How to do two concurrent API calls in swift 4

Question

Thanks in advance for help, I have two API calls, both are concurrent and any call could be success first(I don't want call in sequence), after success of both calls, I have to stop my activity indicator and reload my tableView, Here is my code but I don't know is this the right way or not and how to reload my tableView and stop my activity indicator.

func downloadDetails(){
    let operationQueue: OperationQueue = OperationQueue()
    let operation1 = BlockOperation() {
    WebServiceManager.getAData(format:A, withCompletion: {(data: Any? , error: Error?) -> Void in

          if let success = data {
              DispatchQueue.main.async {
                  (success code)
              }
           }
        })

        let operation2 = BlockOperation() {
        webServiceManager.getBData(format: B, withCompletion: {(data: Any? , error: Error?) -> Void in

                if let success = data {
                    DispatchQueue.main.async {
                       (success code)
                    }
                }
            })
        }
        operationQueue.addOperation(operation2)
    }
    operationQueue.addOperation(operation1)
}
downloadDetails() "calling function"

Answer 1

This is exactly the use case for DispatchGroup. Enter the group for each call, leave the group when the call finishes, and add a notification handler to fire when they're all done. There's no need for a separate operation queue; these are already asynchronous operations.

func downloadDetails(){
    let dispatchGroup = DispatchGroup()

    dispatchGroup.enter()   // <<---
    WebServiceManager.getAData(format:A, withCompletion: {(data: Any? , error: Error?) -> Void in

        if let success = data {

            DispatchQueue.main.async {
                (success code)
                dispatchGroup.leave()   // <<----
            }
        }
    })

    dispatchGroup.enter()   // <<---
    webServiceManager.getBData(format: B, withCompletion: {(data: Any? , error: Error?) -> Void in

        if let success = data {

            DispatchQueue.main.async {
               (success code)
               dispatchGroup.leave()   // <<----
            }
        }
    })

    dispatchGroup.notify(queue: .main) {
        // whatever you want to do when both are done
    }
}

Answer 2

I would use OperationQueue.

It is preferred for long running task and give you control to cancel request, if needed.

At the end of each operation you can check operation count to know remaining operations.

I have added pseudo code.

let operationQueue: OperationQueue = OperationQueue()

func downloadDetails(){

    let operation1 = BlockOperation() { [weak self] in

        guard let strongSelf = self else {
            return
        }

        sleep(2)

        DispatchQueue.main.async {
            strongSelf.handleResponse()
        }

        let operation2 = BlockOperation() { [weak self] in

            guard let strongSelf = self else {
                return
            }

            sleep(2)

            DispatchQueue.main.async {
                strongSelf.handleResponse()
            }
        }
        strongSelf.operationQueue.addOperation(operation2)
    }

    self.operationQueue.addOperation(operation1)
}

func handleResponse() {
    print("OPERATIONS IN PROGRESS: \(self.operationQueue.operations.count)")
    if self.operationQueue.operations.count == 0 {
        print("ALL OPERATIONS ARE COMPLETE")
    }
}

func cancelOperation() {
    self.operationQueue.cancelAllOperations()
}

This prints

OPERATIONS IN PROGRESS: 1
OPERATIONS IN PROGRESS: 0
ALL OPERATIONS ARE COMPLETE

Answer 3

Dispatch group may fail in one case, suppose the first api response returns before entering into the second group. So in that case your notify block will get called without second api. So for preventing that issue you have to add all the enters statement before any api call. Like -

func download(){
    let dispatchGroup = DispatchGroup()
    /// Enter into the group for all the apis from here only.
    dispatchGroup.enter()  
    dispatchGroup.enter()   
    
    ApiManager.shared.data(request: firstRequest, withCompletion: {(data: Any? , error: Error?) -> Void in
        dispatchGroup.leave()  
    })

    ApiManager.shared.data(request: secondRequest, withCompletion: {(data: Any? , error: Error?) -> Void in
        dispatchGroup.leave() 
    })

    dispatchGroup.notify(queue: .main) {
        /// From here you may notify to hide indicator and reload the table.
    }
}