loop array of locations in separate thread - ios

Question

I'm working on an application for the iPhone and I'm keeping track of the users current location. When the didupdateLocations delegate method actually executes i would like to test if the location in the NSArray is in a predefined array the contains other locations, this array mind you could grow over time.
I'm running a for loop within this method to test against my own array of locations but i would like to move that to a separate thread. So in case my own array with multiple locations grows to a large number the for loop does not freeze my UI.
I have tried it like this but I'm getting undesirable results. I understand that the location tracking definitely happens in a separate thread. However those didupdateLocations execute on a separate thread. The Apple doc's are not very clear on the matter. My end goal again is to compare against my array and not lock the UI.
- (void)locationManager:(CLLocationManager *)manager
didUpdateLocations:(NSArray *)thisLocation {
dispatch_queue_t queue= dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);
// get the last object in the array of locations
CLLocation* location = [thisLocation lastObject];
dispatch_async(queue, ^{
[self checkmyArray:location];
});
}
-(void)checkmyArray:(CLLocation *)workingLocation{
NSLog(#"SoundTheAlarm");
int alarm_on_c = 0;
NSUInteger tmp_count = [theData.LocationsObjectArray count];
BOOL alarm;
NSMutableDictionary * tempObject;
CLLocationDistance distance = 0.0;
for (int i = 0; i < tmp_count; i++) {
tempObject= [theData.LocationsObjectArray objectAtIndex:i];
thisLoc = [[tempObject objectForKey:#"onoff"] isEqual:#YES];
if (thisLoc) {
//check if we are near that location
double lat = [[tempObject objectForKey:#"latitude"] doubleValue];
double lon = [[tempObject objectForKey:#"longitude"] doubleValue];
// goal = [[CLLocation alloc] initWithLatitude:40.097771 longitude:-74.941399];
goal = [[CLLocation alloc] initWithLatitude:lat longitude:lon];
// check the destination between current location and goal location - in meters
distance = [goal distanceFromLocation:workingLocation];
NSLog(#"distance %f\n", distance);
}
// if distance from goal is less than 350 meters
if (distance <= 350){
[self scheduleNotification:[tempObject objectForKey:#"name"]];
// turn off tracking for this location
[tempObject setObject:#NO forKey:#"onoff"];
[theData.LocationsObjectArray replaceObjectAtIndex:i withObject:tempObject];
NSIndexPath *path = [NSIndexPath indexPathForRow:i inSection:0];
ExtendedSavedCellTableViewCell *cell = (ExtendedSavedCellTableViewCell *)[self.tableView cellForRowAtIndexPath:path];
cell.switchView.on = NO;
// save the update to the switch to the database as well
NSString *lat = [tempObject objectForKey:#"latitude"];
/*check to determine if the uiswitch is turned off or on.*/
[self fetchedResultsController:#NO lat:lat index:path];
[self displayAlertViewForAlarm:[tempObject objectForKey:#"name"]];
}
-(void)displayAlertViewForAlarm:(NSString *)nameOfLocation{
dispatch_async(dispatch_get_main_queue(), ^(void) {
UIAlertView *myAlert = [[UIAlertView alloc] initWithTitle:#"Destination reached"
message:nameOfLocation
delegate:self
cancelButtonTitle:#"Go Away"
otherButtonTitles:#"Notify", nil];
[myAlert show];
});
}

Answer 1

It's generally a bad idea to use threads in iOS if you can at all avoid it. In your case, I'd implement the function which does the looping to automatically pop out of the loop after too many iterations and then schedule the next chunk of iterating to happen in another pass through the event loop. In other words, something like this:
- (void) checkLocationsStartingAt:(NSNumber)start
{
NSInteger s = (start) ? [start intValue] : 0;
for (i = s; i < list.length; i++) {
if (i > TOO_MANY) {
[self performSelector:#selector(checkLocationsStartingAt:)
withObject:#(i)
afterDelay:0.001];
return;
} else {
// check element at i
}
}
}
see: NSObject Reference

Answer 2

You are checking one location against an array of other locations.
You display an alert for time you are closer than 350m from a location.
You could be within 350m of many items in the list.
You have no code prevent multiple alerts.
So you will sometimes have many alerts.
You have a couple options. One will probably suit your needs better than the others. Possibly one I didn't list.
You could re-use a single UIAlertView instance and check the
visible property on it. If it's already visible, do nothing.
You could break out of the loop once you get a single "hit" on being
<350m.
You could stick in a BOOL which records if you've shown the alert and
check the BOOL every time before you show the alert again.