fix(api): add collection type casting in swift 5.7

[5d]

Issue #

Description

Swift Collection type casting is introduced in Swift 5.8. As our library is still supporting Swift 5.7, adding backward compatibility support for collection casting.

General Checklist

• Added new tests to cover change, if needed
• Build succeeds with all target using Swift Package Manager
• All unit tests pass
• All integration tests pass
• Security oriented best practices and standards are followed (e.g. using input sanitization, principle of least privilege, etc)
• Documentation update for the change if required
• PR title conforms to conventional commit style
• New or updated tests include Given When Then inline code documentation and are named accordingly testThing_condition_expectation()
• If breaking change, documentation/changelog update with migration instructions

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[lawmicha]

minor refactoring comment, let me know if you need another approval if adopting it

[lawmicha]

looks like you can probably reduce this once more

    let hasAuthorizationError: Bool
    let underlyingError: Error?
#if swift(<5.8)
    if let errors = errors.cast(to: AppSyncRealTimeRequest.Error.self) {
        hasAuthorizationError = errors.contains(where: { $0 == .unauthorized})
        underlyingError = errors.first
    } else if let errors = errors.cast(to: GraphQLError.self) {
        hasAuthorizationError = errors.map(\.extensions)
            .compactMap { $0.flatMap { $0["errorType"]?.stringValue } }
            .contains(where: { AppSyncErrorType($0) == .unauthorized })
        underlyingError = GraphQLResponseError<R>.error(errors)
    } else {
        hasAuthorizationError = false
        underlyingError = errors.first
    }
#else
    if let errors = errors as? [AppSyncRealTimeRequest.Error] {
        hasAuthorizationError = errors.contains(where: { $0 == .unauthorized})
        underlyingError = errors.first
    } else if let errors = errors as? [GraphQLError] {
        hasAuthorizationError = errors.map(\.extensions)
            .compactMap { $0.flatMap { $0["errorType"]?.stringValue } }
            .contains(where: { AppSyncErrorType($0) == .unauthorized })
        underlyingError = errors.first
    } else {
        hasAuthorizationError = false
        underlyingError = errors.first
    }
#endif
    return APIError.operationError(
        errorDescription(hasAuthorizationError),
        "",
        underlyingError
    )

Or even further, just use cast everywhere. This would avoid having to duplicate business logic while maintaining the Collection type casting in one place. Once we support 5.8 and up, then we can replace usages of cast with as

func cast<T>(to type: T.Type) -> [T]? {
        #if swift(<5.8)
        self.reduce([]) { partialResult, ele in
            if let partialResult, let ele = ele as? T {
                return partialResult + [ele]
            }
            return nil
        }
        #else
        return self as? [T]
        #endif
    }

[sebaland]

I like the idea of having the compiler directive in the cast function instead

[5d]

Including the directive in the cast func will make unit testing harder. Once we upgrade the Swift tool version to 5.8, removing the code within the annotation will become straightforward.

[sebaland]

Why would it make unit testing harder? We don't need to unit-test that we support both 5.7 and 5.8

[lawmicha]

Chatted with @5d, the CI set up will cause cast to be unit tested against Swift 5.9 so it won't reach the code path. We'd have to refactor the unit test to test against something like:

func cast<T>(to type: T.Type) -> [T]? {
        #if swift(<5.8)
        return castSwift5_7() // update unit tests to test this function instead
        #else
        return self as? [T]
        #endif
    }

We to upgrade to min 5.8 soon so as Di mentioned it will be easier to remove the code later in the current PR