Swift Combine dependent API Calls

Question

I have 3 API call, all of them are being called using Publishers. API A & API B will be called first before API C got called. But API C should be called only when the response from API A & B met some conditions.

Currently I have these functions to get the API response:

func getAPIA() -> AnyPublisher<Response<ResponseA>, Error> { ... }
func getAPIB() -> AnyPublisher<Response<ResponseB>, Error> { ... }
func getAPIC() -> AnyPublisher<Response<ResponseC>, Error> { ... }

I have these line of code, it it possible to

Publishers.Zip3(
    getAPIA(),
    getAPIB(),
    getAPIC() // --> How to make this being called based on the response of A and B
).sink(
    receiveCompletion: { print($0) },
    receiveValue: { [weak self] a, b, c in
        // return value
    }
).store(in: &cancellable)

If the response of A and B met some conditions, then APIC will be called, otherwise not. Is it achievable using Combine? Thanks.

Answer 1

You can use flatMap() to chain async operations so the they are executed in sequence. For example in your code sample you could write:

Publishers.Zip(getAPIA(), getAPIB()).flatMap { (responseA: Response<ResponseA>,
                                                responseB: Response<ResponseB>) in
    // you can call getAPIC() based on some condtion on responseA or responseA
    // and combine all the results in a tuple
    Publishers.Zip3(Just(responseA).setFailureType(to: Error.self),
                    Just(responseB).setFailureType(to: Error.self),
                    getAPIC())
}