How to use a Objective-C #define from Swift

Question

I am migrating a UIViewController class to train a bit with Swift. I am successfully using Objective-C code via the bridging header but I have the need of importing a constants file that contains #define directives.

I have seen in Using Swift with Cocoa and Objective-C (Simple macros) the following:

Simple Macros

Where you typically used the #define directive to define a primitive constant in C and Objective-C, in Swift you use a global constant instead. For example, the constant definition #define FADE_ANIMATION_DURATION 0.35 can be better expressed in Swift with let FADE_ANIMATION_DURATION = 0.35. Because simple constant-like macros map directly to Swift global variables, the compiler automatically imports simple macros defined in C and Objective-C source files.

So, it seems it's possible. I have imported the file containing my constants into the bridging header, but I have no visibility from my .swift file, cannot be resolved.

What should I do to make my constants visible to Swift?

UPDATE:

It seems working with NSString constants, but not with booleans:

#define kSTRING_CONSTANT @"a_string_constant" // resolved from swift
#define kBOOL_CONSTANT YES // unresolved from swift

Try using true instead of YES.

David Berry
– David Berry

2014-06-20 15:47:58 +00:00
Commented Jun 20, 2014 at 15:47 — David Berry
– David Berry, Commented Jun 20, 2014 at 15:47

Peter O. · Accepted Answer · 2016-11-04 00:20:38Z

66

At the moment, some #defines are converted and some aren't. More specifically:

#define A 1

...becomes:

var A: CInt { get }

Or:

#define B @"b"

...becomes:

var B: String { get }

Unfortunately, YES and NO aren't recognized and converted on the fly by the Swift compiler.

I suggest you convert your #defines to actual constants, which is better than #defines anyway.

.h:

extern NSString* const kSTRING_CONSTANT;
extern const BOOL kBOOL_CONSTANT;

.m

NSString* const kSTRING_CONSTANT = @"a_string_constant";
const BOOL kBOOL_CONSTANT = YES;

And then Swift will see:

var kSTRING_CONSTANT: NSString!
var kBOOL_CONSTANT: ObjCBool

Another option would be to change your BOOL defines to

#define kBOOL_CONSTANT 1

Faster. But not as good as actual constants.

edited Nov 4, 2016 at 0:20

Peter O.

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answered Jun 20, 2014 at 11:25

fabrice truillot de chambrier

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5 Comments

Ricky Over a year ago

May I know the reason you use NSString! and not String! ?

foOg Over a year ago

I think it is because you never have to test if a constant is nil before using it, so it is "safe" to unwrap it when used. However I would have let NSString but I am not that confortable with swift at this time :)

Tim Over a year ago

I suppose #defines and consts become constants: i.e. lets

Jose Manuel Abarca Rodríguez Over a year ago

developer.apple.com/library/ios/documentation/Swift/Conceptual/…

Shayne Over a year ago

How does one pass in constants via -DCONSTANT=val on the command line or xcode plist build settings if we change them to let declarations. It kinda misses the whole point of them really.

pkamb · Accepted Answer · 2020-03-19 07:30:07Z

22

Just a quick clarification on a few things from above.

Swift Constant are expressed using the keywordlet

For Example:

let kStringConstant:String = "a_string_constant"

Also, only in a protocol definition can you use { get }, example:

protocol MyExampleProtocol {
    var B:String { get }
}

edited Mar 19, 2020 at 7:30

pkamb

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answered Jul 11, 2014 at 14:34

Joshua

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Comments

Ankish Jain · Accepted Answer · 2014-12-02 09:33:57Z

8

In swift you can declare an enum, variable or function outside of any class or function and it will be available in all your classes (globally)(without the need to import a specific file).

  import Foundation
  import MapKit

 let kStringConstant:String = "monitoredRegions"

  class UserLocationData : NSObject {    
class func getAllMonitoredRegions()->[String]{
     defaults.dictionaryForKey(kStringConstant)
 }

answered Dec 2, 2014 at 9:33

Ankish Jain

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2 Comments

XYZ Over a year ago

Do you think it is necessary to declear the type of kStringConstant which can be inferred by the content provided, "monitoredRegins".

Ankish Jain Over a year ago

Nope not really. Its just a coding practice , any hardcoded string I like to put it on top. Later easily move it out to a constants file.

Amr Angry · Accepted Answer · 2016-11-22 16:18:57Z

1

simple swift language don't need an macros all #define directives. will be let and complex macros should convert to be func

answered Nov 22, 2016 at 16:18

Amr Angry

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1 Comment

Anirudha Mahale Over a year ago

Yes you are correct, Using Imported C Macros in Swift

Ankur Lahiry · Accepted Answer · 2019-04-18 04:00:45Z

-1

The alternative for macro can be global variable . We can declare global variable outside the class and access those without using class. Please find example below

import Foundation
let BASE_URL = "www.google.com"

class test {

}

edited Apr 18, 2019 at 4:00

Ankur Lahiry

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answered Jun 3, 2017 at 15:44

garg

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