add --swift language option

[brodycj]

• add --swift CLI option
• add swift option to library API (not documented at this point)
• update iOS template to generate native module with Swift language implementation, if --swift option is enabled
• update CLI test snapshots

resolves #221

only tested with React Native 0.62 at this point

I would like to give thanks to the following resources for some extremely helpful guidance for a Swift beginner like myself:

• various comments in How can we use swift instead of objective Objective-C ? #221
• https://reactnative.dev/docs/native-modules-ios.html#exporting-swift
• https://medium.com/javascript-in-plain-english/react-native-native-modules-with-swift-6768ea03b3f
• https://nshipster.com/swift-literals/
• https://stackoverflow.com/questions/24074479/how-to-create-a-string-with-format
• https://moduscreate.com/blog/swift-modules-for-react-native/

and some other interesting resources I found:

• https://github.com/rhdeck/react-native-swift (does not seem to be really needed if we generate one -Bridging-Header.h per native module written in Swift as proposed here
• https://teabreak.e-spres-oh.com/swift-in-react-native-the-ultimate-guide-part-1-modules-9bb8d054db03 (it looks like part 2 could potentially help with the native view part)
• https://levelup.gitconnected.com/powering-react-native-with-kotlin-swift-support-e5c1210a4e25 (also useful for Kotlin as discussed in Support Kotlin for Android #352)

TODO items:

• resolve an ugly "main queue setup" warning message from the Metro bundler
• add test cases with swift option enabled
• support native view with Swift language option
• explicitly reject combination of Swift & Apple Networking options
• test with tvOS
• test with other supported React Native versions (0.60, 0.61, 0.63-rc)
• use Stryker Mutator to check for any missing test cases
• add documentation

/cc @dlowder-salesforce

[shaikhaffan]

Hi when are you planning to merge this changes?

[brodycj]

I will try to finish and merge this in the near future.

[brodycj]

A static requiresMainQueueSetup method or some other solution is needed to help avoid an ugly message like this:

Module AwesomeNativeModule requires main queue setup since it overrides `init` but doesn't implement `requiresMainQueueSetup`. In a future release React Native will default to initializing all native modules on a background thread unless explicitly opted-out of.

ref:

• https://stackoverflow.com/questions/50929930/understanding-when-to-use-dispatch-get-main-queue-and-requiresmainqueuesetup-by/51014267#51014267
• How can we use swift instead of objective Objective-C ? #221 (comment)

I think the next step would be to understand why we need to specify this queue setup for Swift but not for Objective-C.

I would also like to see some documentation here: https://reactnative.dev/docs/native-modules-ios#exporting-swift

[fatlinesofcode]

ive been using this via npx brodybits/create-react-native-module#add-swift-option react-native-swift-module --swift --generate-example and works pretty well, i think you should merge it

adding

  static func requiresMainQueueSetup() -> Bool {
    return true
  }

to the Swift file

resolved the requiresMainQueueSetup issue