Multiple calls using Alamofire to different APIs - how to save results to an array?

Question

I'm developing an app that fetches data from few different APIs using Alamofire (each call is done using a function). Then I have to collect all of the results (Double type in my case) to one array to calculate the average.

As long as Alamofire uses asynchronous calls, it is impossible to simply append new value to an array from inside of call.

Here is a function that calls each of functions responsible for fetching data by Alamofire:

func collectData() {
   fetchFromFirstAPI()
   fetchFromSecondAPI()  //etc.
}

And here is an example of one of these functions:

func fetchFromFirstAPI() {
   let APIKey = "XXXXXXXXX"
   let APIURL = "http://urlapi.com/api" as URLConvertible

   let parameters: Parameters = ["APPKEY": APIKey]

   Alamofire.request(APIURL, method: .get, parameters: parameters, encoding: URLEncoding.default).validate().responseJSON { response in
        switch response.result {
        case .success(let data):
            let json = JSON(data)
            if let result = json["main"]["value"].double {
                myArray.append(result)
            } else {
                print("error")
            }
        case .failure:
            print("error")
        }
   }
}

And the array:

var myArray: [Double] = []

How to deal with it?

Why *can't* you simply append each result to an initialised array? Or do you want to know when all the calls have finished? If you're after the latter, see [this answer](http://stackoverflow.com/questions/35906568/wait-until-swift-for-loop-with-asynchronous-network-requests-finishes-executing/35906703#35906703). — paulvs, Dec 05 '16 at 02:01
I know I'll have to create a dispatch group, that's clear for me. Basically I cannot append every result in the way shown above because of async call. As you can see, I was trying to do that but myArray is still empty (outside of Alamofire call). — mrpapa, Dec 05 '16 at 02:06
Why exactly is it impossible? I don't see the problem.. And in the code you posted I can't see how you declared `myArray` — paulvs, Dec 05 '16 at 02:17
I've edited my question with the array declaration code. As Alamofire's README says, 'The result of a request is only available inside the scope of a response closure.'. I'm trying to append it after successful fetching data, but myArray contains appended element only inside the call. — mrpapa, Dec 05 '16 at 02:25
I think I understand your source of confusion, what this statement from the Alamofire docs means is that the result is not available immediately after invoking the request, because the response closure is executed some time (let's say 3 seconds) after the request was sent. All you need to do is ensure `myArray` is declared globally (but within your view controller, for example) and you'll be right to go. — paulvs, Dec 05 '16 at 02:31
@Rob, good point. I've implemented a dispatch group in the meantime and now it simply works. Thanks for your help, I've learned something new :) — mrpapa, Dec 05 '16 at 02:32

score 7 · Accepted Answer · answered Dec 05 '16 at 04:24

You said:

As long as Alamofire uses asynchronous calls, it is impossible to simply append new value to an array from inside of call.

You can actually go ahead and append items to the array inside the completion handlers. And because Alamofire calls its completion handlers on the main queue by default, no further synchronization is needed. And you can use dispatch groups to know when all the requests are done. First, I'd give my methods completion handlers so I know when they're done, e.g.:

func fetchFromFirstAPI(completionHandler: @escaping (Double?, Error?) -> Void) {
    let APIKey = "XXXXXXXXX"
    let APIURL = "http://urlapi.com/api"

    let parameters: Parameters = ["APPKEY": APIKey]

    Alamofire.request(APIURL, parameters: parameters).validate().responseJSON { response in
        switch response.result {
        case .success(let data):
            let json = JSON(data)
            if let result = json["main"]["value"].double {
                completionHandler(result, nil)
            } else {
                completionHandler(nil, FetchError.valueNotFound)
            }
        case .failure(let error):
            completionHandler(nil, error)
        }
    }
}

Where

enum FetchError: Error {
    case valueNotFound
}

And you can then do something like:

func performRequestsAndAverageResults(completionHandler: @escaping (Double?) -> ()) {
    var values = [Double]()
    let group = DispatchGroup()

    group.enter()
    fetchFromFirstAPI { value, error in
        defer { group.leave() }
        if let value = value { values.append(value) }
    }

    group.enter()
    fetchFromSecondAPI { value, error in
        defer { group.leave() }
        if let value = value { values.append(value) }
    }

    group.notify(queue: .main) {
        completionHandler(self.average(values))
    }
}

func average<T: FloatingPoint>(_ values: [T]) -> T? {
    guard values.count > 0 else { return nil }

    let sum = values.reduce(0) { $0 + $1 }
    return sum / T(values.count)
}

Now, you may want to handle errors differently, but this should illustrate the basic idea: Just append the results in the completion handlers and then use dispatch groups to know when all the requests are done, and in the notify closure, perform whatever calculations you want.

Clearly, the above code doesn't care what order these asynchronous tasks finish. If you did care, you'd need to tweak this a little (e.g. save the results to a dictionary and then build a sorted array). But if you're just averaging the results, order doesn't matter and the above is fairly simple.

Multiple calls using Alamofire to different APIs - how to save results to an array?

1 Answers1