带有URLSession的Swift iOS -DispatchGroup锁定了未位于其中的应用程序的其他部分

Question

I have an array of up to 6 images. I use a loop to loop through all of the images, turn them into metadata, send the metadata to Storage and then when done I send the url strings to Firebase Database.

I'm using DispatchGroup to control the loop as the Url is changed to Data so I can send the data to Firebase Storage.

If this loop is happening in tabOne, if i go back and forth to tabTwo or tabThree, when the loop finishes and the alert appears, tabTwo is temporarily locked or tabThree gets temporarily locked for around 2-3 seconds. I cannot figure out where I'm going wrong?

I'm not sure if it makes a difference but I'm using a custom alert instead of the UIAlertController. It's just some UIViews and a button, it's nothing special so I didn't include the code.

var urls = [URL]()
picUUID = UUID().uuidString
dict = [String:Any]()

let myGroup = DispatchGroup()
var count = 0

for url in urls{

    myGroup.enter() // enter group here
    URLSession.shared.dataTask(with: url!, completionHandler: {
            (data, response, error) in
            guard let data = data, let _ = error else { return }

            DispatchQueue.main.async{
                self.sendDataToStorage("\(self.picUUID)_\(self.count).jpg", picData: data)
                self.count += 1
            }
     }).resume()

    // send dictionary data to firebase when loop is done
    myGroup.notify(queue: .main) {
        self.sendDataToFirebaseDatabase()
        self.count = 0
    }
}

func sendDataToStorage(_ picId: String, picData: Data?){

    dict.updateValue(picId, forKey:"picId_\(count)")

    let picRef = storageRoot.child("pics")
    picRef.putData(picData!, metadata: nil, completion: { (metadata, error) in

        if let picUrl = metadata?.downloadURL()?.absoluteString{

             self.dict.updateValue(picUrl, forKey:"picUrl_\(count)")
             self.myGroup.leave() // leave group here
        }else{
             self.myGroup.leave() // leave group if picUrl is nil
        }
    }
}

func sendDataToFirebaseDatabase(){

    let ref = dbRoot.child("myRef")
    ref.updateChildValues(dict, withCompletionBlock: { (error, ref) in
         displaySuccessAlert()
    }
}

Answer 1

I don't know much about Firebase, but you are dispatching your sendDataToFirebaseDatabase method to main queue which probably explains why your UI becomes unresponsive.

Dispatch sendDataToFirebaseDatabase to a background queue and only dispatch your displaySuccessAlert back to main queue.