Including a directory as a search path during compilation with CocoaPods - ios

Question

I'm trying to create an iOS project that includes C code that is shared across other platforms (Android). Some of the third party libraries I'm compiling rely on having certain directories included as search paths. However, no matter what I've tried, I've been unable to the CocoaPods .podspec file to include a given directory in the compilation search path. I'm using CocoaPods 1.8.4.
Whilst this is a Flutter plugin, it's my understanding that this is an issue with CocoaPods/Xcode, since Flutter has merely set up the project files. However, it's worth noting that Flutter sets up an example workspace, which I do not want to modify. I want the Plugin itself to be modified to compile in the requisite code, not the example workspace/project. In other words, I want only the ios/repro_cocoapods_issue.podspec file to be modified.
In case it matters, I used the following Flutter command to create the project:
flutter create -t plugin -i objc -a java repro_cocoapods_issue
I have a full repro up on GitHub here. To reproduce, open the workspace at example/ios/Runner.xcworkspace in Xcode and try to build.
These are the things I've tried in the ios/repro_cocoapods_issue.podspec file:
s.xcconfig = { 'HEADER_SEARCH_PATHS' => '"${PROJECT_DIR}/external/"'}
s.xcconfig = { 'USER_HEADER_SEARCH_PATHS' => '"${PROJECT_DIR}/external/"'}
s.user_target_xcconfig = { 'HEADER_SEARCH_PATHS' => '"${PROJECT_DIR}/external/"'}
s.user_target_xcconfig = { 'USER_HEADER_SEARCH_PATHS' => '"${PROJECT_DIR}/external/"'}
s.pod_target_xcconfig = { 'HEADER_SEARCH_PATHS' => '"${PROJECT_DIR}/external/"'}
s.pod_target_xcconfig = { 'USER_HEADER_SEARCH_PATHS' => '"${PROJECT_DIR}/external/"'}
s.compiler_flags = '-Iexternal/'
s.preserve_paths = 'external/**'
s.preserve_paths = '${PROJECT_DIR}/external/**'
In all cases, I get:
'external_dir.c' file not found
If I change the code to #include ../external/external_dir.c then it works, but this does not help with my actual use case.
How can I configure my podspec so that a specified directory is included in the search path during compilation?
Output of pod env
Stack
CocoaPods : 1.8.4
Ruby : ruby 2.3.7p456 (2018-03-28 revision 63024) [universal.x86_64-darwin18]
RubyGems : 2.5.2.3
Host : Mac OS X 10.14.6 (18G95)
Xcode : 11.1 (11A1027)
Git : git version 2.23.0
Ruby lib dir : /System/Library/Frameworks/Ruby.framework/Versions/2.3/usr/lib
Repositories : master - git - https://github.com/CocoaPods/Specs.git # d13a0869021d940f32f0543a03500755b39be998
Installation Source
Executable Path: /usr/local/bin/pod
Plugins
cocoapods-deintegrate : 1.0.4
cocoapods-plugins : 1.0.0
cocoapods-search : 1.0.0
cocoapods-stats : 1.1.0
cocoapods-trunk : 1.4.1
cocoapods-try : 1.1.0

Answer 1

Answering my own question after finally figuring it out.
Something is aggressively caching the "output" of .podspec files. What was causing the issue is that I was changing the .podspec and cleaning+rebuilding, expecting that to be sufficient to pick up my changes, but it was not. If I instead do a git clean -xdf and then build, my changes are picked up.
I ended up using the properties I expected should work (with slight tweaks to the above that were easy to figure out once the options were actually being used):
s.source_files = [
'Classes/**/*',
'shared_c/**/*'
]
s.public_header_files = [
'Classes/**/*.h',
'shared_c/*.h'
]
s.xcconfig = {
'HEADER_SEARCH_PATHS' => [
'"${PODS_TARGET_SRCROOT}/shared_c/"'
],
'GCC_PREPROCESSOR_DEFINITIONS' => 'SOME_SYMBOL_FOR_THIRD_PARTY_CODE=1 SOME_OTHER_SYMBOL=1'
}
s.dependency 'Flutter'
s.ios.deployment_target = '9.0'
As for why the files are being so aggressively cached, I need to chase that up with the Flutter team (and I'm expecting them to say it's a CocoaPods/XCode issue).

Answer 2

Make sure that every edit to the podspec file also increases its version.
CocoaPods podspecs versions are assumed to be immutable and cached as such. See pod cache list and pod cache clean.