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

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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)

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You use the above setting like so:

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

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