Hey gang ... SKIE question (or I guess maybe techn...

# touchlab-tools

[Bradleycorn]

Hey gang ... SKIE question (or I guess maybe technically a swift question) ... I'm having some confusion consuming this Flow in my swift code:

data class UserInfo(val name: String, val userId: Int)

class UserRepo {
    val userInfo: Flow<UserInfo?> = someFunThatReturnsAUserInfoFlow()
}

In my swift code, I want to get the "first" userId value.
My first attempt went something like this:

let userInfo = await userRepo.userInfo.first(where: { _ in true})
let userId = userInfo?.userId

But this results in an error on the last line:

Value of optional type 'SkieSwiftOptionalFlow<UserInfo>.Element' (aka 'Optional<UserInfo>') must be unwrapped to refer to member 'userId' of wrapped base type 'UserInfo'

That seems odd. It says the value must be unwrapped, but I thought that's what the ?. syntax effectively does? But I guess not.
I can get things working by doing:

let userId = userInfo??.userId

What is this ?? sorcery? I am familiar with ?? as the nil coalescing operator (let myInt = optionalInt ?? 0). But here ?? is doing something different.
How is this working, and what is it doing? Is this documented anywhere?

[Tadeas Kriz]

In Swift it's possible to get Optional<Optional<Value>>, because Optional is an enum with either a value or no value. On JVM every object is optional and the Optional<Value> in Kotlin is just syntactic compile-time safety thing, therefore Optional<Optional<Value>> doesn't mean anything.

IIRC Swift tries to automatically flatten most of nested optionals, but when generics are involved it's not that simple. The .first operator returns Optional<Element>, but since the flow can contain optional elements already, it results in the type being Optional<Optional<Element>>.

The ?? here isn't a nil coalescing operator, but instead two unwraps, one after another.

[Bradleycorn]

Thanks @Tadeas Kriz that does make sense.
This will be fun to explain to my ios devs 😄

[Tadeas Kriz]

You could perhaps declare an extension that'd solve it for you:

extension SkieSwiftOptionalFlow where Element == UserInfo? {
  var first: UserInfo? {
    get async {
      if let value = await first(where: { _ in true }) { return value } else { return nil }
    }
  }
}

That way you can then use await userRepo.userInfo.first to get it

[Tadeas Kriz]

If you make it public, you can put it in src/iosMain/swift and have it part of the API without having to declare it in the consuming code