atxe atxe - 1 month ago 13
Swift Question

How to use a Objective-C #define from Swift

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

Answer

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.

Comments