API calls blocks UI thread Swift

Question

I need to sync web database in my coredata, for which I perform service api calls. I am using Alamofire with Swift 3. There are 23 api calls, giving nearly 24k rows in different coredata entities.

My problem: These api calls blocks UI for a minute, which is a long time for a user to wait.

I tried using DispatchQueue and performing the task in background thread, though nothing worked. This is how I tried :

 let dataQueue = DispatchQueue.init(label: "com.app.dataSyncQueue")
 dataQueue.async {
        DataSyncController().performStateSyncAPICall()
        DataSyncController().performRegionSyncAPICall()
        DataSyncController().performStateRegionSyncAPICall()
        DataSyncController().performBuildingRegionSyncAPICall()

        PriceSyncController().performBasicPriceSyncAPICall()
        PriceSyncController().performHeightCostSyncAPICall()

        // Apis which will be used in later screens are called in background
        self.performSelector(inBackground: #selector(self.performBackgroundTask), with: nil) 

    }

An API call from DataSyncController:

 func performStateSyncAPICall() -> Void {
        DataSyncRequestManager.fetchStatesDataWithCompletionBlock {
            success, response, error in
            self.apiManager.didStatesApiComplete = true 
        }
    }

DataSyncRequestManager Code:

static func fetchStatesDataWithCompletionBlock(block:@escaping requestCompletionBlock) {

    if appDelegate.isNetworkAvailable {

        Util.setAPIStatus(key: kStateApiStatus, with: kInProgress)

        DataSyncingInterface().performStateSyncingWith(request:DataSyncRequest().createStateSyncingRequest() , withCompletionBlock: block)
    } else {
        //TODO: show network failure error
    }
}

DataSyncingInterface Code:

func performStateSyncingWith(request:Request, withCompletionBlock block:@escaping requestCompletionBlock)
 {
    self.interfaceBlock = block
    let apiurl = NetworkHttpClient.getBaseUrl() + request.urlPath!
     Alamofire.request(apiurl, parameters: request.getParams(), encoding: URLEncoding.default).responseJSON { response in

             guard response.result.isSuccess else {
               block(false, "error", nil )
               return
             }

            guard  let responseValue = response.result.value else {
                block (false, "error", nil)
                return
            }
            block(true, responseValue, nil)
        }
}

I know many similar questions have been already posted on Stackoverflow and mostly it is suggested to use GCD or Operation Queue, though trying DispatchQueues didn't work for me.

Am I doing something wrong? How can I not block UI and perform the api calls simultaneously?

Answer 1

DispatchQueue manages the execution of work items. Each work item submitted to a queue is processed on a pool of threads managed by the system.

I usually use NSOperationQueue with Alamofire, but the concepts are similar. When you set up an async queue, you allow work to be performed independently of the main (UI) thread, so that your app doesn't freeze (refuse user input). The work will still take however long it takes, but your program doesn't block while waiting to finish.

You really have only put one item into the queue.

You are adding to the queue only once, so all those "perform" calls wait for the previous one to finish. If it is safe to run them concurrently, you need to add each of them to the queue separately. There's more than one way to do this, but the bottom line is each time you call .async {} you are adding one item to the queue.

dataQueue.async { 
   DataSyncController().performStateSyncAPICall() 
}

dataQueue.async { 
   DataSyncController(). performRegionSyncAPICall l() 
}

Answer 2

You can do this to run on a background thread:

DispatchQueue.global(qos: .background).async {

    // Do any processing you want.

    DispatchQueue.main.async {
        // Go back to the main thread to update the UI.
    }
}