How to declare a constant in swift that can be used in objective c

Question

if I declare the swift constant as a global constant like:

let a = "123"

but the a cannot be found in objective c.

How to solve this?

Answer 1

Swift code:

public class MyClass: NSObject {
    public static let myConst = "aConst"
}

and then in Objective-C:

[MyClass myConst]

Isn't this working as well? As in this works for me.

Also this is somewhat shorter as creating a object first (alloc, init). Making a new function for every constant is... not pretty :/

Update for Swift 4

Because of the changes in Swift 4's Objective-C inference, you need to add the @objc annotation to the declared constant as well. The previous declaration then becomes:

@objcMembers
public class MyClass: NSObject {
    public static let myConst = "aConst"
}

The calling Objective-C code remains the same.

Using @objcMembers makes all constants available (as if you'd write @objc before each constant), but I've had times where the compiler somehow wouldn't generate the corresponding ObjC code.

In those cases I'd suggest adding the @objc decorator before the constant as well.
I.e.: @objc public static let myConst = "aConst"

Answer 2

From Apple Doc:

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:

1. Generics
2. Tuples
3. Enumerations defined in Swift
4. Structures defined in Swift
5. Top-level functions defined in Swift
6. Global variables defined in Swift
7. Typealiases defined in Swift
8. Swift-style variadics
9. Nested types
10. Curried functions

Therefore its not possible to access global variables(Constants) or global functions defined in Swift.

Possible Solutions:

1. From the Apple Document Swift programming language, You can Declare Type Properties as

   class var constant: Int =  {
       return 10
   }()
   

   But currently in Swift(beta-3) Type properties are not supported.

2. You can declare a Class function to get a constant value:

   In Swift:

   class func myConst() -> String {
   
       return "Your constant"
   }
   

Accessing from Objective-C:

 NSString *constantValue = [ClassName myConst];
 NSLog(@"%@", constantValue);

Answer 3

You should not have any problem by using let in Objective-C, next example was made with Xcode 7.2 :

MyClass.swift

import Foundation
import UIKit
@objc class MyClass : NSObject { // <== @objc AND NSObject ARE BOTH NECESSARY!!!
let my_color = UIColor( red:128/255,green:32/255,blue:64/255,alpha:1 ) // <== CONSTANT!!!
}

MyObjectiveC.m

#import "PROJECTNAME-Swift.h" // <== NECESSARY TO RECOGNIZE SWIFT CLASSES!!!
@interface MyObjectiveC ()
@end
@implementation MyObjectiveC
@synthesize tableview; // <== ANY UI OBJECT, JUST AS EXAMPLE!!!
- (void) viewDidLoad () {
MyClass * mc = [ [ MyClass alloc ] init ]; // <== INSTANTIATE SWIFT CLASS!!!
tableview.backgroundColor = mc.my_color;   // <== USE THE CONSTANT!!!
}
@end

PROJECTNAME is the name of your Xcode project, as shown in Project Navigator.

Answer 4

In your swift class,

let constant: Float = -1
class YourClass: NSObject {
class func getMyConstant() -> Float {return constant}
...
}

Clean, build to let xcode prepare this method useable for obj-c. Then at your obj-c class

if ([YourClass getMyConstant] != 0) {
...
}

Answer 5

First of all you need to know about the important of auto-generated Swift header file.

It is the one that will made the magic to transcribe the Swift code to be understandable from Objective-C.

This file is auto-generated by Xcode (do not look for it in your project).

The important of this file is to use the correct name, it can match with your target name, but, may not, it is the product module name. (Search for it in your project settings as "Product module")

You need to import this file on the Objective-C class that you want to use a Swift class and also the Swift class name of your Swift file.

#import <ProductModuleName-Swift.h>
@class MySwiftClassName;

My Swift class should have the prefix @objc and inherit from NSObject:

@objc class MySwiftClassName: NSObject {
   let mySwiftVar = "123"
}

Then you can call your Swift variable from the Objective-C file:

MySwiftClassName *mySwiftClassO = [[MySwiftClassName alloc] init];
NSString *myVar = mySwiftClassO.mySwiftVar;

Make sure to clean and rebuild your project after each change to force regenerate this auto-generated file.

If your Swift header file was auto-generated correctly you can navigate to it by clicking over the import file name and check if all the code you need was properly transcribed.

In the following post you can find more detailed information about this. https://solidgeargroup.com/bridging-swift-objective-c

Answer 6

Classes func don't work. The only solution I have found out is this one:

class YourController: NSObject {

@objc static let shared = YourController()

private override init() { }

@objc class func sharedInstance() -> YourController {
    return YourController.shared
}

@objc let terms = "Your-String-here"

And then on Obj-c file:

[[YourController sharedInstance].terms]