Memory access conflict with structs and observer pattern

Question

I'm implementing an Observer design pattern on a Struct model object. The idea is that I will pass my model down a chain of UIViewController and as each controller modifies it, previous controllers will also be updated with changes to the object.

I'm aware this problem could be solved by using a class instead of struct and modifying the object directly through reference, however I'm trying to learn more about using structs.

struct ModelObject {
    var data: Int = 0 {
        didSet {
            self.notify()
        }
    }
    private var observers = [ModelObserver]()

    mutating func attachObserver(_ observer: ModelObserver){
        self.observers.append(observer)
    }

    private func notify(){
        for observer in observers {
            observer.modelUpdated(self)
        }
    }
}

protocol ModelObserver {
    var observerID: Int { get }
    func modelUpdated(_ model: ModelObject)
}

class MyViewController : UIViewController, ModelObserver {
    var observerID: Int = 1
    var model = ModelObject()

    override func viewDidLoad() {
        self.model.attachObserver(self)
        self.model.data = 777
    }

    func modelUpdated(_ model: ModelObject) {
        print("received updated model")
        self.model = model //<-- problem code
    }
}

Simply put, my model object notifies any observer when data changes by calling notify().

My problem right now is memory access: when data gets set to 777, self.model becomes exclusively accessed, and when it calls notify which calls modelUpdated and eventually self.model = model, we get an error:

Simultaneous accesses to 0x7fd8ee401168, but modification requires exclusive access.

How can I solve this memory access issue?

Answer 1

If you're observing "a thing," then that "thing" has an identity. It's a particular thing that you're observing. You can't observe the number 4. It's a value; it has no identity. Every 4 is the same as every other 4. Structs are values. They have no identity. You should not try to observe them any more than you'd try to observe an Int (Int is in fact a struct in Swift).

Every time you pass a struct to a function, a copy of that struct is made. So when you say self.model = model, you're saying "make a copy of model, and assign it to this property." But you're still in an exclusive access block because every time you modify a struct, that also makes a copy.

If you mean to observe ModelObject, then ModelObject should be a reference type, a class. Then you can talk about "this particular ModelObject" rather than "a ModelObject that contains these values, and is indistinguishable from any other ModelObject which contains the same values."