Observing.swift

import Foundation

class Observer: NSObject {
    private let recipe: Recipe

    init(recipe: Recipe) {
        self.recipe = recipe
    }

    deinit {
        NotificationCenter.default.removeObserver(self)
    }

    func observe() {
        NotificationCenter.default.addObserver(
            self,
            selector: #selector(receive(clipDidUpdateNotification:)),
            name: Recipe.ClipNotification.didUpdate.name,
            object: recipe
        )

        NotificationCenter.default.addObserver(
            self,
            selector: #selector(receive(clipDidRemoveNotification:)),
            name: Recipe.ClipNotification.didRemove.name,
            object: recipe
        )

        NotificationCenter.default.addObserver(
            self,
            selector: #selector(receive(clipDidAddNotification:)),
            name: Recipe.ClipNotification.didAdd.name,
            object: recipe
        )
    }

    func receive(clipDidUpdateNotification: Notification) {
        print("clip did update")
    }

    func receive(clipDidRemoveNotification: Notification) {
        print("clip did remove")
    }

    func receive(clipDidAddNotification: Notification) {
        print("clip did add")
    }
}

class Recipe {
    enum ClipNotification {
        case didUpdate
        case didRemove
        case didAdd

        var name: Notification.Name {
            switch self {
            case .didUpdate:
                return Notification.Name("recipe.clip.didUpdate")
            case .didRemove:
                return Notification.Name("recipe.clip.didRemove")
            case .didAdd:
                return Notification.Name("recipe.clip.didAdd")
            }
        }
    }

    init(clip: Clip) {
        self.clip = clip
    }

    var clip: Clip? {
        didSet {
            switch (oldValue, clip) {
            case let (oldValue?, clip?) where oldValue != clip:
                NotificationCenter.default.post(name: ClipNotification.didUpdate.name, object: self, userInfo: ["oldValue": oldValue, "clip": clip])
            case let (oldValue?, nil):
                NotificationCenter.default.post(name: ClipNotification.didRemove.name, object: self, userInfo: ["oldValue": oldValue])
            case let (nil, clip?):
                NotificationCenter.default.post(name: ClipNotification.didAdd.name, object: self, userInfo: ["clip": clip])
            default:
                break
            }
        }
    }
}

struct Clip: Equatable {
    let id: Int

    static func == (left: Clip, right: Clip) -> Bool {
        return left.id == right.id
    }
}

let clip = Clip(id: 1)
var recipe = Recipe(clip: clip)

let observer = Observer(recipe: recipe)
observer.observe()

recipe.clip = nil // "clip did remove"
recipe.clip = Clip(id: 2) // "clip did add"
recipe.clip = Clip(id: 3) // "clip did update"