Convert asynchronously large no of GPS data to city/country - ios

Question

Background: I have a list of contacts, which are retrieved on an asynchronous queue from a cloud based database. Once done, I dispatch back to the main queue and show these contacts in a TableView.
Besides names and other details, each contact object has GPS coordinate properties (latitude and longitude). I want to use these GPS coordinates to retrieve the name of the city and country of each contact and update the TableView showing that information in the local language of the device the user has.
Problem: The problem I am trying to overcome is that I have a few hundred contacts. Initially, I used concurrent queues to get the city/country strings. But I paused the app in XCode and realised the disaster of 300+ threads created. So I changed the code running each lookup of the city/country on the same serial queue.
The issue now is that only for about 50 of the contacts the data is updated. I do not know why and not sure even if the queue is serial. Debugging shows there are still 100+ “serial queues” created. I expected one at the time. What am I doing wrong? Thanks
The code for the class, in which I have my TableView is as follows:
dispatch_async(dispatch_get_global_queue(Int(QOS_CLASS_USER_INITIATED.rawValue), 0)) {
// get or update the contactsList
activeUser.contactsList.getAllContacts()
// once we have the contacts we go back to the main queue
dispatch_async(dispatch_get_main_queue()) {
// and refresh tableview to show the contacts
self.tableView.reloadData()
// now we refresh the locations of the contacts
for index in 0...activeUser.contactsList.listOfContacts.count
{
// set local variables for lat and long
let lat = activeUser.contactsList.listOfContacts[index].latitude
let long = activeUser.contactsList.listOfContacts[index].longitude
// call location service method with completion handler
MyLocationServices().updateLocationToLocalLanguage(lat, longitude: long, completionHandler:
{ (city, country) -> () in
// update contact's details
activeUser.contactsList.listOfContacts[index].city = city
activeUser.contactsList.listOfContacts[index].country = country
// refresh each time the table to show updated contact data
self.tableView.reloadData()
})
}
}
The method within the MyLocationServices class looks as follows:
func updateLocationToLocalLanguageDispatched(latitude: String, longitude: String, completionHandler: (city: String, country: String) -> ())
{
let serialQ = dispatch_queue_create("AddressUpdateQ", DISPATCH_QUEUE_SERIAL)
dispatch_async(serialQ)
{
var cityString = "NA"
var countryString = "NA"
let group = dispatch_group_create()
dispatch_group_enter(group)
let location = CLLocation(latitude: Double(latitude)!, longitude: Double(longitude)!)
self.geocoder.reverseGeocodeLocation(location)
{
(placemarks, error) -> Void in
if let placemarks = (placemarks as [CLPlacemark]!) where placemarks.count > 0
{
let placemark = placemarks[0]
if ((placemark.addressDictionary!["City"]) as? String != nil) { cityString = ((placemark.addressDictionary!["City"]) as? String)! }
if ((placemark.addressDictionary!["Country"]) as? String != nil) { countryString = ((placemark.addressDictionary!["Country"]) as? String)! }
}
dispatch_group_leave(group)
}
dispatch_group_wait(group, DISPATCH_TIME_FOREVER)
dispatch_async(dispatch_get_main_queue())
{
completionHandler(city: cityString, country: countryString)
}
}
}

Answer 1

I have solved the issue and documenting hoping it is helpful in case others face similar problems.
(1) Serial Queue: I have realised serial queue initiation should not happen within the method, but enclose the for-loop. Thus the following code should be removed from the method and put around the for-loop. But this does not solve the fact that I only get about 50 results and nothing for the remaining GPS data of the contacts.
let serialQ = dispatch_queue_create("AddressUpdateQ", DISPATCH_QUEUE_SERIAL);
dispatch_async(serialQ)
{ [for index....] }
(2) Location: The second issue is related to Apple's Geocoder, which happens online. Officially, the number of requests are limited. The documentation states that
one "should not send more than one geocoding request per minute".
Applications should be conscious of how they use geocoding. Geocoding
requests are rate-limited for each app, so making too many requests in
a short period of time may cause some of the requests to fail.
Checking the error code I get, it is Error Domain=kCLErrorDomain Code=2. It seems the service is denied and I cannot make further requests.
(3) Solutions: There are two solutions in my view. One is to update 30 or so users, wait some time, and then dispatch after that another queue to get again some data. The second solution is to live with having all data in English, make a string that each user saves in the cloud. So for other users retrieving contacts, it's just a matter of fetching a string from the database rather than checking GPS data "on the fly".