One way to let a subclass inherit a method referencing its own type in Swift

static-class-method-that-references-subclass-type.swift

// NOTE: this first attempt has been superseded by a slightly better one:
// https://gist.github.com/masonmark/967ec16161d6e0b26e146f32c2f35898

// Mason 2019-09-28: This is one way to allow subclasses to inherit a
// method with a parameter whose type refers to the actual subclass type
// (and not the parent class's type).
//
// One purpose here is to define a base class, that can have various
// subclasses, and enable all of the subclasses to inherit a class method
// named configure(), which creates an instance of the subclass and then
// configures the instance and its various subclass-specific properties.
//
// This turned out not to be so straighforward, so there is some convoluted
// set up code to make it work.
//
// I am not sure this is the best way to do this in Swift 5.1; it is just
// the first way I found that worked.

protocol BaseConfigurable {
    
    // static func configure( configurator: (Self) -> Void) -> Self;
    //
    // We cannot have the above definition in the protocol itself. It
    // must be defined only in the protocol extension, otherwise we get
    // error:
    //
    // Protocol 'BaseConfigurable' requirement 'configure(configurator:)' cannot
    // be satisfied by a non-final class ('Configurable') because it uses 'Self'
    // in a non-parameter, non-result type position
}

extension BaseConfigurable where Self: Configurable {
    
    static func configure( _ configurator: (Self) -> Void) -> Self {
        let result = Self.init();
        configurator(result);
        return result;
    }
}

class Configurable: BaseConfigurable {
    
    required init() {
        
        // We need this required init, otherwise we get this error:
        //
        // Constructing an object of class type 'Self' with a metatype
        // value must use a 'required' initializer
    }
}

class Person: Configurable {
    var name = "Alice"
    var age  = 0
    
    var bio: String {
        "My name is \(name), and I am a \(Self.self). I am \(age) years old."
    }
}

class PoliceOfficer: Person {
    var badgeNumber: String = "0000000"
}

class Lieutenant: PoliceOfficer {
    var hasBeard   = true
    var cigarCount = 0
}

class FireFighter: Person {
    var helmetSize: Int?
    var hasLicenseToDriveFireEngine = false
}


let lisa = Person.configure() { lisa in
    lisa.name = "Lisa"
    lisa.age = 39
}

let fred = Lieutenant.configure() { fred in
    fred.name = "Fred"
    fred.age = 57
    fred.hasBeard = false
}

// let biff = FireFighter.configure() { biff in
//     biff.hasLicenseToDriveFireEngine = true;
//     biff.helmetSize = 15
//     biff.age = 21
//     biff.name = "Biff Wigginberger Jr."
// }
// instead of the above, do it another way for example:

func configureBiff(biff: FireFighter) {
    biff.hasLicenseToDriveFireEngine = true;
    biff.helmetSize = 15
    biff.age = 21
    biff.name = "Biff Wigginberger Jr."
}

let biff = FireFighter.configure(configureBiff)


print(lisa.bio)
print(fred.bio)
print(biff.bio)

// prints:
//    My name is Lisa, and I am a Person. I am 39 years old.
//    My name is Fred, and I am a Lieutenant. I am 57 years old.
//    My name is Biff Wigginberger Jr., and I am a FireFighter. I am 21 years old.