Apply property wrapper in superclass owned property

Question

I have created my own property wrapper for the theming of UI components like UIView, UILabel etc.

 class MyUIViewController: UIViewController {
    @Theme private override var view: UIView! // it doesnt work!!!
    @Theme private var myCustomView: UIView! // it works!!
 }

in this case, i will get a compile error "Cannot override with a stored property 'view'"

I know that the view is a property of UIViewController. Do you know if there is any possible way to apply the property wrapper to a stored(superclass) property? any suggestions would be appreciated :) thanks a lot!

`MyUIViewController` inherits from `UIViewController` which doesn't declare `myCustomView` property, so you should get an error on `myCustomView` too (something like "does not override any property from superclass") and your code shouldn't works. — rraphael, Apr 22 '20 at 08:52

score 0 · Answer 1 · answered Apr 22 '20 at 09:42

I found a way to do that but it's more like a hack than a good implementation (so I would not recommend it), and I haven't fully tested it (as it really on the UIViewController view loading mechanism, this can lead to some undefined behavior).

That said, in the property wrapper documentation you can find a "translation example" that explains how property wrapper works.

@Lazy var foo = 1738

// translates to:

private var _foo: Lazy<Int> = Lazy<Int>(wrappedValue: 1738)
var foo: Int {
  get { return _foo.wrappedValue }
  set { _foo.wrappedValue = newValue }
}

So we can imitate this to manually wrap a superclass property.

Note that doing this on the view property is a bit special as the view is not loaded during the view controller initialization, but more like a lazy var.

@propertyWrapper
struct Theme<WrappedValue: UIView> {
    var wrappedValue: WrappedValue?
}

class Controller: UIViewController {
    override func loadView() {
        super.loadView()
        _view.wrappedValue = view
    }

    private var _view: Theme<UIView> = .init()
    override var view: UIView! {
        get {
            if _view.wrappedValue == nil {
                // This is a trick I would not recommend using, but basically this line
                // forces the UIViewController to load its view and trigger the
                // loadView() method.
                _ = super.view
            }
            return _view.wrappedValue
        }
        set {
            _view.wrappedValue = newValue
        }
    }
}

I made the wrapped value in the property wrapper optional because the view property is nil during the initialization process (as the view is not yet loaded)

Apply property wrapper in superclass owned property

1 Answers1