LayoutInvalidating.swift

@propertyWrapper
public final class LayoutInvalidating<Value> where Value: Equatable {

    public static subscript<EnclosingSelf>(
      _enclosingInstance observed: EnclosingSelf,
      wrapped wrappedKeyPath: ReferenceWritableKeyPath<EnclosingSelf, Value>,
      storage storageKeyPath: ReferenceWritableKeyPath<EnclosingSelf, LayoutInvalidating>
    ) -> Value where EnclosingSelf: UIView {
      get {
        return observed[keyPath: storageKeyPath].stored
      }
      set {
        let oldValue = observed[keyPath: storageKeyPath].stored
        observed[keyPath: storageKeyPath].stored = newValue

        if newValue != oldValue {
            observed.setNeedsLayout()
        }
      }
    }

    public var wrappedValue: Value {
      get { fatalError("called wrappedValue getter") }
      set { fatalError("called wrappedValue setter") }
    }

    public init(wrappedValue: Value) {
        self.stored = wrappedValue
    }

    // MARK: - Private

    private var stored: Value
}

// Test for Playgrounds

public class SomeView: UIView {

    @LayoutInvalidating public var volume: CGFloat = 0.0

}

let a = SomeView(frame: .zero)
a.volume = 20.0