FR: Support NS_ERROR_ENUM for better error handling in Swift

[liamnichols]

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Feature proposal

• Firebase Component: All

👋 I'm trying to use the Firebase iOS SDK (in particular, FirebaseAuth) in a Swift project and I noticed that it was quite hard to check errors returned by the library methods.. For example, to switch over some common scenarios I ended up writing code a bit like this:

let nsError: NSError = error
if nsError.domain == AuthErrorDomain, let code = AuthErrorCode(rawValue: nsError.code) {
    switch code {
    case .invalidEmail:
        startInvalidEmailFlow()
        return
    default:
        break
    }
}
startGenericErrorFlow()

It's great that all of the constants have Swift names but I was really hoping that I could use errors in a more Swift friendly way. I dug back through some docs and realised that what we are missing is the NS_ERROR_ENUM in order to have the errors better declared in Swift.

Some related links:

https://github.com/apple/swift-evolution/blob/master/proposals/0112-nserror-bridging.md#importing-error-types-from-objective-c
https://developer.apple.com/documentation/swift/cocoa_design_patterns/handling_cocoa_errors_in_swift#2993724

If we declared FIRAuthErrorDomain using the NSErrorDomain typedef and switched FIRAuthErrorCode enum to an NS_ERROR_ENUM enum with the name FIRAuthError (drop the 'Code') then we could start using the errors like the following in Swift:

switch error {
case AuthError.invalidEmail:
    startInvalidEmailFlow()
default:
    startGenericErrorFlow()
}

I imagine that it's not such a simple task, but i believe that this would really help a lot of users handle errors much more reliably because right now the alternative options aren't great. Thanks!

[yuchenshi]

I've tracked this feature request internally at b/183415773.

[a-25]

@liamnichols, @yuchenshi: Hello.
Made a PR for it here - 07d55e1
But it contains breaking changes.

So the objective-c-style code:
if nsError.domain == AuthErrorDomain, let code = AuthErrorCode(rawValue: nsError.code)

should be converted into:
if nsError.domain == AuthErrorDomain, let code = AuthErrorCode.Code(rawValue: nsError.code)

And swift variant mentioned by @liamnichols is also working:

switch error {
case AuthErrorCode.invalidEmail:
    startInvalidEmailFlow()
default:
    startGenericErrorFlow()
}

Do you think it is OK to merge it or I'd better fix the breaking changes?

[paulb777]

@a-25 Thanks for the analysis and PR!

We're currently ramping up on a project to modernize the Firebase Swift APIs and improving error handling should be a part of it.

I'll mark your PR for the next breaking change release, and we can also consider a more incremental approach like #9007 that fixes error handling in a Swift wrapper library.

[ryanwilson]

Fixed in the Firebase 9 branch which will be merged to master in the near future. Closing now, thanks!