Cannot convert Generic value to AnyObject, when it is constrained by AnyObject - swift

Question

The error is Cannot convert value of type '(O?, ObservedType) -> Void' to expected argument type '(AnyObject?, ObservedType) -> Void, but I find this curious since O is constrained as AnyObject.
For context, I'm creating my own Observable class, but this question is actually about the specific error message above rather than how I might use any other third-party framework to use observables. That is, how can I properly cast my completion handler in this case.
public class Observable<ObservedType> {
struct Observer<ObservedType> {
weak var observer: AnyObject?
let completion: (AnyObject?, ObservedType) -> Void
}
private var observers: [Observer<ObservedType>]
public var value: ObservedType? {
didSet {
if let _ = value {
notifyObservers()
}
}
}
public init(_ value: ObservedType? = nil) {
self.value = value
observers = []
}
public func observe<O: AnyObject>(forward object: O?, completion: #escaping (O?, ObservedType) -> Void) {
observers.append(Observer(observer: object, completion: completion)) // error here
if let value = value {
completion(object, value)
}
}
private func notifyObservers() {
for observer in observers {
if let value = value {
DispatchQueue.main.async { observer.completion(nil, value) }
}
}
}
}
Is it possible to cast my completion handler in this case, or in some way equate O and AnyObject

Answer 1

According to your types, I can pass any object I want to the first parameter of Observer.completion. But the function you're assigning to .completion can only accept some specific type O.
You have to change completion to (AnyObject?, ObservedType) -> Void.
public func observe<O: AnyObject>(forward object: O?, completion: #escaping (AnyObject?, ObservedType) -> Void) {
^^^^^^^^^^
And the function you pass will have to deal with the fact that it can be passed anything. I suspect that this will break your whole system. But I don't believe this style of Observable is going to work, anyway, because of exactly these kinds of type problems.
There's really no good way to directly store the Observer inside the Observable. You're not currently using it, but I assume you want it for something like removing the observer. There are ways to do that, but you can't store the observer itself. You can return a unique identifier (UUID, for example), or you can work with ObjectIdentifiers or you can pass back "remove this item" closures that the observer must call. But you generally don't want to store the observer directly (and definitely not as an AnyObject).
I recommend using Combine for this, since that's what it's designed for. Or if you need to support older iOS versions, see this experiment for ways to make this work, or this experiment for a simplified version closer to what you're trying to do here.