Swift - class method which must be overridden by subclass

You have two options:

1. Use a Protocol

Define the superclass as a Protocol instead of a Class

Pro: Compile time check for if each "subclass" (not an actual subclass) implements the required method(s)

Con: The "superclass" (protocol) cannot implement methods or properties

2. Assert in the super version of the method

Example:

class SuperClass {
    func someFunc() {
        fatalError("Must Override")
    }
}

class Subclass : SuperClass {
    override func someFunc() {
    }
}

Pro: Can implement methods and properties in superclass

Con: No compile time check

The following allows to inherit from a class and also to have the protocol's compile time check :)

protocol ViewControllerProtocol {
    func setupViews()
    func setupConstraints()
}

typealias ViewController = ViewControllerClass & ViewControllerProtocol

class ViewControllerClass : UIViewController {

    override func viewDidLoad() {
        self.setup()
    }

    func setup() {
        guard let controller = self as? ViewController else {
            return
        }

        controller.setupViews()
        controller.setupConstraints()
    }

    //.... and implement methods related to UIViewController at will

}

class SubClass : ViewController {

    //-- in case these aren't here... an error will be presented
    func setupViews() { ... }
    func setupConstraints() { ... }

}

There isn't any support for abstract class/ virtual functions, but you could probably use a protocol for most cases:

protocol SomeProtocol {
    func someMethod()
}

class SomeClass: SomeProtocol {
    func someMethod() {}
}

If SomeClass doesn't implement someMethod, you'll get this compile time error:

error: type 'SomeClass' does not conform to protocol 'SomeProtocol'

Another workaround, if you don't have too many "virtual" methods, is to have the subclass pass the "implementations" into the base class constructor as function objects:

class MyVirtual {

    // 'Implementation' provided by subclass
    let fooImpl: (() -> String)

    // Delegates to 'implementation' provided by subclass
    func foo() -> String {
        return fooImpl()
    }

    init(fooImpl: (() -> String)) {
        self.fooImpl = fooImpl
    }
}

class MyImpl: MyVirtual {

    // 'Implementation' for super.foo()
    func myFoo() -> String {
        return "I am foo"
    }

    init() {
        // pass the 'implementation' to the superclass
        super.init(myFoo)
    }
}