Using a generic Swift class in Objective-C

Question

Let's say I define a generic class in Swift, similar to the following:

class MyArray<T> {
    func addObject(object: T) {
        // do something... hopefully
    }
}

(I know there is a slightly better implementation of array, this is merely an example.)

In Swift I can now use this class very easily:

let a = MyArray<String>()
a.addObject("abc")

With Xcode 7, we now have generics in Objective-C, so I would assume that I could use this class in Objective-C:

MyArray<NSString*> *a = [[MyArray<NSString*> alloc] init];
[a addObject:@"abc"];

However, MyArray is never added to my Project-Swift.h file. Even if I change it to inherit from NSObject, it still doesn't appear in my Swift.h file.

Is there any way to create a generic Swift class and then use it in Objective-C?

Update: If I try to inherit from NSObject and annotate with @objc:

@objc(MyArray)
class MyArray<T>: NSObject {
    func addObject(object: T) {
        // do something... hopefully
    }
}

I get the following compiler error:

Generic subclasses of '@objc' classes cannot have an explicit '@objc' attribute because they are not directly visible from Objective-C.

Does this mean there is no way to use a generic Swift class in Objective-C?

How does one indirectly reference the class?

@MrBeardsley I just tried it and it gave an interesting compiler error (see above). — Senseful
– Senseful, Commented Sep 4, 2015 at 23:29

Cœur · Accepted Answer · 2017-06-30 12:59:05Z

31

Swift generic types cannot be used in Objective-C.

https://developer.apple.com/library/content/documentation/Swift/Conceptual/BuildingCocoaApps/MixandMatch.html#//apple_ref/doc/uid/TP40014216-CH10-ID136

This excludes Swift-only features such as those listed here:

Generics

...

edited Jun 30, 2017 at 12:59

Cœur

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answered Sep 5, 2015 at 0:37

newacct

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4 Comments

Senseful Over a year ago

Was that document updated for Xcode 7? Since there are now generics in Objective-C, wouldn't it not be considered a Swift-only feature?

newacct Over a year ago

@Senseful: Lightweight generics in Objective-C are different from and much more limited than Swift generics.

finneycanhelp Over a year ago

@newacct do you know a webpage that describes the limitations?

Kim Over a year ago

The list of Swift-only exclusions is removed in the documentation for Swift 2.2.

zoul · Accepted Answer · 2018-08-30 08:15:17Z

9

There is a workaround for some situations, when you need to import Swift generic class into Objective-C.

Let's say you have Swift REST service, which uses Generics. I'm using Moya framework and that's how my service looks like:

 class AuthService: BaseService<AuthAPI> {
    func register(email: String)  {
       // ... 
    }
 }

It's inherited from base service using generics, so I can not use it directly in my Objective-C code.

So here is a workaround:

Lets create AuthServiceProtocol:

@objc protocol AuthServiceProtocol {
    func register(email: String)
}

Then let's create a factory for service (or a singleton method, no difference):

@objc class Services: NSObject {
    static let authService: AuthServiceProtocol = AuthService()
}

Then I'm able to call my generic Swift auth service from Objective-C code:

- (void)someObjcMethod {
    [Services.authService registerWithEmail:self.email];
}

edited Aug 30, 2018 at 8:15

zoul

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answered Jul 12, 2017 at 11:58

medvedNick

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3 Comments

Parv Bhasker Over a year ago

I successfully call the Services in my obj-c class But it gives error when i tried to use authService. "Property 'authService' not found on object of type 'Services ". Plese help me out to solve this issue

RMP Over a year ago

Do you have to do class AuthService: BaseService<AuthAPI>, AuthServiceProtocol ?

hariszaman Over a year ago

what if a registerWithEmail wants to return some kind of generic class :)

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