Swift conditional compiling didn't work properly when I use macro defined in objective-c

Question

I define a simple macro in objective-c header file, and import this header file into Swift through project bridging header. I was able to use this macro as a constant in Swift, but when I use it to do conditional compiling, it doesn't work properly.

I create a simple project in Xcode 10.2.1 and add some code to reproduce it. In ViewController.h

#define TEST_FLAG 1

@interface ViewController : UIViewController
@end

In ViewController.m

#import "testMacro-Swift.h"

- (void)viewDidLoad {
    [super viewDidLoad];

    SwiftClass *s = [[SwiftClass alloc] init];
    [s printMSG];

#if TEST_FLAG
    NSLog(@"Objc works.");
#endif
}

In testMacro-Bridging-Header.h

#import "ViewController.h"

SwiftFile

@objc class SwiftClass: NSObject {
    @objc func printMSG() {
        print("Macro \(TEST_FLAG)")
        #if TEST_FLAG
        print("compiled XXXxXXXXX")
        #endif
    }
}

Console Output

Macro 1
2019-07-03 14:38:07.370231-0700 testMacro[71724:11911063] Objc works.

I expected compiled XXXxXXXXX to be printed after Macro 1, but it not.

I am curious why this will happen. My project is mixed with objc and swift. I don't want to declare a same flag in swift.

I do import objc file in bridging header. you can see print("Macro \(TEST_FLAG)") runs, and print correct TEST_FLAG — Kan Chen

Anatoli P Anatoli P · Accepted Answer · 2019-07-04T00:09:54

Based on this Apple article, https://developer.apple.com/documentation/swift/imported_c_and_objective-c_apis/using_imported_c_macros_in_swift, simple C (and Objective-C) macros are imported into Swift as global constants. This is demonstrated by the output from your line

print("Macro \(TEST_FLAG)")

The snippet

    #if TEST_FLAG
    print("compiled XXXxXXXXX")
    #endif

uses a different TEST_FLAG, which is a Swift preprocessor flag. You could define it under Build Settings -> Active Compilation Conditions as TEST_FLAG or under Build Settings -> Other Swift Flags as -DTEST_FLAG.

The above explains why this happens. I can't think of a simple way to avoid defining the same flag separately for Objective-C and Swift preprocessor in Xcode. If you just want to control whether some Swift code is executed based on the TEST_FLAG, you can do something like this:

if TEST_FLAG != 0 {
        print("compiled XXXxXXXXX")
}

However, if you want to control compilation of the code, then you may have to use separate TEST_FLAGs for Objective-C and Swift and make sure they are consistent. To aid in making them consistent, you could set the TEST_FLAG used by Objective-C code in Other C Flags, which allow you to define different flags for different SDKs, Architectures, and build types (Release/Debug). Active Compilation Conditions allow the same flexibility.

Another trick to facilitate consistency between (Objective-)C and Swift compiler flags is to create a new user-defined build setting: click on + to the left of the search box under Build Settings.

Say, call it COMMON_TEST_FLAG and set its value to TEST_FLAG. Then add -D$(COMMON_TEST_FLAG) to Other C Flags and Other Swift Flags. Now when you build your code TEST_FLAG will be defined in both Objective-C and Swift code within your target. If you don't want it to be defined, just change the value of COMMON_TEST_FLAG to something else. A couple of things to watch for, though:

You cannot make COMMON_TEST_FLAG empty: this will cause the other flags to be just -D, leading to a build error.
Make sure the value of COMMON_TEST_FLAGdoes not conflict with macros defined elsewhere.

Swift conditional compiling didn't work properly when I use macro defined in objective-c

2 Answers