如何使我的ReferenceWritableKeyPath类通用

Question

I am using ReferenceWritableKeyPath to modify properties in a model class. This works fine, however, I now want to make my code a bit more generic so that I can modify properties from different classes.

Here is my current code:

final class MyListener {
    private let reference: String
    private var listener: DatabaseHandle?
    private let backendClient: BackendClient
    private weak var model: MyModel?

    func listenToAndUpdate<T>(_ property: ReferenceWritableKeyPath<MyModel, T?>) {
        guard listener == nil else {
            return
        }
        listener = backendClient.listenToRtdbProperty(reference) { [weak self] (result: Result<T, RequestError>) in
            switch result {
            case .success(let value):
                self?.model?[keyPath: property] = value
            case .failure:
                self?.model?[keyPath: property] = nil
            }
        }
    }
}

I essentially need to have something generic in the model property, but also in the Root of the ReferenceWritableKeyPath . How can I do this? I tried using generics on the class but it wouldn't allow me to have a weak reference to a generic property.

Answer 1

You were on the right track by making the MyListener class generic, but you also need to put a type constraint on the generic type to ensure that it can only be a class, which can have a weak reference. All classes conform to AnyObject , so it is a sufficient type constraint for your case.

final class MyListener<Model: AnyObject> {
    private let reference: String
    private var listener: DatabaseHandle?
    private let backendClient: BackendClient
    private weak var model: Model?

    func listenToAndUpdate<T>(_ property: ReferenceWritableKeyPath<Model, T?>) {
        guard listener == nil else {
            return
        }
        listener = backendClient.listenToRtdbProperty(reference) { [weak self] (result: Result<T, RequestError>) in
            switch result {
            case .success(let value):
                self?.model?[keyPath: property] = value
            case .failure:
                self?.model?[keyPath: property] = nil
            }
        }
    }
}