x/mobile: enable building frameworks for Catalyst

[losh11]

As of macOS 10.15, users can build iPad apps to run on macOS using Catalyst. However if you build a framework using gomobile, even though the framework has support for x86_64, Xcode will display an error.

To fix this, all you need to do is target clang for the macOS sdk & add a cflag with -target x86_64-apple-ios13.0-macabi. Fully tested example below.

Gomobile should support specifying a target catalyst. In x/mobile/cmd/gomobile/env.go, add a new case "catalyst" for function envInit which looks like this:

case "catalyst":
    clang, cflags, err = envClang('macosx')
    cflags += " -target x86_64-apple-ios13.0-macabi"

[losh11]

@hyangah if y'all are okay with this, I can implement?

[agnivade]

All yours !

[damiandizeo]

@losh11 added a new case "catalyst" for function envInit , but when I do 'gomobile bind' and create the .framework, Xcode still displaying error in when building on macOS Catalyst. Am I missing something ?

[losh11]

@damiandizeo I'm not able to check exactly what you've done without a full set of diffs. This likely means your framework is not build for x86_64h as required by catalyst.

[damiandizeo]

@losh11 thanks, adding the cflag '-target x86_64-apple-ios13.0-macabi', was able to compile the framework and now runs under Mac Catalyst

[losh11]

Great to hear @damiandizeo, if you or anyone else has time to implement this, that would be awesome. Unfortunately I no longer have the time to do this myself.

[dpwiese]

I looked at this quickly last night, and had a bit of trouble coming up with a solution as elegant as I had hoped. The environment is configured in envInit() for each architecture in allArchs. Support for catalyst needs to provide an additional configuring, but for the existing amd64 architecture. I don't think it makes sense to add catalyst to allArchs as a new architecture, as it is not. Treating it as such seems like a poor choice, and furthermore would require many changes throughout to support it if added as a new "architecture".

Rather, when setting up the environment in envInit() I've simply included it as an additional case alongside the existing true architectures. I've opened a (draft) PR with the current proposed solution here: golang/mobile#45

Feedback is appreciated, after which I'll mark ready for review. 🚀