Category for obj-c class derived from swift

Question

I am running project with lots of legacy code with objc and swift. I've been using objc MPOldKeychainManager , which is now deprecated and swift's NewKeychainManager is to be used.

The problem is following: MPOldKeychainManager had some categories written and I don't want to rewrite them as swift extensions. What I've done is:

• naming the class derived from the NewKeychainManager (visible in " myTarget-Swift.h ") to " MPOldKeychainManager "
• removing the objc declaration of MPOldKeychainManager

...hoping that the categories will still work.

objc(MPOldKeychainManager)
class NewKeychainManager: KeychainManager {
}

Unfortunately, old extensions can't see the MPOldKeychainManager (derived from swift), even though I've updated the imported header to myTarget-Swift.h

#import "myTarget-Swift.h" //previously - objc "MPOldKeychainManager.h"
@interface MPOldKeychainManager (Authentication)

Question: is it possible to use categories for objc classes derived from swift?

• I have already tried totally new naming
• I have already tried loads of clean-builds

Answer 1

In case you haven't seen it, here is a useful resource for migrating from Objective-C to Swift: https://developer.apple.com/documentation/swift/migrating_your_objective_c_code_to_swift . Among other things, it states that one cannot subclass a Swift class in Objective-C. What you are trying to do is specify a different Objective-C name, MPOldKeychainManager , for the NewKeychainManager Swift class.

This will, actually, work if you add an ampersand before objc , like so:

@objc(MPOldKeychainManager)
class NewKeychainManager: KeychainManager {
}

You can then use all your existing categories in Objective-C. You will, however, have a problem using them in Swift, because to be usable in Swift they need to be available in the bridging header, and you won't be able to use the class' Objective-C name ( MPOldKeychainManager ) in the bridging header.

You can, however, write an Objective-C wrapper class that will have a method corresponding to each category method and also taking a NewKeychainManager pointer. A wrapper method can then delegate to the category method, which is available to Objective-C code, so you won't have to re-implement your category methods in Swift.

Let's say an Objective-C category has method authenticateUser: :

@interface MPOldKeychainManager (Authentication)

-(void)authenticateUser:(int32_t)uid;

@end

The method could be wrapped as follows:

@interface OldKCMWrapper : NSObject

+(void)authenticateUser:(int32_t)uid withManager:(NewKeychainManager*)inst;

@end

This interface declaration must be available, directly or indirectly, via the bridging header. Then, somewhere in your Objective-C code, the wrapper could be implemented thus:

@implementation OldKCMWrapper

+(void)authenticateUser:(int32_t)uid withManager:(MPOldKeychainManager*)inst {
    [inst authenticateUser:uid];
}

@end

The wrapper can then be used in Swift code, eg:

let kcm = NewKeychainManager()
OldKCMWrapper.authenticateUser(321, with: kcm)

In fact, the wrapper could be used in a Swift extension of NewKeychainManager . You would still have a Swift extension with equivalents of all the Objective-C category methods, but you would not have to re-implement their code in Swift: methods in the extension would simply delegate to the wrapper.

Hopefully this is helpful. There are other ways of implementing this idea, possibly more elegant.