How to make a synchronous request using Alamofire?

Question

I am trying to do a synchronous request using Alamofire. I have looked on Stackoverflow and found this question: making an asynchronous alamofire request synchronous.

I saw that the accepted answer uses completion to make Alamofire request synchronous but I cannot make it to work. This is my simplified code:

func loadData(completion: (Bool)) -> (Int, [String], [String], [String]){

    Alamofire.request(url!, method: .get, parameters: nil, encoding: JSONEncoding.default, headers: nil).responseJSON { response in

        switch(response.result) {
        case .success(_):
            if let JSON = response.result.value as! [[String : AnyObject]]!{
                 //Here I retrieve the data
            }

            completion(true)
            break

        case .failure(_):
            print("Error")
            completion(false)
            break  
        }
   }

   return (numberRows, nameArray, ageArray, birthdayArray)
}

With this code I am getting an error when trying to make completion(bool value). The error that I am getting is the following:

Cannot call value of non-function type 'Bool'

I have tried using a lot of examples using completion to get the values synchronously (because I need to retrieve the data before to show it on a table and at the same time get the number of rows of that table) without success.

How can I use that completion to get a synchronous response?

Thanks in advance!

Answer 1

Updated:

You can use a Semaphore to freeze the calling thread until the task has returned a value: Ref

func performSynchronously(request: URLRequest) -> (data: Data?, response: URLResponse?, error: Error?) {
        let semaphore = DispatchSemaphore(value: 0)

        var data: Data?
        var response: URLResponse?
        var error: Error?

        let task = self.dataTask(with: request) {
            data = $0
            response = $1
            error = $2
            semaphore.signal()
        }

        task.resume()
        semaphore.wait()

        return (data, response, error)
    }

Now, let’s say that we wanted to render the items loaded by the above WWDCItemsLoader within a SwiftUI view. An initial idea on how to do that might be to do something like this: Ref

struct WWDCItemsList: View {
    var loader: WWDCItemsLoader
    @State private var loadingState = LoadingState<[WWDCItem]>.idle

    var body: some View {
        switch loadingState {
        case .idle:
            Color.clear.onAppear(perform: loadItems)
        case .loading:
            ProgressView()
        case .loaded(let items):
            List(items) { item in
                // Rendering each item
                ...
            }
        case .failed(let error):
            ErrorView(error: error, reloadHandler: loadItems)
        }
    }

    private func loadItems() async {
        loadingState = .loading
        
        do {
            let items = try await loader.load()
            loadingState = .loaded(items)
        } catch {
            loadingState = .failed(error)
        }
    }
}

Old Answer: (Swift 2.0)

when you use completion handler do not use return.

func loadData(completion: @escaping (_ number: Int, _ strArr1: [String], _ strArr2: [String], _ strArr3: [String]) -> ()){

  Alamofire.request(url!, method: .get, parameters: nil, encoding: JSONEncoding.default, headers: nil).responseJSON { response in
    
    switch(response.result) {
    case .success(_):
        if let JSON = response.result.value as! [[String : AnyObject]]!{
            //Here I retrieve the data
        }
        completion(number: numberRows, strArr1 : nameArray, strArr2 : ageArray, strArr3: birthdayArray)
        break
        
    case .failure(_):
        print("Error")
        completion(number: numberRows, strArr1 : nameArray, strArr2 : ageArray, strArr3: birthdayArray)
        break
    }
  }
}

loadData (completion: { (number, strArr1, strArr2, strArr3) in
    // do it
    // for exapmple
    self.number = number
    self.strArr1 = strArr1
    // and so on
    
})

or if you want return any value in closure you must use completion handler for return any value or some thing like, for example if you want return Boolean value:

func loadData(completion:(number: numberRows, strArr1 : nameArray, strArr2 : ageArray, strArr3: birthdayArray) -> (Bool))

and in the loadData

loadData( completion: { ( number, strArr1, strArr2, strArr3 ) -> (Bool) in
       # code 
       return False
})

or some think else.

I use swift 3. but if you want another version of swift careful about External Parameter Names and internal parameter names, like: @escaping (_ number: Int, _ strArr1: [String], _ strArr2: [String], _ strArr3: [String]) -> ())

if you want set external parameter names, just need drop _ and set name for parameters.