How one model can notify another model for specific property has changed?

Question

Context, an iOS app using UIKit and MVC.

Model A has a singleton object, and one of its property value (foo in this case) can be changed during the runtime. Model B has a property, Model C, that is initialized using the property value of Model A.

class ModelA {
    private(set) var foo: CustomObjectClassName

    static let shared = ModelA()
}

class ModelB {
    private var bar: ModelC

    init() {
        self.bar = ModelC(ModelA.shared.foo)
    }

    // TODO: Observer `foo` value change in Model A and then
    // reinit ModelC to replace the old `bar` object
}

What is the common design pattern or mechanism that should be used to let Model B know about property change in Model A and re-initialize its own property?

I found two patterns that can be used; however, the more I read about them, they seem to be designed for communication between Model and Controller.

• NotificationCenter (Notification & Observer)
• Key-value Observing

Related Information

• Using Key-Value Observing in Swift, https://developer.apple.com/documentation/swift/cocoa_design_patterns/using_key-value_observing_in_swift

• NotificationCenter, https://developer.apple.com/documentation/foundation/notificationcenter

Answer 1

Thanks for El Tomato comment, delegation can be a pattern be used.

A delegation pattern would work since in my case Model A is only used by a single entity Model B.

protocol ModelADelegate {
    func fooDidChange() -> Void
}

class ModelA {
    public var delegate: ModelADelegate?

    private(set) var foo: CustomObjectClassName {
        didSet {
            delegate.fooDidChange()
        }
    }
   
    static let shared = ModelA()
}

class ModelB, ModelADelegate {
    private var bar: ModelC

    init() {
        ModelA.shared.delegate = self

        self.bar = ModelC(ModelA.shared.foo)
    }

    func fooDidChange() {
        self.bar = ModelC(ModelA.shared.foo)    
    }
}