Pass values from Objective-C to Swift class [duplicate] - ios

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?

Answer 1

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];

Answer 2

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.

Answer 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"

Answer 4

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