How to append values in xcconfig variables?

Question

I\'m using Xcode and .xcconfig files. I\'m trying to append some values in the preprocessor definitions, but I simply can\'t make it work.

I tried the following (as

Answer 1

As stated in other answers, prior to Xcode 10, xcconfig files could not simply inherit and extend each other's values. But,

Since Xcode 10, xcconfig now work as one might expect them to : $(inherited) actually expand to the previously defined value of the variable.

When an .xcconfig file contains multiple assignments of the same build setting, later assignments using $(inherited) or $() will inherit from earlier assignments in the .xcconfig. The legacy build system caused every use of $(inherited) or $() to skip any other values defined within the .xcconfig. To detect whether your .xcconfig is affected by this improvement, running defaults write com.apple.dt.XCBuild EnableCompatibilityWarningsForXCBuildTransition -bool YES in Terminal will cause Xcode to generate a warning about this situation.

(Xcode 10 beta 1 release notes)

So for example, given two simple .xcconfig files:

// Generic.xcconfig
OTHER_SWIFT_FLAGS = $(inherited) -DMY_GENERIC_FLAG

// Debug.xcconfig
#include "Generic.xcconfig"
OTHER_SWIFT_FLAGS = $(inherited) -DMY_DEBUG_FLAG

Assuming your project uses Debug.xcconfig for its Debug configuration, you'll get the expected value -DMY_GENERIC_FLAG -DMY_DEBUG_FLAG for OTHER_SWIFT_FLAGS.

(instead of just -DMY_DEBUG_FLAG in Xcode 9 and earlier releases)

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The new behavior is pretty straightforward: $(inherited) is simply replaced by the previously defined value of the variable, if any.

So in the previous example, if we expand the #include statement, we'll get the following xcconfig file:

// Merged xcconfig files after resolving #include
OTHER_SWIFT_FLAGS = -DMY_GENERIC_FLAG
OTHER_SWIFT_FLAGS = $(inherited) -DMY_DEBUG_FLAG

• On the first line OTHER_SWIFT_FLAGS value is -DMY_GENERIC_FLAG ( $(inherited) expands to nothing, because this is the first definition of OTHER_SWIFT_FLAGS we encounter¹).
• On the second line OTHER_SWIFT_FLAGS if overwritten, and its value is now -DMY_GENERIC_FLAG -DMY_DEBUG_FLAG (its previous value + the newly added flag).

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On a more complex xcconfig setup, things could look like this:

// First.xcconfig
OTHER_SWIFT_FLAGS = $(inherited) -DMY_FIRST_FLAG

// Second.xcconfig
OTHER_SWIFT_FLAGS = $(inherited) -DMY_SECOND_FLAG

// Last.xcconfig
#include "Generic.xcconfig"
OTHER_SWIFT_FLAGS = $(inherited) -DMY_LAST_FLAG

// Merge.xcconfig
#include "First.xcconfig"
#include "Second.xcconfig"
OTHER_SWIFT_FLAGS = $(inherited) -DMY_INTERMEDIATE_FLAG
#include "Last.xcconfig"

We'll assume this time we're using the Merge.xcconfig in our configuration.

The resolved value for OTHER_SWIFT_FLAGS will then be -DMY_FIRST_FLAG -DMY_SECOND_FLAG -DMY_INTERMEDIATE_FLAG -DMY_LAST_FLAG.

This might be surprising at first, but it actually makes sense: once the #include are resolved, we end up with this xcconfig:

OTHER_SWIFT_FLAGS = $(inherited) -DMY_FIRST_FLAG
OTHER_SWIFT_FLAGS = $(inherited) -DMY_SECOND_FLAG
OTHER_SWIFT_FLAGS = $(inherited) -DMY_INTERMEDIATE_FLAG
OTHER_SWIFT_FLAGS = $(inherited) -DMY_LAST_FLAG

The final resolved value is then the one defined on the last line, which is -DMY_LAST_FLAG plus the value it inherited from the previous line -DMY_INTERMEDIATE_FLAG etc etc.

Note that naturally, if you forget $(inherited) in one of the definitions, you'll break the inheritance chain and only get the values from the bottom definitions, up the definition without $(inherited).

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¹ One may expect the xcconfig file to inherit previous values defined at the Project level, but it doesn't seem to be the case

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