Swift globals and global functions in objective c

31
votes

the documentation says:

Global constants defined in C and Objective-C source files are automatically imported by the Swift compiler as Swift global constants.

But it doesn't say anything about the other way around. I need to define a global swift constant and be able to see it one the objective c side like a global c constant. Like on the swift side define:

public let CARDS = ["card1", "card2"]

and see use it on the objective c side like

NSLog(@"Cards count: %d", [CARDS count])

What should I do? I've already imported the swift automatically generated header like:

#import "MyProject-Swift.h"

and in Xcode if I command-click on it, it takes me to the correct place in the swift code, but at compile time I get: 

'User of undeclared Identifier CARDS'

on my objective c side.

3
objective-cswiftglobal-variables
Globals defined in Swift cannot currently be used with Objective-C – aleclarson

3 Answers

45
votes

Here is the document about it

You’ll have access to anything within a class or protocol that’s marked with the @objc attribute as long as it’s compatible with Objective-C. This excludes Swift-only features such as those listed here:

  • Generics
  • Tuples
  • Enumerations defined in Swift
  • Structures defined in Swift
  • Top-level functions defined in Swift
  • Global variables defined in Swift
  • Typealiases defined in Swift
  • Swift-style variadics
  • Nested types
  • Curried functions

Global variables (including constants) are unaccessible from Objective-C.

Instead, you have to declare a class which has accessors for the global constants.

// Swift
public let CARDS = ["card1", "card2"]

@objc class AppConstant {
   private init() {}
   class func cards() -> [String] { return CARDS }
}

// Objective-C
NSArray *cards = [AppConstant cards];
3
votes

Nice answer by @rintaro, but another alternative simple Swift answer for constants that can be used in both Swift and Objective-C:

@objcMembers
class MyConstants: NSObject {
    static let kMyConstant1 = "ConstantValue1";
    static let kMyConstant2 = "ConstantValue2";
    static let CARDS = ["card1", "card2"]
}

You can access this on both Swift and Objective-C by:

MyConstants.kMyConstant1 // this will return "ConstantValue1"
MyConstants.CARDS // this will return array ["card1", "card2"]
0
votes

Swift global functions (ie. swift top-level functions) cannot be accessed by objC. Period. End of story.

See above answer from rintaro, to wit...
"This excludes Swift-only features such as those listed here: . . . Top-level functions defined in Swift"

boom