Instantiating class from Reference instead of String

Question

Let's suppose I've a class

class Test {

}

Then in another part of the program I could do

   let testClass = NSClassFromString( "Test" ) as NSObject.Type
    let testInstance = testClass()

Is there a way to do the same thing from reference of Test itself, pass Test as a reference and then create an object from the reference

By reference I mean pass the Test itself and receiving part of the code would be able to create object out of it.

just like we used to do in Objective-C

TestClassRef = [Test class]

[[TestClassRef alloc] init]

Edit:

protocol ISomeProtocol {
    func action ()
    func init() 
}

class Test
    var classesMap = Dictionary<String,ISomeProtocol>()

    func init () {
        self.classesMap = Dictionary<String,ISomeProtocol>()
    }

   func myAction (name:String ) {
        var myClass:ISomeProtocol;

        if let myClass = self.classesMap[ name ] {
            var myInstance = myClass() as ISomeProtocol
            myInstance.action()
        } else {
            return
        }

    }
}

Community · Accepted Answer · 2017-05-23 12:21:22Z

0

Yes, but you need to make some promises about the class. For example, the equivalent of your objC code is this:

let testClass: AnyClass = Test.self
let test = testClass()

That won't compile because Swift has no way to know that AnyClass has an available init(). You need to promise that it can be trivially constructed:

protocol Trivial {
   init()
}

Which means that your class must conform to that:

final class Test: Trivial {
    required init() {}
}

And then you're free to pass around and use references to the class:

func make<T: Trivial>(type: T.Type) -> T {
  return type()
}

let myType = Test.self

let something = make(myType)

(I wrap this into a function called make because some simpler examples without a function will currently cause the compiler to crash. But most of the time you want to pass this to a function anyway.)

Some more discussion on this issue coming from other directions:

edited May 23, 2017 at 12:21

CommunityBot

11 silver badge

answered Sep 28, 2014 at 17:13

Rob Napier

301k37 gold badges492 silver badges660 bronze badges

Sign up to request clarification or add additional context in comments.

10 Comments

user2727195 Over a year ago

why class Test is marked as final? is it necessary? can you make an instance without involving the function make?

user2727195 Over a year ago

let's say they all are stored inside a dictionary associated by a string (key), we retrieve by key and create an object out of it

Rob Napier Over a year ago

See the linked discussions on why it is easiest if it's final. It doesn't have to be, but then it needs a "required" initializer. If you want to create objects based on strings, see the discussion here for my recommendations: stackoverflow.com/questions/25864108/…

Rob Napier Over a year ago

As I noted, you can do this without an extra function, but in some cases it will crash the compiler today. If it's not crashing the compiler, then that's fine.

user2727195 Over a year ago

we are getting close, so assuming no crash, how would you rewrite (my edit) to make it work

|

Collectives™ on Stack Overflow

Instantiating class from Reference instead of String

1 Answer 1

10 Comments

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

10 Comments

Your Answer

Sign up or log in

Post as a guest

Linked

Related