Implement Swift protocol with Objective-C property getter

Your UILabel and UIControl extensions satisfy the way you created your protocol because they already have a property called highlighted whose getter accessor method is isHighlighted.

Your HighlightableView does not satisfy the adoption of your Highlightable protocol because you have the @objc(isHighlighted) requirement on your getter.

You have to use computed properties to satisfy this. However, this means that you also need a backing store for the highlighted property. Something like private var _highlighted = false.

In your case, since this is undesirable, you could remove the @objc attribute on your protocol.

protocol Highlightable: class {
    var highlighted: Bool { get set }
}

extension UILabel: Highlightable { }
extension UIControl: Highlightable { }

class HighlightableView: UIView, Highlightable {
    var highlighted = false
}

let label = UILabel()
label.isHighlighted // Prior to iOS 10, this property is called "highlighted"

let view = HighlightableView()
view.highlighted

let highlightables: [Highlightable] = [ label, view ]

for highlightable in highlightables {
    print(highlightable.highlighted)
}

// Prints:
// false
// false

However the property names would not be consistent across the concrete types.

Here is an alternative approach:

@objc protocol Highlightable: class {
    var isHighlighted: Bool { @objc(isHighlighted)get @objc(setHighlighted:)set }
}

extension UILabel: Highlightable { }
extension UIControl: Highlightable { }

class HighlightableView: UIView, Highlightable {
    private var _isHighlighted = false
    var isHighlighted: Bool {
        @objc(isHighlighted) get {
            return _isHighlighted
        }
        @objc(setHighlighted:) set {
            _isHighlighted = newValue
        }
    }
}

let label = UILabel()
label.isHighlighted = true

let view = HighlightableView()
view.isHighlighted

let highlightables: [Highlightable] = [ label, view ]

for highlightable in highlightables {
    print(highlightable.isHighlighted)
}

// Prints:
// false
// false

This exposes a consistent isHighlighted property across all of the concrete types while still conforming to Highlightable. The drawback here is the @objc attribute is more pervasive in a context where is should not be necessary. That is, the @objc attribute is not being used to expose a Swift protocol to Objective-C code.

EDIT:

Looking at the iOS 10 API diffs for Swift (and doing some testing in Xcode 7.2), UILabel and UIControl's isHighlighted property was previously named highlighted. Using the code above while linking against iOS SDK 9.3 or lower will result in compile time errors.

In the first example, these errors can be fixed by renaming the label.isHighlighted line to label.highlighted.

In the second example, these errors can be fixed by renaming all instances of isHighlighted to highlighted (except for those within the brackets of @objc attributes).

9.3 to iOS 10.0 API Differences: https://developer.apple.com/library/content/releasenotes/General/iOS10APIDiffs/index.html