Can not update UI while making GMSMarkers

Question

Function which initialize GMSMarkers:

func makeMarkers (progressHandler: ((String) -> Void)? = nil) {
    progressHandler?("Инициализируем маркеры...")
    for device in self.devices {
        progressHandler?("Инициализируем маркеры: для устройства \(device.name!)...")
        if let position = self.positions.findById(device.positionId) {
            if let marker = DeviceMarker(device, at: position) {
                self.deviceMarkers.append(marker)
            }
        }
    }
    for geofence in self.geofences {
        progressHandler?("Инициализируем маркеры: для геометки \(geofence.name!)...")
        if let marker = GeofenceMarker(geofence) {
            self.geofenceMarkers.append(marker)
        }
    }

    self.locationManager.requestLocation()

    progressHandler?("Инициализируем маркеры: для пользователя \(self.loggedUser.name!)...")
    if let userLocation = self.locationManager.location {
        self.userMarker = UserMarker(self.loggedUser, at: userLocation)
    } else {
        self.userMarker = UserMarker(self.loggedUser, at: CLLocation(latitude: 48, longitude: 44))
    }
    progressHandler?("Маркеры инициализированы...")
}

As you can see I want inform user about progress of this operation using progressHandler.

But during execution of that function I can not see any of that messages. Snipped where I call function makeMarkers:

                        ...
            DB.geofences.server.getAll() { geofences in
                self.statusLabel.text = "Получили геометки \(geofences.count)..."
                DB.devices.server.getAll() { devices in
                    self.statusLabel.text = "Получили устройства \(devices.count)..."
                    DB.positions.server.getAll() { positions in
                        self.statusLabel.text = "Получили позиции \(positions.count)..."

                        DB.devices.client.insert(devices)
                        self.statusLabel.text = "Записали устройства..."
                        DB.positions.client.insert(positions)
                        self.statusLabel.text = "Записали позиции..."
                        DB.geofences.client.insert(geofences)
                        self.statusLabel.text = "Записали геометки..."

                        MapManager.shared.makeMarkers() { status in
                            self.statusLabel.text = status
                        }
                        ...

And if I change view messages using print() all messages will be shown. So, question is: why UI changes are blocked during execution of my function and how get rid of that?

UPDATE: Sure, I do everything in main thread.

UPDATE 2: Code snippet where I'm calling my function updated. Last message which I can see before map controller shows is "Получили устройства...", it is strange, because I'm sure I get positions as well

Maybe I misunderstand the syntax, but aren't you setting your progressHandler to nil? — matiastofteby, Oct 02 '17 at 10:47
Did you tried to do force it works on main thread, like `DispatchQueue.main.async { self.statusLabel.text = status }` `statusLabel` shows "Записали геометки..." or not? — Taras Chernyshenko, Oct 02 '17 at 11:57
@mathiastofteby: progressHandler set to nil in declaring function, which means default value, when I call my function progressHandler is not nil. — zzheads, Oct 02 '17 at 12:03
@zzheads but label shows "Записали геометки..." or not? — Taras Chernyshenko, Oct 02 '17 at 12:10
Nope, strange, but last message is "Получили устройства ...".. — zzheads, Oct 02 '17 at 13:04
Think I have to post here snippet of code where I call that func — zzheads, Oct 02 '17 at 13:04

score 2 · Answer 1 · answered Oct 02 '17 at 20:37

Well, that occurs because UI updates happen in the main thread. But when you change your label's text this change does not happen immediately. Instead, all UI changes get scheduled and only get rendered when the app finishes the current pass through the event loop.

Since you are performing time-consuming tasks in the same main loop, it becomes blocked and changes won't happen until those tasks are finished and your function returns.

You may check these articles to get more detailed explanation on how event loops work: Run Loops, Main event loop

I suggest you move all time-consuming tasks (such as database queries) to a background thread and call the main thread from there when you actually want to update your UILabel.

Can not update UI while making GMSMarkers

1 Answers1