Assigning a class dynamically from a class name string

First of all, a general advice, avoid using `Any` as far as you can.

You can declare a protocol `Animal`, and make all your animal classes conform to it:

protocol Animal: class {
    init()
    
    func move()
    
    //...
}

extension Tiger: Animal {}
extension Elephant: Animal {}
extension Gorilla: Animal {}

And declare the variable `animal` as `Animal`:

var animal: Animal?

And second, you could use `NSClassFromString` in Swift, but in most cases you want to use such feature, there may be a better alternative.

For example, use an Array of classes, rather than an Array of Strings:

var classes: [Animal.Type] = [Elephant.self, Gorilla.self, Tiger.self]
var classIndex = 2

func setupAnimal() {
    let cls = classes[classIndex]
    animal = cls.init()
    addChild(animal!)
}

func moveAnimal() {
    animal?.move()
}

You are not showing the whole code, so you may need some modifications, but please try.

Thanks for the response, OOPer 🙂!

I tried a bunch of stuff, but definitely creating an entity for animal itself helped, and it made things more streamlined in general. So, I proceeded to make a new class Animal, that the animal type classes subclass. I have my reasons for subclassing rather than using a protocol. In fact, this whole animal theme is only here to make things more understandable.

Using a common parent class made things a lot more dynamic and simple. However, I still couldn't get the class-from-string thing to work. I'm trying to avoid hardcoding the collection of Animal subclasses, because the idea is that they come from data and it is assumed that those classes exist. I tried NSClassFromString and Bundle.main.classNamed(), but I just couldn't get them cast and interpreted properly.

My current code:

var animal: Animal?

var classNames = ["Elephant", "Gorilla", "Tiger"]
var classIndex = 2

setupAnimal()
moveAnimal()

func setupAnimal() {
     let animalName = classNames[classIndex]

     if animalName == "Elephant" {
          animal = Elephant()
     } else if animalName == "Gorilla" {
          animal = Gorilla()
     } else if animalName == "Tiger" {
          animal = Tiger()
     }

     addChild(animal?)
}

func moveAnimal() {
     animal?.move()
}

What I'm still trying to do is to get rid of the remaining conditionals by determining the class name from the string. So, if animalName == "Tiger" { animal = Tiger() } should become something along the lines of animal = NSClassFromString(classNames[classIndex]), but I don't know what exactly.

A simple way is to have class as @objC (thus need to be NSObject).

Here is an example:

@objc(Animal)

class Animal: NSObject {
//    var name: String
//
//    init(named: String) {
//        self.name = named
//    }
}

@objc(Elephant)
class Elephant: Animal {
}

var cls = NSClassFromString("Animal")!
print(cls)
cls = NSClassFromString("Elephant")!
print(cls)

you get

Animal

Elephant

And

cls = NSClassFromString("Elephant")!.superclass()!
print(cls)

gets

Animal

Interacting with text data looks like the right reason to use `NSClassFromString`.

When you want to get a class from its class name in Swift, you need a fully qualified class name, which means, a module name prefixed.

x NSClassFromString("Tiger")

o NSClassFromString("MyModuleName.Tiger")

(Unless you give @objc names as suggested by Claud31.)

So, you can write something like this:

extension String {
    func toMyModuleClass() -> AnyClass? {
        struct My {
            static let moduleName = String(reflecting: Animal.self).prefix{$0 != "."}
        }
        return NSClassFromString("\(My.moduleName).\(self)")
    }
}

(You may define `moduleName` with a String literal, if you prefer. Assuming `Animal` and its all subclasses exist in the same module.)

And your Animal class needs to have a required initializer to instatiate an instance from its subclasses:

class Animal {
    required init() {
        //...
    }
    
    func move() {
        //...
    }
    
    //...
}
class Elephant: Animal {
    //...
}
class Gorilla: Animal {
    //...
}
class Tiger: Animal {
    //...
}

And then, you can write your `setupAnimal` like this:

func setupAnimal() {
    if let animalClass = classNames[classIndex].toMyModuleClass() as? Animal.Type {
        animal = animalClass.init()
        addChild(animal!)
    } else {
        print("Class \(classNames[classIndex]) does not exist")
    }
}

Or, you can put fully qualified names in your data resource.

I found this discussion on Swift forum really interesting for this post.

h ttps://forums.swift.org/t/pitch-fully-qualified-name-syntax/28482

Thanks guys! I tried using Animal.Type to set the class coming from NSClassFromString. The Animal class now also has a required initializer. I didn't really understand module name and fully qualified name stuff. Where should the module name be coming from?

Currently the code I have crashes at the NSClassFromString line, assumedly because it can't find the class. Perhaps the missing module name is a reason for that. I'm actually working in SpriteKit, so these classes happen to be subclasses of SKNode: SKNode -> Animal -> Tiger.

Current code:

var animal: Animal?

var classNames = ["Elephant", "Gorilla", "Tiger"]
var classIndex = 2

setupAnimal()
moveAnimal()

func setupAnimal() {
     let animalName = classNames[classIndex]

     animal = (NSClassFromString(animalName) as! Animal.Type).init()

     addChild(animal?)
}

func moveAnimal() {
     animal?.move()
}

It is not so difficult once understood.

Assume your project's name is MyProject, and you have some Swift classes:

class Animal {
    required init() {
        //...
    }
    
    func move() {
        //...
    }
    
    //...
}
class Elephant: Animal {
    //...
}

Then you can try:

    print(NSClassFromString("Elephant"))
    print(NSClassFromString("MyProject.Elephant"))

    print(NSStringFromClass(Elephant.self))

In this case, "MyProject.Elephant" is the full name of the class Elephant and `NSClassFromString` works only on full names.

You need to prefix `MyProject.` to make `NSClassFromString` work.

Ooooh okay 🙂. This was actually familiar from declaring custom class names in Scene Editor. Great, thanks so much, dudes! Everything works now and is sufficiently dynamic with no hardcoding.

Big ups!