无法使DispatchGroup在Swift中正常工作

Question

Need to wait for some loop of api calls to be finished before updating the UI, but couldn't yet find out why my DispatchGroup.notify() is getting executed before the http calls:

override func viewDidAppear(_ animated: Bool) {

    if isFirstLoad {

        ProgressIndicator.shared.showProgressView(self.view)

        let serviceUrl = MobnerServices.service_base+"GetTrainersAround?lat=\(self.currLatitude!)&lon=\(self.currLongitude!)&radius=50"

        Alamofire.request(serviceUrl).responseJSON{ response in
            do{
                guard let responseData = response.data else{
                    print("No data received.")
                    ProgressIndicator.shared.hideProgressView()
                    return
                }

                let dispatchGroup = DispatchGroup()
                let decoder = JSONDecoder()

                let retrievedTrainers = try decoder.decode([STTrainer].self, from: responseData)

                self.trainers = retrievedTrainers
                self.isFirstLoad = false

                for trainer in self.trainers{
                    dispatchGroup.enter()

                    let getUserPictureUrl = MobnerServices.service_base+"GetUserPicture?filename=\(trainer.profilePicturePath)"

                    Alamofire.request(getUserPictureUrl).responseImage { response in
                        guard let image = response.result.value else {
                            print(response.error!.localizedDescription)
                            return
                        }

                        self.trainersPictures.append(STTrainerPicture(userId: trainer.userId, profilePicture: image))
                    }

                    dispatchGroup.leave()
                }

                dispatchGroup.wait()

                ProgressIndicator.shared.hideProgressView()
                self.tableView.reloadData()

            }
            catch{
                print(error.localizedDescription)
            }
        }

    }
}

Does anyone have some tip here? Thanks in advance!

Answer 1

Your issue is that you are calling dispatchGroup.leave() in the wrong place. It needs to be inside the completion handler of the async call.

let dispatchGroup = DispatchGroup()
let decoder = JSONDecoder()

let retrievedTrainers = try decoder.decode([STTrainer].self, from: responseData)

self.trainers = retrievedTrainers
self.isFirstLoad = false

for trainer in self.trainers{
    dispatchGroup.enter()

    let getUserPictureUrl = MobnerServices.service_base+"GetUserPicture?filename=\(trainer.profilePicturePath)"

    Alamofire.request(getUserPictureUrl).responseImage { response in
        guard let image = response.result.value else {
            print(response.error!.localizedDescription)
            dispatchGroup.leave()
            return
        }

        self.trainersPictures.append(STTrainerPicture(userId: trainer.userId, profilePicture: image))
        dispatchGroup.leave()
    }
}

dispatchGroup.notify(queue: .main) {
    ProgressIndicator.shared.hideProgressView()
    self.tableView.reloadData()
}

You also need to perform the UI updates on the main queue.