: SchedulingServiceDelegate?
    
    private var _requests: [SchedulingRequest] = [] {
        didSet {
            NotificationCenter.default.post(name: .SchedulingServiceDidChange, object: self)
            delegate?.schedulingServiceDidChange(self)
        }
    }
    
    private var _scheduledRequests: [SchedulingRequest: Timer] = [:]
    
    public private(set) var isPaused: Bool = true
    
    public var scheduledRequests: [SchedulingRequest] {
        return _requests.sorted(by: { $0.date < $1.date })
    }
    
    @discardableResult
    public func schedule(identifier: String = UUID().uuidString, date: Date, using closure: ((SchedulingRequest) -> Void)? = nil) -> SchedulingRequest {
        var request = SchedulingRequest(identifier: identifier, date: date, notification: .closure)
        request.handler = closure
        schedule(request: request)
        return request
    }
    
    @discardableResult
    public func schedule(identifier: String = UUID().uuidString, date: Date, notification: Notification.Name) -> SchedulingRequest {
        let request = SchedulingRequest(identifier: identifier, date: date, notification: notification)
        schedule(request: request)
        return request
    }
    
    private func schedule(request: SchedulingRequest) {
        if _requests.contains(request) {
            // if we have an existing request, invalidate its timer
            _scheduledRequests[request]?.invalidate()
        } else {
            // otherwise append it
            _requests.append(request)
            delegate?.schedulingService(self, didAdd: request)
        }
        
        guard !isPaused else { return }
        
        // if we're not paused, start the timer
        enqueueTimer(for: request)
    }
    
    fileprivate func handleRequest(_ request: SchedulingRequest) {
        removeRequest(request)
        
        // execute the handler if it was set, and post the notification
        request.handler?(request)
        
        if request.notification != .closure {
            // if a valid name was specified, post a notification
            NotificationCenter.default.post(name: request.notification, object: request)
        }
        
        // send a catch-all notification as well for all requests
        NotificationCenter.default.post(name: .SchedulingServiceDidCompleteRequest, object: request)
        delegate?.schedulingService(self, didComplete: request)
    }
    
    public func cancel(request: SchedulingRequest) {
        removeRequest(request)
        NotificationCenter.default.post(name: .SchedulingServiceDidCancelRequest, object: request)
        delegate?.schedulingService(self, didCancel: request)
    }
    
    public func cancelAllRequests() {
        for request in scheduledRequests {
            cancel(request: request)
        }
    }
    
    public func resume() {
        isPaused = false
        
        _requests.forEach {
            enqueueTimer(for: $0)
        }
    }
    
    public func pause() {
        isPaused = true
        
        _scheduledRequests.values.forEach {
            $0.invalidate()
        }
    }
    
    private func enqueueTimer(for request: SchedulingRequest) {
        guard request.date >= Date() else {
            // if the date is in the past, handle it immediately
            handleRequest(request)
            return
        }
        
        let timer = Timer(fire: request.date, interval: 0, repeats: false) { [weak self] _ in
            self?.handleRequest(request)
        }
        
        _scheduledRequests[request] = timer
        RunLoop.main.add(timer, forMode: .defaultRunLoopMode)
    }
    
    private func removeRequest(_ request: SchedulingRequest) {
        guard let index = _requests.index(of: request) else { return }
        
        _requests.remove(at: index)
        _scheduledRequests[request]?.invalidate()
        _scheduledRequests[request] = nil
    }
    
}

extension SchedulingService: CustomStringConvertible {
    
    public var description: String {
        return "\(type(of: self)) (\(isPaused ? "Paused" : "Running")) | \(scheduledRequests.count) pending request(s)"
    }
    
}

extension SchedulingService: CustomDebugStringConvertible {
    
    public var debugDescription: String {
        var string = description
        
        for request in _requests {
            string.append("\n▹ \(request)")
        }
        
        return string
    }
    
}

extension SchedulingService: Codable {
    
    public convenience init(from decoder: Decoder) throws {
        self.init()
        
        let container = try decoder.singleValueContainer()
        _requests = try container.decode(Array<SchedulingRequest>.self)
    }
    
    public func encode(to encoder: Encoder) throws {
        var container = encoder.singleValueContainer()
        try container.encode(_requests)
    }

}

public struct SchedulingRequest: Codable {
    
    public enum CodingKeys: String, CodingKey {
        case identifier
        case date
        case notification
    }
    
    public let identifier: String
    public let date: Date
    public let notification: Notification.Name
    
    fileprivate var handler: ((SchedulingRequest) -> Void)? = nil
    
    fileprivate init(identifier: String = UUID().uuidString, date: Date, notification: Notification.Name) {
        self.identifier = identifier
        self.date = date
        self.notification = notification
    }
    
}

extension SchedulingRequest: Hashable {
    
    public var hashValue: Int {
        return identifier.hashValue
    }
    
    public static func ==(lhs: SchedulingRequest, rhs: SchedulingRequest) -> Bool {
        return lhs.identifier == rhs.identifier
    }
    
}

extension SchedulingRequest: CustomStringConvertible {
    
    public var description: String {
        return "Request (\(identifier)) | \(date)"
    }
    
}

extension Notification.Name: Codable {
    public static let SchedulingServiceDidCompleteRequest = Notification.Name(rawValue: "SchedulingServiceDidHandleRequest")
    public static let SchedulingServiceDidCancelRequest = Notification.Name(rawValue: "SchedulingServiceDidCancelRequest")
    public static let SchedulingServiceDidChange = Notification.Name(rawValue: "SchedulingServiceDidChange")
    
    // used to specify an empty notification for closure based requests
    fileprivate static let closure = Notification.Name(rawValue: "")
}

SchedulingServiceStore.swift

//
//  ScheduleStore.swift
//  Scheduling
//
//  Created by Shaps Benkau on 20/02/2018.
//  Copyright © 2018 152percent Ltd. All rights reserved.
//

import Foundation

public final class SchedulingServiceStore {
    
    public let service: SchedulingService
    
    public init(service: SchedulingService? = nil) {
        if let service = service {
            self.service = service
        } else {
            if let data = try? Data(contentsOf: SchedulingServiceStore.storeUrl),
                let service = try? JSONDecoder().decode(SchedulingService.self, from: data) {
                self.service = service
            } else {
                self.service = SchedulingService()
            }
        }
        
        NotificationCenter.default.addObserver(forName: .SchedulingServiceDidChange, object: nil, queue: .main) {
            [weak self] note in
            self?.save()
        }
    }
    
    public func save() {
        do {
            let encoder = JSONEncoder()
            let data = try encoder.encode(service)
            try data.write(to: SchedulingServiceStore.storeUrl)
        } catch {
            print(error)
        }
    }
    
    private static var storeUrl: URL {
        let path = NSSearchPathForDirectoriesInDomains(.documentDirectory, .userDomainMask, true).first!
        return URL(fileURLWithPath: path).appendingPathComponent("SchedulingService.json")
    }
    
}

This micro-library also provides nice output for debugging purposes.