Combine+UIKit.swift

import Combine
import UIKit

public protocol CombineCompatible {}

// MARK: - UIControl
public extension UIControl {
    final class Subscription<SubscriberType: Subscriber, Control: UIControl>: Combine.Subscription where SubscriberType.Input == Control {
        private var subscriber: SubscriberType?
        private let input: Control

        public init(subscriber: SubscriberType, input: Control, event: UIControl.Event) {
            self.subscriber = subscriber
            self.input = input
            input.addTarget(self, action: #selector(eventHandler), for: event)
        }

        public func request(_ demand: Subscribers.Demand) {}

        public func cancel() {
            subscriber = nil
        }

        @objc private func eventHandler() {
            _ = subscriber?.receive(input)
        }
    }

    struct Publisher<Output: UIControl>: Combine.Publisher {
        public typealias Output = Output
        public typealias Failure = Never

        let output: Output
        let event: UIControl.Event

        public init(output: Output, event: UIControl.Event) {
            self.output = output
            self.event = event
        }

        public func receive<S>(subscriber: S) where S: Subscriber, Self.Failure == S.Failure, Self.Output == S.Input {
            let subscription = Subscription(subscriber: subscriber, input: output, event: event)
            subscriber.receive(subscription: subscription)
        }
    }
}

extension UIControl: CombineCompatible {}

public extension CombineCompatible where Self: UIControl {
    func publisher(for event: UIControl.Event) -> UIControl.Publisher<UIControl> {
        .init(output: self, event: event)
    }
}

// MARK: - UIBarButtonItem
public extension UIBarButtonItem {
    final class Subscription<SubscriberType: Subscriber, Input: UIBarButtonItem>: Combine.Subscription where SubscriberType.Input == Input {
        private var subscriber: SubscriberType?
        private let input: Input

        public init(subscriber: SubscriberType, input: Input) {
            self.subscriber = subscriber
            self.input = input
            input.target = self
            input.action = #selector(eventHandler)
        }

        public func request(_ demand: Subscribers.Demand) {}

        public func cancel() {
            subscriber = nil
        }

        @objc private func eventHandler() {
            _ = subscriber?.receive(input)
        }
    }

    struct Publisher<Output: UIBarButtonItem>: Combine.Publisher {
        public typealias Output = Output
        public typealias Failure = Never

        let output: Output

        public init(output: Output) {
            self.output = output
        }

        public func receive<S>(subscriber: S) where S: Subscriber, Self.Failure == S.Failure, Self.Output == S.Input {
            let subscription = Subscription(subscriber: subscriber, input: output)
            subscriber.receive(subscription: subscription)
        }
    }
}

extension UIBarButtonItem: CombineCompatible {
    public convenience init(image: UIImage?, style: UIBarButtonItem.Style, cancellables: inout Set<AnyCancellable>, action: @escaping () -> Void) {
        self.init(image: image, style: style, target: nil, action: nil)
        self.publisher.sink { _ in action() }.store(in: &cancellables)
    }

    public convenience init(image: UIImage?, landscapeImagePhone: UIImage?, style: UIBarButtonItem.Style, cancellables: inout Set<AnyCancellable>, action: @escaping () -> Void) {
        self.init(image: image, landscapeImagePhone: landscapeImagePhone, style: style, target: nil, action: nil)
        self.publisher.sink { _ in action() }.store(in: &cancellables)
    }

    public convenience init(title: String?, style: UIBarButtonItem.Style, cancellables: inout Set<AnyCancellable>, action: @escaping () -> Void) {
        self.init(title: title, style: style, target: nil, action: nil)
        self.publisher.sink { _ in action() }.store(in: &cancellables)
    }

    public convenience init(barButtonSystemItem systemItem: UIBarButtonItem.SystemItem, cancellables: inout Set<AnyCancellable>, action: @escaping () -> Void) {
        self.init(barButtonSystemItem: systemItem, target: nil, action: nil)
        self.publisher.sink { _ in action() }.store(in: &cancellables)
    }
}

public extension CombineCompatible where Self: UIBarButtonItem {
    var publisher: UIBarButtonItem.Publisher<UIBarButtonItem> {
        .init(output: self)
    }
}

Examples.swift

import UIKit
import Combine


class ViewController: UIViewController {

    var cancellables: Set<AnyCancellable> = []
    let button1 = UIButton()
    let button2 = UIButton()

    override func viewDidLoad() {
        super.viewDidLoad()
                
        // No good
        let textField = UITextField()
        textField.publisher(for: \.text)
            .map { _ in (textField.text ?? "").count == 4 }
            .assign(to: \.isEnabled, on: button1)
            .store(in: &cancellables)

        // OK
        textField.publisher(for: .allEditingEvents)
            .map { _ in (textField.text ?? "").count == 4 }
            .assign(to: \.isEnabled, on: button1)
            .store(in: &cancellables)

        // OK
        let completion: (Notification) -> Void = { _ in }

        NotificationCenter.default.publisher(for: UIResponder.keyboardWillShowNotification)
            .receive(on: DispatchQueue.main)
            .sink { [weak self] in completion($0) }
            .store(in: &cancellables)

        // OK
        button1.publisher(for: .touchUpInside)
            .sink(receivedValue: { 
               // do something
            })
            .store(in: &cancellables)
      
        // OK
        let buttonEvent: PassthroughSubject<UIControl, Never> = .init()
        button1.publisher(for: .touchUpInside)
            .subscribe(buttonEvent)
            .store(in: &cancellables)

        // OK
        Publishers.Merge(button1.publisher(for: .touchUpInside), button2.publisher(for: .touchUpInside))
            .subscribe(buttonEvent)
            .store(in: &cancellables)

    }
}