How to expose existing property on Obj-C class using an extension protocol in Swift

Question

In Swift 1.1 we were able to have code like below compile and work where we exposed existing Objective-C properties through a protocol added by an extension. We also had a few where the property is handled by the extension.

@objc protocol Enableable: class {
    var enabled: Bool { get set }
}

let DisabledAlpha: CGFloat = 0.5
let EnabledAlpha: CGFloat = 1.0

extension UIButton: Enableable {}

extension UIImageView: Enableable {
    var enabled: Bool {
        get {
            return alpha > DisabledAlpha
        }
        set(enabled) {
            alpha = enabled ? EnabledAlpha : DisabledAlpha
        }
    }
}

When trying to compile this code using XCode 6.3 and Swift 1.2, we get the following error Type 'UIButton' does not conform to the protocol 'Enableable' . The UIImageView extension seems to compile fine.

Is there any way to expose these sort of existing properties from an Objective-C type or do we have to implement a proxying property with a different name?

Answer 1

The compiler error message

note: Objective-C method 'isEnabled' provided by getter for 'enabled' does not match the requirement's selector ('enabled')

gives a hint about the problem. The enabled property of UIButton is inherited from UIControl and in Objective-C declared as

@property(nonatomic, getter=isEnabled) BOOL enabled

Therefore the protocol method has to be

@objc protocol Enableable: class {
    var enabled: Bool { @objc(isEnabled) get set }
}

and the implementation (similarly as in Swift 1.2 error on Objective-C protocol using getter ):

extension UIImageView: Enableable {
    var enabled: Bool {
        @objc(isEnabled) get {
            return alpha > DisabledAlpha
        }
        set(enabled) {
            alpha = enabled ? EnabledAlpha : DisabledAlpha
        }
    }
}