Class name as a function parameter in swift

Question

Is there a way to refer to a class inside of a function that is placed in another class?

Simplified, class number one would look like this:

class ClassOne: UITableViewController, NSFetchedResultsControllerDelegate {

    override func viewWillAppear(animated: Bool) {
        super.viewWillAppear(animated)

        Client().fetchFunction(ClassOne)

    }

    func otherFunction(){
        ...
    }

}

Class number two looks like this:

public class ClassTwo: NSManagedObject, NSFetchedResultsControllerDelegate {

    func fetchFunction(originClass: ???.Type){

        originClass.otherFunction()

    }

}

My attempt to solve the problem does not work, indicated by the "???" placed above. All types I have input for "???" resulted in error messages.

Any ideas how to solve the problem? I also tried to follow Class as function parameter in Swift but it does not seem to work for my case.

what exactly would you want to allow to get passed in? An instance? The class itself? Where would be the sense in the later one? — luk2302
– luk2302, Commented Jan 11, 2016 at 22:05

Aaron Rasmussen · Accepted Answer · 2016-01-11 22:47:58Z

2

You can do what you want using a combination of protocols and generic functions. Something like this should work:

protocol SomeProtocol {
    static func myFunc()
}

class ClassOne: SomeProtocol {
    class func myFunc() { print("Hello") }
}

class ClassTwo {

    func test<T: SomeProtocol>(type: T.Type) {
        T.myFunc()
    }

}

ClassTwo().test(ClassOne.self)   // Prints "Hello"

Like some of the other answerers, I am having a hard time understanding what you are trying to do - I assumed that you were trying to invoke a class function, otherwise there would be no need to pass the type - you would just pass an instance of the object. If you really want to pass an instance, then modify the code like this:

protocol SomeProtocol {
    func myFunc()
}

class ClassOne: SomeProtocol {
    func myFunc() { print("Hello") }
}

class ClassTwo {

    func test<T: SomeProtocol>(object: T) {
        object.myFunc()
    }

}

ClassTwo().test(ClassOne())   // Prints "Hello"

edited Jan 11, 2016 at 22:47

answered Jan 11, 2016 at 22:34

Aaron Rasmussen

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1 Comment

Timo Krall Over a year ago

Great suggestion :-) The second one works for functions just as I wanted it to, thanks!

Cristik · Accepted Answer · 2016-01-11 22:11:18Z

2

Class names as parameters is not a good idea, at least not in your case, and you can achieve similar results via a more clean approach, without losing the type safety that Swift has.

You can achieve this more clearly by adding a protocol:

protocol MyProtocol() {
    public func otherFunction()
}

which can be used in ClassTwo:

public class ClassTwo: NSManagedObject, NSFetchedResultsControllerDelegate {

    func fetchFunction(obj: MyProtocol){
        obj.otherFunction()
    }
}

You can then add an extension to ClassOne that conforms with that protocol:

extension ClassOne: MyProtocol {
    public func otherFunction() {
        // do your magic
    }
}

You can apply this technique to all classes that need to have fetch support.

answered Jan 11, 2016 at 22:11

Cristik

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1 Comment

Timo Krall Over a year ago

Thank you, looks amazing! I will try this and the other approaches mentioned on this page after waking up, it's midnight in this time zone.

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