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

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.

Answer 2

Note that making synchronous requests is highly discouraged by Apple, for reasons noted here .

In this example I'm simplifying the call, if you have more information, such as the content of the cells, I suggest you take a look at SwiftyJSON and return the entire JSON Blob, then parse it in the relevant methods (numberOfRows, etc.).

class TableViewJSONAsynchCalls: UIViewController, UITableViewDelegate, UITableViewDataSource {
    var tableView = UITableView()
    var numberOfRows = 0;

    override func viewDidLoad() {
        loadData { (didCompleteRequest) in
            if (didCompleteRequest) {
                tableView.delegate = self
                tableView.dataSource = self
                tableView.reloadData()
            } else {
                // Handle error if data was not loaded correctly
            }
        }
    }

    func tableView(_ tableView: UITableView, numberOfRowsInSection section: Int) -> Int {
        return numberOfRows;
    }

    func tableView(_ tableView: UITableView, cellForRowAt indexPath: IndexPath) -> UITableViewCell {
        return UITableViewCell(style: UITableViewCellStyle.default, reuseIdentifier: "cell")
    }

    func tableView(_ tableView: UITableView, didSelectRowAt indexPath: IndexPath) {
        print("selected")
    }

    func loadData(completion: (Bool) -> Void) {
        // Make asynchronous call using alamofire
        // This simulates you parsing the JSON and setting the relevant variables, 
        // personally I would recommend you return a JSON blob and then 
        // parse it in the relevant methods.
        sleep(2)
        // If call is successful
        self.numberOfRows = 10
        completion(true)

    }
}

Answer 3

You can convert any method to synchronous using something like this:

func getName() -> (String?, Error?) { //an async call is in there
    let semaphore = DispatchSemaphore(value: 0)
    var name: String? // result to return
    var error: Error? // error to throw
    service.getUserName().subscribe { result in //call alamofire or anything
        switch(result) {
          case .success(let res): name = res.name
          case .failure(let err): error = err
        }
    } onFailure: { err in
        error = err
    }.disposed(by: bag)
    semaphore.wait()
    return (name, error)
}