How to use Swift struct in Objective-C

Question

Simply I have a struct that stores the application constants as below:

struct Constant {

    static let ParseApplicationId = "xxx"
    static let ParseClientKey = "xxx"

    static var AppGreenColor: UIColor {
        return UIColor(hexString: "67B632")
    }
}

These constants can be use in Swift code by calling Constant.ParseClientKey for example. But in my code, it also contains some Objective-C classes. So my question is how to use these constants in the Objective-C code?

If this way to declare constants is not good then what is the best way to create global constants to be used in both Swift and Objective-C code?

Please follow common swift code style and use a lowercase letter to start your let/var identifiers. — Nikolai Ruhe
– Nikolai Ruhe, Commented Oct 3, 2014 at 14:37
@NikolaiRuhe Would this not be the correct style for static properties of a struct? Much like UIControlEvents.TouchUpInside? — Luke Rogers
– Luke Rogers, Commented Aug 24, 2015 at 11:38
@NikolaiRuhe Take a look at the Swift declaration: developer.apple.com/library/ios/documentation/UIKit/Reference/… — it's definitely a struct. — Luke Rogers
– Luke Rogers, Commented Aug 25, 2015 at 10:01
@LukeRogers That's because of Swift 2.0's changed way of how to import NS_OPTIONS style enums. Semantically UIControlEvent is still an enumeration type. — Nikolai Ruhe
– Nikolai Ruhe, Commented Aug 25, 2015 at 11:02
@NikolaiRuhe LukeRogers: Dinh didn't ask about that issue, and most of us viewers didn't click here to read about it. — original_username
– original_username, Commented Oct 3, 2016 at 21:23

qix · Accepted Answer · 2021-06-03 07:39:32Z

135

Sad to say, you can not expose struct, nor global variables to Objective-C. see the documentation, which states in part:

Use Classes When You Need Objective-C Interoperability

If you use an Objective-C API that needs to process your data, or you need to fit your data model into an existing class hierarchy defined in an Objective-C framework, you might need to use classes and class inheritance to model your data. For example, many Objective-C frameworks expose classes that you are expected to subclass.

As of now, IMHO, the best way is something like this:

let ParseApplicationId = "xxx"
let ParseClientKey = "xxx"
let AppGreenColor = UIColor(red: 0.2, green: 0.7, blue: 0.3 alpha: 1.0)

@objc class Constant: NSObject {
    private init() {}

    class func parseApplicationId() -> String { return ParseApplicationId }
    class func parseClientKey() -> String { return ParseClientKey }
    class func appGreenColor() -> UIColor { return AppGreenColor }
}

In Objective-C, you can use them like this:

NSString *appklicationId = [Constant parseApplicationId];
NSString *clientKey = [Constant parseClientKey];
UIColor *greenColor = [Constant appGreenColor];

edited Jun 3, 2021 at 7:39

qix

8,0641 gold badge63 silver badges69 bronze badges

answered Oct 3, 2014 at 14:27

rintaro

52k14 gold badges135 silver badges140 bronze badges

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

8 Comments

khunshan Over a year ago

Accessible to Swift also?

Fabian Over a year ago

@khunshan Yes. Accessible directly ParseClientKey, or via the class Constant.clientKey()

William Hu Over a year ago

Should use @objc class Constant: NSObject

ElectroBuddha Over a year ago

This works but in Swift 4, I also needed to put @objc in front of every class func to be able to call them from Objective C code.

user1898712 Over a year ago

@ElectroBuddha You can do @objcMembers on the class to reveal the whole class to Objective-C code.

|

Cœur · Accepted Answer · 2019-04-04 02:51:25Z

Why not create a file with both a struct and an @objc class, something like this:

import UIKit

extension UIColor {
    convenience init(hex: Int) {
        let components = (
            R: CGFloat((hex >> 16) & 0xff) / 255,
            G: CGFloat((hex >> 08) & 0xff) / 255,
            B: CGFloat((hex >> 00) & 0xff) / 255
        )
        self.init(red: components.R, green: components.G, blue: components.B, alpha: 1)
    }
}

extension CGColor {
    class func colorWithHex(hex: Int) -> CGColorRef {
        return UIColor(hex: hex).CGColor
    }
}

struct Constant {
    static let kParseApplicationId = "5678"
    static let kParseClientKey = "1234"
    static var kAppGreenColor: UIColor { return UIColor(hex:0x67B632) }
    static var kTextBlackColor: UIColor { return UIColor(hex:0x000000) }
    static var kSomeBgBlueColor: UIColor { return UIColor(hex:0x0000FF) }
    static var kLineGrayCGColor: CGColor { return CGColor.colorWithHex(0xCCCCCC) }
    static var kLineRedCGColor: CGColor { return CGColor.colorWithHex(0xFF0000) }
}


@objc class Constants: NSObject {
    private override init() {}

    class func parseApplicationId() -> String { return Constant.kParseApplicationId }
    class func parseClientKey() -> String { return Constant.kParseClientKey }
    class func appGreenColor() -> UIColor { return Constant.kAppGreenColor }
    class func textBlackColor() -> UIColor { return Constant.kTextBlackColor }
    class func someBgBlueColor() -> UIColor { return Constant.kSomeBgBlueColor }
    class func lineGrayCGColor() -> CGColor { return Constant.kLineGrayCGColor }
    class func lineRedCGColor() -> CGColor { return Constant.kLineRedCGColor }
}

For use in Objective-C files add this when you need to use constants:

#import "ProjectModuleName-Swift.h"

Swift usage:

self.view.backgroundColor = Constant.kAppGreenColor

Objective-C usage:

self.view.backgroundColor = [Constants appGreenColor];

This way you can update colors, default text, web service urls for whole app in one place.

It wasn't immediately clear to me that we could scroll your answer to discover the objective-c part!

Anil Arigela · Accepted Answer · 2020-01-15 08:52:38Z

3

Though this might be late or redundant, I could make it work with the following code :

@objcMembers class Flags: NSObject {
    static let abcEnabled = false
    static let pqrEnabled = false
    .
    .
    .
}

Obviously, to use in objc c code, you have to do #import "ProjectModuleName-Swift.h"

answered Jan 15, 2020 at 8:52

Anil Arigela

4462 silver badges8 bronze badges

Comments

koira · Accepted Answer · 2016-07-18 16:21:42Z

1

You should make the let statements private if you want to make other Swift types in your code to access these constants only via class:

private let AppGreenColor = UIColor(red: 0.2, green: 0.7, blue: 0.3 alpha: 1.0)

@objc class Constant {
    class func appGreenColor() -> UIColor { return AppGreenColor }
}

In Swift, you can use them like this:

UIColor *greenColor = Constant.appGreenColor

The following line will not compile anymore now since the let statement is private:

UIColor *greenColor = appGreenColor

answered Jul 18, 2016 at 16:21

koira

1,2239 silver badges10 bronze badges

Collectives™ on Stack Overflow

How to use Swift struct in Objective-C

4 Answers 4

8 Comments

1 Comment

Comments

Comments

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

4 Answers 4

8 Comments

1 Comment

Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Linked

Related