Main Thread Checker: UI API called on a background thread: -[UIApplication applicationState]

Question

I am using google maps in Xcode 9 beta, iOS 11.

I am getting an error outputted to the log as follows:

Main Thread Checker: UI API called on a background thread: -[UIApplication applicationState] PID: 4442, TID: 837820, Thread name: com.google.Maps.LabelingBehavior, Queue name: com.apple.root.default-qos.overcommit, QoS: 21

Why would this be occurring as I am almost certain I'm not altering any interface elements from the main thread in my code.

 override func viewDidLoad() {

    let locationManager = CLLocationManager()


    locationManager.requestAlwaysAuthorization()


    locationManager.requestWhenInUseAuthorization()

        if CLLocationManager.locationServicesEnabled() {

            locationManager.desiredAccuracy = kCLLocationAccuracyNearestTenMeters
            locationManager.startUpdatingLocation()
        }

      viewMap.delegate = self

     let camera = GMSCameraPosition.camera(withLatitude: 53.7931183329367, longitude: -1.53649874031544, zoom: 17.0)


        viewMap.animate(to: camera)


    }

    func locationManager(manager: CLLocationManager, didUpdateLocations locations: [CLLocation]) {
        let locValue:CLLocationCoordinate2D = manager.location!.coordinate
        print("locations = \(locValue.latitude) \(locValue.longitude)")
    }

    func mapView(_ mapView: GMSMapView, willMove gesture: Bool) {


    }

    func mapView(_ mapView: GMSMapView, idleAt position: GMSCameraPosition) {

        if(moving > 1){
            moving = 1
        UIView.animate(withDuration: 0.5, delay: 0, animations: {

            self.topBarConstraint.constant = self.topBarConstraint.constant + (self.topBar.bounds.height / 2)

            self.bottomHalfConstraint.constant = self.bottomHalfConstraint.constant + (self.topBar.bounds.height / 2)

            self.view.layoutIfNeeded()
        }, completion: nil)
    }
         moving = 1
    }


    // Camera change Position this methods will call every time
    func mapView(_ mapView: GMSMapView, didChange position: GMSCameraPosition) {
        moving = moving + 1
        if(moving == 2){


            UIView.animate(withDuration: 0.5, delay: 0, animations: {


                self.topBarConstraint.constant = self.topBarConstraint.constant - (self.topBar.bounds.height / 2)

                self.bottomHalfConstraint.constant = self.bottomHalfConstraint.constant - (self.topBar.bounds.height / 2)


                self.view.layoutIfNeeded()
            }, completion: nil)
        }
        DispatchQueue.main.async {

            print("Moving: \(moving) Latitude: \(self.viewMap.camera.target.latitude)")
            print("Moving: \(moving)  Longitude: \(self.viewMap.camera.target.longitude)")
        }
    }

Answer 1

It's hard to find the UI code which is not executed in main thread sometimes. You can use the trick below to locate it and fix it.

1. Choose Edit Scheme -> Diagnostics, tick Main Thread Checker.

   Xcode 11.4.1

   Click the small arrow next to the Main Thread Checker to create a Main Thread Checker breakpoint.

   Previous Xcode

   Tick on Pause on issues.

2. Run your iOS application to reproduce this issue. (Xcode should pause on the first issue.)

3. Wrap the code that modify the UI in DispatchQueue.main.async {}

Answer 2

First, make sure your invocations of google maps and ui changes are called from the main thread.

You can enable Thread Sanitizer option in Xcode using below steps:

You can put offending lines on the main thread with the following:

DispatchQueue.main.async {
    //Do UI Code here. 
    //Call Google maps methods.
}

Also, update your current version of google maps. Google maps had to make a couple of updates for the thread checker.

For the question: "Why would this be occurring?" I think Apple added an assertion for an edge case which Google then had to update their pod for.

Answer 3

Wrap the lines of code that modify the UI in DispatchQueue.main.async {} in order to make sure they execute on the main thread. Otherwise, you may be calling them from a background thread, where UI modifications are not allowed. All such lines of code must be executed from the main thread.

Answer 4

Refer this link https://developer.apple.com/documentation/code_diagnostics/main_thread_checker

For me this worked when I called from block.

Answer 5

I think the solution is already given, For my issue is Keyboard on the way.

UIKeyboardTaskQueue may only be called from the main thread

Answer 6

Choose scheme -> Diagnotics, remove main thread checker, then the warning will disappear. scheme editor