Swift: how to use PREPROCESSOR Flags (like `#if DEBUG`) to implement API keys?

Apple included full support for Swift preprocessor flags as of Xcode 8, so it's no longer necessary to set these values in "Other Swift Flags".

The new setting is called "Active Compilation Conditions", which provides top-level support for the Swift equivalent of preprocessor flags. You use it in exactly the same way as you would "Other Swift Flags", except there's no need to prepend the value with a "-D" (so it's just a little cleaner).

From the Xcode 8 release notes:

Active Compilation Conditions is a new build setting for passing conditional compilation flags to the Swift compiler. Each element of the value of this setting passes to swiftc prefixed with -D, in the same way that elements of Preprocessor Macros pass to clang with the same prefix. (22457329)

You use the above setting like so:

#if DEBUG
    let accessToken = "DebugAccessToken"
#else
    let accessToken = "ProductionAccessToken"
#endif

UPDATED: Xcode 8 now supports this automatically, see @DanLoewenherz's response above.

Prior to Xcode 8, you could still use Macros in the same way:

#if DEBUG
let apiKey = "KEY_A"
#else
let apiKey = "KEY_B"
#endif

However in order for them to be picked up by Swift, you need to set "Other Swift Flags" in your target's Build Settings:

• Open Build Settings for your target
• Search for "other swift flags"
• Add the macros you wish to use, preceded by the -D flag