iPhone performSelectorInBackground - iphone

Question

I have this problem:
Here is a part of my appDelegate file where I create a "performSelectorInBackground" method.
- (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions {
[self addSplash];
[self getLocation];
[self performSelectorInBackground:#selector(backgroundOp) withObject:nil];
return YES;
}
First I add some splash screen, the I get a location and the call background method.
This is content of background method:
- (void) backgroundOp
{
NSAutoreleasePool *pool = [[NSAutoreleasePool alloc] init];
[self createEditableCopyOfDatabaseIfNeeded];
[self initTempData];
[self initApp];
[self checkDataVersion];
}
[self setAppStrings];
[self performSelectorOnMainThread:#selector(resultOp) withObject:nil waitUntilDone:YES];
[pool release];
}
I download some data, check version of data, setup strings for application and the call on main thread method to create a tab bar controller code here:
- (void) resultOp
{
tabBarController.delegate = self;
[self.window addSubview:tabBarController.view];
[self addTabBarArrow];
[self.window makeKeyAndVisible];
[self removeSplash];
}
Here I create a tab bar controller and remove splash screen. Then start my firstViewController.
Problem is that in my firstViewController I show a current location, but it is wrong. Sometimes is correct but very often is wrong.
Where is a problem ? Is there any option how to check if background thread end ? Or something other solution for my problem (I need only: show splash with activity indicator and some messages (this messages are changed in method e.g. init, get location etc.), then I need get location, the remove splash and show firstViewController) ... thanks a lot
Edit: Here is code for location:
- (void) getLocation
{
splashScreenController.splashLabel.text = #"Localization ...";
locationManager = [[CLLocationManager alloc] init];
locationManager.delegate = self;
locationManager.distanceFilter = kDistanceFilter;
locationManager.desiredAccuracy = kCLLocationAccuracyHundredMeters;
[locationManager startUpdatingLocation];
}

Answer 1

Keep in mind, that the longer the location update runs, the more accurate it gets. The first 'hit' for the location isn't always the best one (most of the time it is the wrong one).
Maybe you could show your code where you have the events of the CLLocationManager.
Also, how much is the wrong position off of the right position? I think the AGPS thing first quickly checks its location by using WiFi hotspots nearby and after that gets more accurate by using the GPS chip.

Answer 2

Wherever you are using the location you acquire (which I can't really see from the code you posted), you should have checks on the horizontalAccuracy property of the newLocation you are receiving (and verticalAccuracy as well if altitude matters). You can say something like
if(newLocation.horizontalAccuracy < 100) {
//do something with newLocation
//because it is accurate to 100 meters
}
If you do not do these types of checks, you can get some really inaccurate locations, up to three or four kilometers from your real location at first.
Also, when using multiple threads, data integrity can sometimes become a problem. You need to make sure that variables are not being accessed and changed at the same time in multiple methods, or who knows if you will get the correct output.
Also, it is important to note that all of the methods called within backgroundOp will also be preformed in the background, even without explicitly calling them that way. Use
[self performSelectorOnMainThread:foo withObject:foobar waitUntilDone:NO];
to return to the main thread.
Edit:
viewDidLoad {
[super viewDidLoad];
iterations = -5;
}
- (void)locationManager:(CLLocationManager *)manager didUpdateToLocation:(CLLocation
*)newLocation fromLocation:(CLLocation *)oldLocation {
iterations++;
if(iterations > 0) {
if(newLocation.horizontalAccuracy < 50) {
//do something with location with radius of uncertainty
//of less than 50
}
}