RxSwift withLatestFrom with resultSelector doesn't compile

Question

I have a Driver of type Bool and a BehaviorRelay of type Page (which is a custom enum).

enum Page {
    case option1(CustomClass1, CustomClass2)
    case option2(CustomClass3)
    case option3(CustomClass4)

    var property1: CustomClass2? {
        switch self {
        case .option1(_, let custom):
            return custom
        case .option2, .option3:
            return nil
        }
    }
}

I have the Driver<Bool> in another ViewModel.

class ViewModel1 {
    struct Output {
        let hasItems: Driver<Bool>
    }

    let output: Output

    init() {
        let hasItemsRelay: BehaviorRelay<Bool> = BehaviorRelay<Bool>(value: false)

        self.output = Output(
            hasItems: hasItemsRelay.asDriver()
        )
    }
}

And I have a BehaviorRelay<Page?> in my base class.

class ViewModel2 {
    let currentPageRelay: BehaviorRelay<Page?> = BehaviorRelay<Page?>(value: nil)

    init() {
        self.currentPageRelay = BehaviorRelay<Page?>(value: nil)
    }
}

In ViewModel2 class I'm trying to catch an event on the hasItems driver of ViewModel1.Input and when I get an event, I need the current value of currentPageRelay and later on do stuff with it. So basically withLatestFrom is the thing I need to use.

class ViewModel2 {

   private func test() {
       let customViewModel: ViewModel1 = ViewModel1()

       customViewModel
           .output
           .hasItems
           .withLatestFrom(currentPageRelay) { ($0, $1) }
           .map { (hasItems, page) -> (CustomClass2, Bool)? in 
               guard let property1 = page?.property1 else { return nil }
               return (property1, hasItems)
           }
           .unwrap()
           .drive(onNext: { (property1, hasItems) in 
               // do stuff
           }
           .disposed(by: disposeBag)
   }
}

Xcode completely loses it on the withLatestFrom. No code completion and it gives the following compile error: Expression type '(Bool, _)' is ambiguous without more context

I'm completely in the dark about this one. I've already tried everything, providing the correct classes in the parameter list below it, so that it knows what to expect etc, but no luck so far.

Tieme · Accepted Answer · 2020-01-28T09:23:58.950

I think because .withLatestFrom requires both types it operates on to be of the same observable trait. So both should be either Observable, Driver, Signal, etc.

If you want to keep your Driver in your viewModel a Driver you could add an .asObservable() after the .hasItems:

class ViewModel2 {
    let currentPageRelay: BehaviorRelay<Page?> = BehaviorRelay<Page?>(value: nil)
    let disposeBag = DisposeBag()

    init() {
        // self.currentPageRelay = BehaviorRelay<Page?>(value: nil)
    }

    private func test() {
        let customViewModel: ViewModel1 = ViewModel1()

        customViewModel
            .output
            .hasItems
            .asObservable()
            .withLatestFrom(currentPageRelay) { ($0, $1) }
            .map { (hasItems, page) -> (CustomClass2, Bool)? in
                guard let property1 = page?.property1 else { return nil }
                return (property1, hasItems)
            }
            .asDriver(onErrorJustReturn: nil)
            .drive(onNext: {
                guard let (property1, hasItems) = $0 else {
                    return
                }
                // do stuff
            })
            .disposed(by: disposeBag)
    }
}

Or add a .asDriver() to currentPageRelay in the withLatestFrom(..):

customViewModel
    .output
    .hasItems
    .withLatestFrom(currentPageRelay.asDriver()) { ($0, $1) }
    .map { (hasItems, page) -> (CustomClass2, Bool)? in
        guard let property1 = page?.property1 else { return nil }
        return (property1, hasItems)
    }
    .drive(onNext: {
        guard let (property1, hasItems) = $0 else {
            return
        }
        // do stuff
    })
    .disposed(by: disposeBag)

I had this issue now and googled it, landed on this page and was like, alright! It describes exactly what my problem is! And then I realized it was my own post from last year :facepalm. Does anyone have a clue why it doesn't compile? Thanks for the answer btw. I'm just super curious why it doesn't compile. — Charlotte1993, Jan 22 '20 at 15:18
Haha, that happens to me all the time too‍♂️. I've added some clarification, does that help? — Tieme, Jan 28 '20 at 09:24

RxSwift withLatestFrom with resultSelector doesn't compile

1 Answers1