Protocol Extension on an Objc Protocol

Swift protocol extension in Objective-C class

Objective-C protocols cannot have default implementations.

Objective-C protocols can have real optional methods/properties, unlike Swift protocols, which only have required methods/properties. The workaround for this in Swift is the use of a default implementation, however, sadly those cannot be seen in Objective-C.

I would suggest creating a pure Swift protocol and for all Objective-C classes that want to extend this, write the conformance in Swift, then create @objc wrapper functions in Swift that call the default protocol implementations - if it needs to be called, if it doesn't need to be called, simply ignore it.

Something along the lines of:

protocol SwiftProtocol {
    func requiredFunc()
    func optionalFunc()
}

extension SwiftProtocol {
    func optionalFunc() {}
}

@objc extension ObjcClass: SwiftProtocol {
    @objc func requiredFunc() {
        print("do something")
    }

    // This will call the default implementation - can be omitted if you don't need to call the default implementation from Objective-C
    @objc func objc_optionalFunc() {
        optionalFunc()
    }
}

How to use @objc protocol with optional and extensions at the same time?

I think this is not possible in swift (because of the way it bridges to @objc protocols). But this is a work around(using Obj-c associated objects) to solve the unrecognized selector sent to instance... problem.

fileprivate class AssociatedObject: NSObject {
    var closure: (() -> ())? = nil

    func trigger() {
        closure?()
    }
}

// Keys should be global variables, do not use, static variables inside classes or structs.
private var associatedObjectKey = "storedObject"

protocol CustomProtocol: class {
    func setup()
}

extension CustomProtocol where Self: NSObject {

    fileprivate var associatedObject: AssociatedObject? {
        get {
            return objc_getAssociatedObject(self, &associatedObjectKey) as? AssociatedObject
        }

        set {
            objc_setAssociatedObject(self, &associatedObjectKey, newValue, .OBJC_ASSOCIATION_RETAIN)
        }
    }


    func setup() {
        let object = AssociatedObject()
        object.closure = { [weak self] in // Do not forget to use weak in order to avoid retain-cycle
            self?.functionToCallIndirectlyWithSelector()
        }

        let selector = #selector(object.trigger)
//      Uncomment next line to test it's functionality
        object.perform(selector)

//      Here, you must add selector to the target which needs to call the selector, for example:
//      refreshControl.addTarget(object, action: selector, forControlEvents: .valueChanged)

        self.associatedObject = object
    }

    func functionToCallIndirectlyWithSelector() {
        print("Function got called indirectly.")
    }
}


class CustomClass: NSObject, CustomProtocol {}

let instance = CustomClass()

instance.setup()

I added Self: NSObject constraint to be able to test it's functionality in playground, I'm not sure if it's necessary or not.

Protocol extension with button and selector

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.

Using @objc in Swift 5 protocols

I would use the closure form of notification observation rather than a selector/method:

protocol TrackScreenshot {
    func registerObserver(handler: (()->Void)?)
    func removeObservers()
}

extension TrackScreenshot where Self: ScreenTracking {
    func registerObserver(handler: (()->Void)?) {
        NotificationCenter.default.addObserver(forName: UIApplication.userDidTakeScreenshotNotification, object: nil, queue: nil) { (notification) in
            handler?()
        }
    }
    
    func removeObservers() {
        NotificationCenter.default.removeObserver(self, name: UIApplication.userDidTakeScreenshotNotification, object: nil )
    }
}

Then your use is something like:

self.registerObserver { [weak self] in
    guard let self = self else {
        return
    }
    print("Screen shot")'
}

How to extend protocols / delegates in Objective-C?

The syntax for creating a protocol that implements another protocol is as such:

@protocol NewProtocol <OldProtocol>
- (void)foo;
@end

If you want to call a method in NewProtocol on a pointer typed as OldProtocol you can either call respondsToSelector:

if ([object respondsToSelector:@selector(foo)])
    [(id)object foo];

Or define stub methods as a category on NSObject:

@interface NSObject (NewProtocol)
- (void)foo;
@end
@implementation NSObject (NewProtocol)
- (void)foo
{
}
@end

How does protocol extension work in Swift?

I don't think there is even a need to inherit from NSObject if you are making a BaseClass to be inherited by other classes.

You can simply add classTag in the BaseClass itself, i.e.

class BaseClass {
    var classTag: String {
        return String(describing: type(of: self))
    }
}

class SubClass: BaseClass {
    func someFunc() {
        print(self.classTag)
    }
}

Another option can be to use protocol and protocol extension and provide the default definition of classTag, i.e.

protocol SomeProtocol {
    var classTag: String { get }
}

extension SomeProtocol {
    var classTag: String {
        return String(describing: type(of: self))
    }
}

Then, you can conform SomeProtocol to the classes wherever required, i.e.

class SubClass: SomeProtocol {
    func someFunc() {
        print(self.classTag)
    }
}

In any case, inheriting from NSObject is unnecessary since you don't need any NSObject specific functionality for that.

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