PropertyWrapper: Derived properties

Derived.swift

@propertyWrapper
struct Derived<Instance, Value> {
    var wrappedValue: Value {
        get { fatalError() }
        set { fatalError() }
    }

    private let getter: (Instance) -> Value
    private let setter: (Instance, Value) -> Void

    init(_ keyPath: ReferenceWritableKeyPath<Instance, Value>) {
        getter = { $0[keyPath: keyPath] }
        setter = { $0[keyPath: keyPath] = $1 }
    }

    static subscript(
        _enclosingInstance instance: Instance,
        wrapped wrappedKeyPath: ReferenceWritableKeyPath<Instance, Value>,
        storage storageKeyPath: ReferenceWritableKeyPath<Instance, Self>
    ) -> Value {
        get { instance[keyPath: storageKeyPath].getter(instance) }
        set { instance[keyPath: storageKeyPath].setter(instance, newValue) }
    }
}

Usage.swift

class Parent: UIView {
    private let label = UILabel()

    @Derived(\Parent.label.text) var text: String?
}

let instance = Parent()
instance.text // nil
instance.text = "foo"
instance.text // foo