Simple observable class for Swift

Observable.swift

import Foundation

class ObservableModel: NSObject {

	private var observers: [(keypath: String, block: (Any?) -> Void)] = []

	// keypath has to exist and has to have dynamic property to be notified
	func observe<T>(keypath: String, with block: @escaping (T?) -> Void) {
		guard let _ = value(forKeyPath: keypath) as? T else {
			fatalError("value at \(keypath) is not of type \(T.self)")
		}

		observers.append((keypath: keypath, block: { block($0 as? T) }))
		addObserver(self, forKeyPath: keypath, options: .new, context: nil)
	}

	override func observeValue(forKeyPath keyPath: String?, of object: Any?, change: [NSKeyValueChangeKey : Any]?, context: UnsafeMutableRawPointer?) {
		guard let keypath = keyPath else { return }
		observers.filter { $0.keypath == keypath }.forEach {
			$0.block((object as AnyObject).value(forKeyPath: keypath))
		}
	}

	deinit {
		observers.forEach { removeObserver(self, forKeyPath: $0.keypath) }
	}
}

// how to use

class Model: ObservableModel {
	dynamic var title: String = "Default"
}

class Holder {
	private var title: String?

	func present(model: Model) {
		title = model.title
		model.observe(keypath: #keyPath(Model.title), with: didUpdate(title:))
	}

	func didUpdate(title: String?) {
		self.title = title
	}

	func debug() {
		print(title)
	}
}


let model = Model()
let holder = Holder()
holder.present(model: model)
holder.debug()
model.title = "New title"
holder.debug()