KVOChangeNotifier

KVOChangeNotifier.swift

import Foundation

class KVOChangeNotifier: NSObject {
    weak var object: AnyObject?
    let keyPaths: [String]
    var dependentObject: AnyObject
    let dependentKeyPaths: [String]
    init(object: AnyObject, keyPaths: [String], dependentObject: AnyObject, dependentKeyPaths: [String]) {
        self.object = object
        self.keyPaths = keyPaths
        self.dependentObject = dependentObject
        self.dependentKeyPaths = dependentKeyPaths
        super.init()
        dependentKeyPaths.forEach {
            dependentObject.addObserver(self, forKeyPath: $0, options: [.initial,.new,.prior], context: nil)
        }
    }
    deinit {
        dependentKeyPaths.forEach {
            dependentObject.removeObserver(self, forKeyPath: $0)
        }
    }
    override func observeValue(forKeyPath keyPath: String?, of object: Any?, change: [NSKeyValueChangeKey : Any]?, context: UnsafeMutableRawPointer?) {
        let isPrior = change?[.notificationIsPriorKey] as? Bool ?? false
        if isPrior {
            keyPaths.forEach {
                self.object?.willChangeValue(forKey: $0)
            }
        } else {
            keyPaths.reversed().forEach {
                self.object?.didChangeValue(forKey: $0)
            }
        }
    }
}

This helper enables computed properties to emit KVO change notifications when their dependencies change.

e.g.:

fullNameNotifier = KVOChangeNotifier(self, [#keyPath(ViewModel.fullName)], person, [#keyPath(Person.firstName), #keyPath(Person.lastName)])

addObserver(self, forKeyPath: #keyPath(ViewModel.fullName), options: [.new, .initial], context: nil)