NotificationObservingContext.swift

//
//  NotificationObservingContext.swift
//  Created by Christopher Fuller for Southern California Public Radio
//  Original version at https://github.com/SCPR/swift-experiments/blob/master/source/NotificationObservable.swift
//  Updated by Christopher Fuller for Swift 3 compatibility
//
//  Copyright (c) 2016 Southern California Public Radio
//
//  Permission is hereby granted, free of charge, to any person obtaining a copy
//  of this software and associated documentation files (the "Software"), to deal
//  in the Software without restriction, including without limitation the rights
//  to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
//  copies of the Software, and to permit persons to whom the Software is
//  furnished to do so, subject to the following conditions:
//
//  The above copyright notice and this permission notice shall be included in all
//  copies or substantial portions of the Software.
//
//  THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
//  IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
//  FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
//  AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
//  LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
//  OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
//  SOFTWARE.
//

protocol ObservableNotification {

    static var name: Notification.Name { get }

    init?()

}

extension ObservableNotification {

    static var name: Notification.Name {
        return Notification.Name(String(describing: self))
    }

}

protocol UserInfoTransformableNotification: ObservableNotification {

    var userInfo: [AnyHashable : Any]? { get }

    init?(userInfo: [AnyHashable : Any]?)

}

extension UserInfoTransformableNotification {

    init?() {
        self.init()
    }

}

extension NotificationCenter {

    func post<T: ObservableNotification>(_ notification: T, object: AnyObject? = nil) {
        let closure = {
            self.post(name: type(of: notification).name, object: object)
        }
        Thread.isMainThread ? closure() : DispatchQueue.main.sync { closure() }
    }

    func post<T: UserInfoTransformableNotification>(_ notification: T, object: AnyObject? = nil) {
        let closure = {
            self.post(name: type(of: notification).name, object: object, userInfo: notification.userInfo)
        }
        Thread.isMainThread ? closure() : DispatchQueue.main.sync { closure() }
    }

}

extension NotificationQueue {

    func enqueue<T: ObservableNotification>(_ notification: T, object: AnyObject? = nil, postingStyle: NotificationQueue.PostingStyle, coalesceMask: NotificationQueue.NotificationCoalescing? = nil, dequeue: Bool = false) {
        let closure = {
            if dequeue {
                self.dequeue(type(of: notification), object: object, coalesceMask: coalesceMask)
            }
            self.enqueue(
                Notification(name: type(of: notification).name, object: object, userInfo: nil),
                postingStyle: postingStyle,
                coalesceMask: (coalesceMask ?? [ .onName, .onSender ]),
                forModes: nil
            )
        }
        Thread.isMainThread ? closure() : DispatchQueue.main.sync { closure() }
    }

    func enqueue<T: UserInfoTransformableNotification>(_ notification: T, object: AnyObject? = nil, postingStyle: NotificationQueue.PostingStyle, coalesceMask: NotificationQueue.NotificationCoalescing? = nil, dequeue: Bool = false) {
        let closure = {
            if dequeue {
                self.dequeue(type(of: notification), object: object, coalesceMask: coalesceMask)
            }
            self.enqueue(
                Notification(name: type(of: notification).name, object: object, userInfo: notification.userInfo),
                postingStyle: postingStyle,
                coalesceMask: (coalesceMask ?? [ .onName, .onSender ]),
                forModes: nil
            )
        }
        Thread.isMainThread ? closure() : DispatchQueue.main.sync { closure() }
    }

    func dequeue<T: ObservableNotification>(_ notifications: T.Type, object: AnyObject? = nil, coalesceMask: NotificationQueue.NotificationCoalescing? = nil) {
        let closure = {
            self.dequeueNotifications(
                matching: Notification(name: notifications.name, object: object),
                coalesceMask: Int((coalesceMask ?? [ .onName, .onSender ]).rawValue)
            )
        }
        Thread.isMainThread ? closure() : DispatchQueue.main.sync { closure() }
    }

}

class NotificationObserver {

    private weak var observingContext: NotificationObservingContext!

    fileprivate weak var object: AnyObject?

    private let closure: NotificationObservingContext.NotificationPosted

    fileprivate init(observingContext: NotificationObservingContext, object: AnyObject?, closure: @escaping NotificationObservingContext.NotificationPosted) {
        self.observingContext = observingContext
        self.object = object
        self.closure = closure
    }

    @objc fileprivate func observe(notification: Notification) {
        closure(self, notification)
    }

    func remove() {
        observingContext.removeObserver(self)
    }

}

class NotificationObservingContext {

    typealias NotificationPosted = (_ observer: NotificationObserver, _ notification: Notification) -> Void
    typealias ObservableNotificationPosted<T> = (_ observer: NotificationObserver, _ object: AnyObject?, _ notification: T) -> Void

    fileprivate let notificationCenter: NotificationCenter

    fileprivate var observers = [NotificationObserver]()

    init(notificationCenter: NotificationCenter) {
        self.notificationCenter = notificationCenter
    }

    convenience init() {
        self.init(notificationCenter: NotificationCenter.default)
    }

    deinit {
        removeObservers()
    }

    @discardableResult func observe(_ name: Notification.Name?, object: AnyObject? = nil, closure: @escaping NotificationPosted) -> NotificationObserver {
        let observer = NotificationObserver(observingContext: self, object: object, closure: closure)
        addObserver(observer, name: name, object: object)
        return observer
    }

    @discardableResult func observe<T: ObservableNotification>(object: AnyObject? = nil, closure: @escaping ObservableNotificationPosted<T>) -> NotificationObserver {
        let observer = NotificationObserver(observingContext: self, object: object) {
            observer, notification in
            let object = (notification.object as AnyObject?)
            if let notification = T() {
                closure(observer, object, notification)
            }
        }
        addObserver(observer, name: T.self.name, object: object)
        return observer
    }

    @discardableResult func observe<T: UserInfoTransformableNotification>(object: AnyObject? = nil, closure: @escaping ObservableNotificationPosted<T>) -> NotificationObserver {
        let observer = NotificationObserver(observingContext: self, object: object) {
            observer, notification in
            let object = (notification.object as AnyObject?)
            if let notification = T(userInfo: notification.userInfo) {
                closure(observer, object, notification)
            }
        }
        addObserver(observer, name: T.self.name, object: object)
        return observer
    }

    func removeObservers(object: AnyObject) {
        observers.filter({ $0.object === object }).forEach { removeObserver($0) }
    }

    func removeObservers() {
        observers.forEach { removeObserver($0) }
    }

}

private extension NotificationObservingContext {

    func addObserver(_ observer: NotificationObserver, name: Notification.Name?, object: AnyObject?) {
        observers.append(observer)
        notificationCenter.addObserver(observer, selector: #selector(NotificationObserver.observe), name: name, object: object)
    }

    func removeObserver(_ observer: NotificationObserver) {
        if let index = observers.index(where: { $0 === observer }) {
            let observer = observers.remove(at: index)
            notificationCenter.removeObserver(observer)
        }
    }

}

Example Usage

class Example {

    private let context = NotificationObservingContext()

    init() {

        // Observe notification name:
        context.observe(.UIApplicationDidBecomeActive) {
            observer, notification in
            print(notification)
            // Only if you want a one time notification, you may remove the observer in the closure:
            observer.remove()
        }

        // If you want to remove an observer later, you may keep a reference to it:
        let observer = context.observe(.UIApplicationDidReceiveMemoryWarning) {
            observer, notification in
            print(notification)
        }

        // Arbitary example of removing that observer later:
        DispatchQueue.main.asyncAfter(deadline: .now() + 5) {
            observer.remove()
        }

        // Observe custom notification:
        context.observe(object: self) {
            (observer, object, notification: ExampleNotification) in
            // Access custom notification properties:
            print(notification.dateA)
            print(notification.dateB)
        }

        let notification = ExampleNotification(dateA: .distantPast, dateB: .distantFuture)

        // Post custom notification:
        NotificationCenter.default.post(notification, object: self)

        // Enqueue custom notification:
        NotificationQueue.default.enqueue(notification, object: self, postingStyle: .asap)

        // If you want to remove all observers of a particular object later, you can do that too:
        DispatchQueue.main.asyncAfter(deadline: .now() + 5) {
            [ weak self] in
            guard let _self = self else { return }
            _self.context.removeObservers(object: _self)
        }

    }

    deinit {
        // Nothing to do here since the removal of observers occurs automatically when context is deallocated.
    }

}

struct ExampleNotification: UserInfoTransformableNotification {

    private struct Key {
        static var dateA = "dateA"
        static var dateB = "dateB"
    }

    private(set) var dateA: Date
    private(set) var dateB: Date

    var userInfo: [AnyHashable: Any]? {
        return [
            Key.dateA: dateA,
            Key.dateB: dateB
        ]
    }

    init?(userInfo: [AnyHashable: Any]?) {
        guard
            let dateA = userInfo?[Key.dateA] as? Date,
            let dateB = userInfo?[Key.dateB] as? Date
            else {
                return nil
        }
        self.init(dateA: dateA, dateB: dateB)
    }

    init(dateA: Date, dateB: Date) {
        self.dateA = dateA
        self.dateB = dateB
    }

}

// If you do not need custom notification properties, use ObservableNotification instead:

struct AnotherExampleNotification: ObservableNotification {}