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I wrote about using Preprocessor Codes in 2014, and also all the way back in 2009.

This is an update to using it in Swift (vs Objective-C days).

DEBUG flag

It is very common scenario where you want certain code to run only for the debug mode. For example Admob has a tool for debugging their SDK integrations.

You want code like this:

#if DEBUG
import GoogleMobileAdsMediationTestSuite
#endif

func foo() {
    #if DEBUG
    GoogleMobileAdsMediationTestSuite.present(withAppID: "abc", on: self, delegate: nil)
    #endif
}

Build Settings

The DEBUG flag is not automatically provided. Do not confuse with the debug build configuration.

But we do want the debug build configuration to have the debug flag.

1. Go to Project > Build Settings > Other Swift Flags
2. Add -DDEBUG for the Debug build configuration
3. Do NOT add for the Release build configuration

If your flag name is BANANA, then add -DBANANA.

The biggest difference is that we now add to swift flags.

For Objective-C, we add to Preprocessor Macros. So if you do have Objective-C codes and want to use the same flag, then you must add to Preprocessor Macros too.

NEW flag for Swift

Active Compilation Conditions was introduced in Xcode 8. You need not pass in -D as swiftc will prefix it for you.

What about cocoapods library?

The Swift flag for a project or target will not apply to pods.

You have to set it manually. But since every pod install will reset your changes, you should add a post hook to Podfile like this:

post_install do |installer|
    installer.pods_project.targets.each do |target|
        if target.name == 'Armchair'
            target.build_configurations.each do |config|
                if config.name == 'Debug'
                    config.build_settings['OTHER_SWIFT_FLAGS'] = '-DDebug'
                    else
                    config.build_settings['OTHER_SWIFT_FLAGS'] = ''
                end
            end
        end
    end
end

You could add for every pods. In the config above, we add only for the pod Armchair.