在后台跟踪时间以处理位置更新

Question

I have two constraints in my app, one is user location and the other one is time. Following is the simple location implementation.

func determineMyCurrentLocation() {
    locationManager = CLLocationManager()
    locationManager.delegate = self
    locationManager.allowsBackgroundLocationUpdates = true
    locationManager.desiredAccuracy = kCLLocationAccuracyHundredMeters
    locationManager.requestAlwaysAuthorization()
    locationManager.distanceFilter = 20

    if CLLocationManager.locationServicesEnabled() {
        locationManager.startUpdatingLocation()
    }
}

func locationManager(_ manager: CLLocationManager, didUpdateLocations locations: [CLLocation]) {
    print("*******Location updated*******")
}

With this code, I expect to get triggered every time the user location changes by 20m (in background as well). However, I also need to track the time the user stays in a particular location. Since i need to track the background case, I cannot use timer.

I followed https://www.raywenderlich.com/143128/background-modes-tutorial-getting-started to try background task but as stated in the article, the background time allowed that I got was around 3 min (which is variable). So i believe I cannot go with this.

How can I solve this?

EDIT: I also need to make an api call if the user stays in a location for X minutes. So for that case, it would not be feasible to wait for location updates and calculate the time differences. I could previously solve this by removing the distance filter and continuously checking the location and comparing the time and location. I guess continuous location tracking will get the app rejected, that's why I went for filter. But i am not sure if it will still get rejected or not since iOS will need to track the location for filter as well.

Answer 1

You can use simple Date objects to track the time spent between location updates even if your app is in the background. Simply declare a Date property for your class, update its value from func locationManager(_:, didUpdateLocations:) and compare that to the current time.

// Declare this in a scope that can be accessed from `didUpdateLocations` and where its value won't be released from memory
var lastUpdateTimestamp = Date()

func locationManager(_ manager: CLLocationManager, didUpdateLocations locations: [CLLocation]) {
    let now = Date()
    let timeSinceLastUpdate = now.timeIntervalSince(lastUpdateTimestamp)
    print("\(timeSinceLastUpdate) seconds passed since last location update")
    lastUpdateTimestamp = now
}

Response to the question edit: making an API call after X minutes have passed in the background without a location update is not possible, since there's no supported background mode for executing arbitrary functions at specific points in time. Getting rid of the distance filter as you explained in your edit could be a valid alternative, however if your app is only using location updates to be able to make the API calls and isn't actually doing anything with those locations, your app might indeed get rejected.

Answer 2

When you receive a new location update, check the time difference from the previous update.. giving you the time that they were at the last location.

Depending on what you are doing with these location updates, this might just be pulling the latest update from some DB, or posting to an API.

An example..

func locationManager(_ manager: CLLocationManager, didUpdateLocations locations: [CLLocation]) {
    print("*******Location updated*******")
    let lastUpdateDate = // get the last datetime you received information

    let currentDate = Date()
    let minutesDifference = let componentsLeftTime = Calendar.current.dateComponents([.minute], from: lastUpdateDate, to: currentDate)
}