PromiseKit 6 iOS chaining

Question

I am trying to chain some API calls and I think I am confusing some concepts. Would love some clarification & code samples.

I have implemented these functions...

func promiseFetchPayments(for accountId: String) -> Promise <[OperationResponse]> {
        return Promise <[OperationResponse]> { seal in
            
            payments(for: accountId) { (records, error) in
                
                if let recs = records {
                    seal.resolve(.fulfilled(recs))
                    return
                }
                
                if let e = error {
                    seal.reject(e)
                    return
                }
            }
        }
    }

and

func payments(for accountId: String, completion: @escaping (_ records: [OperationResponse]?, _ error: Error?) -> Void) {
        stellar.payments.getPayments(
            forAccount: accountId,
            order: Order.descending,
            limit: 10
        ) { response in
            switch response {
            case .success(let paymentsResponse):
                
                DispatchQueue.main.async {
                    completion(paymentsResponse.records, nil)
                }

            case .failure(let error):
                DispatchQueue.main.async {
                    completion(nil, error)
                }
            }
        }
    }

I am trying to use it like so:

firstly {
            promiseFetchPayments(for: "XXX")
        }.done { records in
            print(records)
        } .catch { error in
            print(error)
        }

Now this actually ^^^ works OK!!!! My problem is I want to be able to change done to then and be able to chain another function / response or many more.

But the error I keep getting is:

Cannot conform to Thenable.

I am looking for something very similar to this (I know the syntax isn't right just logically follow the chain....

firstly {
            stellar.promiseFetchPayments(for: "")
        }.done { records in
            print(records)
        }.then {
            // call some other method 
        }.done { data in 
          // more data 
        }.catch { error in
            print(error)
        }

Is this actually possible? Can't seem to get any tutorials on the interwebs to compile. Seems Swift compiler really doesn't like PMK syntax or something.

Any ideas?

Might want to consider switching over to Combine if you are not backwards compatible beyond iOS 13. — matt, Feb 08 '21 at 21:29
@matt I was trying to exactly this with combine. But I found it quite difficult to use & super unintuitive. Really didn't jive with the syntax either, hard to wrap your head around. I am going to start reading your website https://www.apeth.com/UnderstandingCombine/toc.html and see maybe it can teach me better. — Adam Dahan, Feb 09 '21 at 15:57
Let me know if it helps! It is _meant_ as a teaching tool. Also note that I have several times explained here how to chain async calls. See esp. https://stackoverflow.com/questions/59428026/how-to-replicate-promisekit-style-chained-async-flow-using-combine-swift/59430629?r=SearchResults&s=1|59.2365#59430629 — matt, Feb 09 '21 at 18:57

score 0 · Accepted Answer · answered Feb 08 '21 at 20:52

The problem is because you're chaining off of a done, which doesn't like that you're trying to then do a call to then off of that.

Instead, you'll want to save the promise and use it for the later calls. You can do something like this:

let promise = firstly {
    stellar.promiseFetchPayments(for: "")
}

promise.done { records in
    print(records)
}

promise.then {
    // call some other method 
}.done { data in 
    // more data 
}.catch { error in
    print(error)
}

You can even return that promise from a method to use in other places, or pass it around to another method.

Ah that's great gavin. Thank you. – Adam Dahan Feb 09 '21 at 15:55 — Adam Dahan, Feb 09 '21 at 15:55

PromiseKit 6 iOS chaining

1 Answers1