Protocol extension with button and selector

Question

I have some protocol:

@objc protocol SomeProtocol { }

that I extend for UIViewController instances. In this extension I want to create and add a button, whose selector is also defined in the protocol:

extension SomeProtocol where Self: UIViewController {

    func addSomeButton() {
        let someButton = UIButton()
        someButton.addTarget(self, #selector(someButtonPressed), for: .touchUpInside)
        view.addSubview(someButton)
    }

    @objc func someButtonPressed() {
    }

}

However, I get the error @objc can only be used with members of classes, @objc protocols, and concrete extensions of classes at the definition of someButtonPressed.

Is there any way to achieve this using protocols?

Thanks in advance for any suggestions!

why not `extension UIViewController` ?? tried `@objc protocol` — Shehata Gamal, Jan 11 '19 at 22:59
@Sh_Khan Hi. It was an `@objc` protocol (fixed the typo now). I'm trying to move to Swift protocols for doing things like this, so that I don't have a very length `UIViewController` with lots of extensions. Do you know of a way to do this, please? — ajrlewis, Jan 11 '19 at 23:04

score 3 · Accepted Answer · answered Jan 12 '19 at 01:30

A workaround is adding a closure sleeve to the UIButton instead of a target action as in shown here https://stackoverflow.com/a/41438789/5058116 and copied below for convenience.

typealias Closure = () -> ()

///
class ClosureSleeve {
    let closure: Closure
    init(_ closure: @escaping Closure) {
        self.closure = closure
    }
    @objc func invoke () {
        closure()
    }
}

extension UIControl {
    func addAction(for controlEvents: UIControl.Event = .touchUpInside, _ closure: @escaping Closure) {
        let sleeve = ClosureSleeve(closure)
        addTarget(sleeve, action: #selector(ClosureSleeve.invoke), for: controlEvents)
        objc_setAssociatedObject(self, String(format: "[%d]", arc4random()), sleeve, objc_AssociationPolicy.OBJC_ASSOCIATION_RETAIN)
    }
}

Then simply replace:

someButton.addTarget(self, #selector(someButtonPressed), for: .touchUpInside)

with:

someButton.addAction { [weak self] in
        self?.someButtonPressed()
    }

and hey presto.

Import Accelerate · Answer 2 · 2019-01-11T23:29:47.160

-1

You need to supply a Selector requirement in the protocol. This is because you can only apply the @objc attribute to an NSObject. The only reason to mark a protocol @objc is for optional methods.

@objc protocol SomeProtocol {
    var action: Selector { get }
}

Change the extension to this:

extension SomeProtocol where Self: UIViewController {
    func addSomeButton() {
        let someButton = UIButton()
        someButton.addTarget(self, action: action, for: .touchUpInside)
        view.addSubview(someButton)
    }
}

Now, this works:

extension UIViewController: SomeProtocol {
    @objc func buttonPressed(sender: UIButton) {
        print("Button pressed")
    }
    var action: Selector {
        return #selector(buttonPressed(sender:))
    }
}

Usage:

let vc: myViewController: MyViewController!
func doSomething() {
  vc.addSomeButton()
}

edited Jan 11 '19 at 23:29

answered Jan 11 '19 at 23:03

Import Accelerate

457
3
13

Thanks for you suggestion. I'd like to keep the implementation of `buttonPressed` *inside* the protocol extension, please. This is to minimise the length of the `UIViewController` so I'm not adding lots of extensions. – ajrlewis Jan 11 '19 at 23:06
You will not have to. Just define it in `UIViewController` and all the subclasses will get it for free. – Import Accelerate Jan 11 '19 at 23:08
Thanks. So there's no way to include this solely in the protocol? – ajrlewis Jan 11 '19 at 23:12
No. As I said in my answer, the `@objc` attribute requires `NSObject`. There is no way to guarantee a Swift protocol only is for `NSObject`s. :( – Import Accelerate Jan 11 '19 at 23:14
@ImportAccelerate Yes you can with the newest toolchain. However I tried it out and thats not working. objc attributes are just not allowed in protocol extensions. – J. Doe Jan 11 '19 at 23:20
I know that. What is your confusion? – Import Accelerate Jan 11 '19 at 23:23
@j-doe is there another way to do this perhaps without extending `UIViewControllers`? – ajrlewis Jan 11 '19 at 23:25
@ImportAccelerate I did read that bit, but thanks for further explaining. – ajrlewis Jan 11 '19 at 23:26
There is no other way – Import Accelerate Jan 11 '19 at 23:28
@Import-Accelerate See my answer if you're interested. – ajrlewis Jan 12 '19 at 01:31
Interesting solution but feels kinda of hacky. I like pure Swift. – Import Accelerate Jan 12 '19 at 01:55

Protocol extension with button and selector

2 Answers2