Adding print to Just changes behaviour of Combine sequence

Question

myurik2 OP

Created Mar ’21

Replies 4

Boosts 0

Views 1.2k

Participants 4

Code block below produces different results depending on if print is commented or not:

Code Block import Combine
var cancellables = Set<AnyCancellable>()
enum FruitError: Error {
    case bad
}
var fruit = "apple"
func getFruit() -> String {
    fruit
}
Just(getFruit)
    //.print()
    .map{$0()}
    .flatMap { value -> AnyPublisher<String, FruitError> in
        print("in flat map \(value)")
        if value == "apple" {
            fruit = "pineapple"
            return Fail(error: FruitError.bad).eraseToAnyPublisher()
        }
        return Just("banana").setFailureType(to: FruitError.self).eraseToAnyPublisher()
    }
    .retry(1)
    .sink(receiveCompletion: { print($0) }, receiveValue: {print($0)})
    .store(in: &cancellables)

Without print:

Code Block in flat map apple
in flat map apple
failure

With print:

Code Block in flat map apple
in flat map pineapple
banana
receive finished
finished
receive finished
Why does print or any other debug operator(breakpoint or handleEvent) change behaviour of sequence?

Playground without print

Playground with print

Boost

Answer 1

SpartaScience OP

Mar ’21

using .share() instead of a .print() does the same trick for me.

0

Answer 2

squirrel987 OP

Mar ’21

How about Deferred?

Deferred{Just(getFruit())} allows retry with just value and without need of extra map.

Code Block Deferred{Just(getFruit())}
    .flatMap { value -> AnyPublisher<String, FruitError> in
        print("in flat map \(value)")
        if value == "apple" {
            fruit = "pineapple"
            return Fail(error: FruitError.bad).eraseToAnyPublisher()
        }
        return Just("banana").setFailureType(to: FruitError.self).eraseToAnyPublisher()
    }
    .retry(1)
    .sink(receiveCompletion: { print($0) }, receiveValue: {print($0)})
    .store(in: &cancellables)

1

Answer 3

myurik2 OP

Mar ’21

Yes, Deferred allows to call Just on retry but doesn't explain why print changes behaviour of sequence. Anyway, @squirrel987 thank you for suggestion!

0

Answer 4

rnikander OP

Mar ’21

Sorry I don't have an answer, I think it's a great question. I played with your example code and I'm just as puzzled. I hope someone else can answer.

0