Add module stability by enabling library evolution support

[yo2bh]

Enabled the BUILD_LIBRARY_FOR_DISTRIBUTION flag introduced in Xcode 11 for supporting framework distribution for creating the XCFramework.

[mxcl]

Seems to break Swift 4.1.52 compilation for some reason. See Travis.

https://travis-ci.org/mxcl/PromiseKit/jobs/627150231

[RomanPodymov]

Seems to break Swift 4.1.52 compilation for some reason. See Travis.

https://travis-ci.org/mxcl/PromiseKit/jobs/627150231

I think I had the same problem in #1121. Probably we should restart the checks.

[mxcl]

I restarted it twice already, but I’ll try again.

[mxcl]

Tried 4 times now, seems consistent.

[mxcl]

Notably this is the only build that is Xcode 11 and uses xcodebuild to build it, so basically, seems like this flag fails.

[yo2bh]

@mxcl Is there any way to resolve this issue which we're facing in Travis CI?

[mxcl]

I am unfamiliar with the BUILD_LIBRARY_FOR_DISTRIBUTION flag, so don’t know. Certainly not being able to build against a platform we support will prevent us merging this.

[yo2bh]

@mxcl - This flag is used to enabling library evolution support.
i.e Swift Library Evolution makes it so that you can link an application against one version of a Swift library, then upgrade the library to a newer version without rebuilding the application.

and

one more thing to create XCFramework, BUILD_LIBRARY_FOR_DISTRIBUTION flag must be set in a build setting.

We can set this flag in Podspec as well but I did not find the.podspec file in the repo.
As /PromiseKit.podspec is in .gitignore path, can you please commit this file so that I'll make changes in .podspec accordingly.

@mxcl - Can you please help me out for this.

[mxcl]

Yeah I read about this recently at the Swift blog.

I don’t really know what to say, if it won’t build against a platform we support I cannot merge it into v6.

[RomanPodymov]

Look what I found at Library Evolution in Swift:

Library evolution support should only be used when a framework is going to be built and updated separately from its clients.

PromiseKit is an open source project, so it does not make sense to build it with BUILD_LIBRARY_FOR_DISTRIBUTION = YES.

[yo2bh]

@RomanPodymov & @mxcl -

What should we do if we have to going build separately to the client? Is there any way to do this?

[mxcl]

What should we do if we have to going build separately to the client? Is there any way to do this?

Modify the xcodeproj with a script.

[RomanPodymov]

Hello @yo2bh
You can use https://github.com/CocoaPods/Xcodeproj and set BUILD_LIBRARY_FOR_DISTRIBUTION = YES for any project. But for PromiseKit let's keep BUILD_LIBRARY_FOR_DISTRIBUTION = NO like in other open source projects.