Combine Publisher for CADisplayLink

CADisplayLinkPublisher.swift

//
//  CADisplayLinkPublisher.swift
//  SwiftUIDemo
//
//  Created by Ahmed Khalaf on 8/19/20.
//
// Credits: https://www.avanderlee.com/swift/custom-combine-publisher/

import UIKit
import Combine

final class CADisplayLinkSubscription<SubscriberType: Subscriber>: Subscription where SubscriberType.Input == CADisplayLink {
    
    // MARK: - Public
    init(subscriber: SubscriberType) {
        self.subscriber = subscriber
        self.displayLink = .init(target: self, selector: #selector(handleDisplayLink))
        self.displayLink?.add(to: .main, forMode: .common)
    }
    // MARK: - Subscription
    func request(_ demand: Subscribers.Demand) {
        
    }
    func cancel() {
        subscriber = nil
        displayLink?.invalidate()
        displayLink = nil
    }
    // MARK: - Private
    private var subscriber: SubscriberType?
    private var displayLink: CADisplayLink?
    @objc private func handleDisplayLink() {
        guard
            let displayLink = displayLink
        else {
            return
        }
        _ = subscriber?.receive(displayLink)
    }
}

struct CADisplayLinkPublisher: Publisher {
    typealias Output = CADisplayLink
    typealias Failure = Never
        
    func receive<S>(subscriber: S) where S : Subscriber, Self.Failure == S.Failure, Self.Output == S.Input {
        subscriber.receive(subscription: CADisplayLinkSubscription(subscriber: subscriber))
    }
}

extension CADisplayLink {
    static var publisher: CADisplayLinkPublisher {
        CADisplayLinkPublisher()
    }
}