Update the User Interface with dispatch_get_global_queue and Web Call - iphone

Question

I am trying to learn to use dispatch_get_global_queue to perform a web call asynchronously. Basically, I need to update the User Interface depending on the user's location. I started using dispatch_get_global_queue because the user interface was unresponsive while the web call was performed.
Now, my problem is that the buttons do not appear correctly when buttonUpdate is called. One button is supposed to be black, and the other is green. However, now they both appear the default blue. Also, the words/title of the buttons do not appear until I click on one of them. Anyone know why? Thank you!
I have this method called preButtonUpdate that calls the buttonUpdate method asynchronously:
-(void)preButtonUpdate {
dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT,0), ^{
[self buttonUpdate];
});
}
The preButtonUpdate method is called in viewDidAppear and appReturnsActive:
- (void)viewDidAppear:(BOOL)animated {
[super viewDidAppear:animated];
[self preButtonUpdate];
//other code
}
- (void)appReturnsActive {
[myTimer invalidate];
myTimer = nil;
//ONLY WANT TO PERFORM THE UPDATE IF VIEWING THE PROFILE VIEW CONTROLLER
UIViewController *currentVC = self.navigationController.visibleViewController;
NSString * name = NSStringFromClass([currentVC class]);
if ([name isEqualToString:#"ProfileViewController"]) {
[self preButtonUpdate];
}
}
Then, the preButtonUpdate calls the buttonUpdate method, which performs the web query and updates the user interface:
-(void) buttonUpdate {
[locationManagerProfile stopUpdatingLocation];
NSString *userLatitude =[(PDCAppDelegate *)[UIApplication sharedApplication].delegate
getUserLatitude];
NSString *userLongitude =[(PDCAppDelegate *)[UIApplication sharedApplication].delegate
getUserLongitude];
NSString *placeLatitude = [[NSUserDefaults standardUserDefaults]
stringForKey:#"savedLatitude"];
NSString *placeLongitude = [[NSUserDefaults standardUserDefaults]
stringForKey:#"savedLongitude"];
NSString *distanceURL = [NSString stringWithFormat:#"http://www.website.com/page.php?
lat1=%#&lon1=%#&lat2=%#&lon2=%#",userLatitude, userLongitude, placeLatitude,
placeLongitude];
NSData *distanceURLResult = [NSData dataWithContentsOfURL:[NSURL
URLWithString:distanceURL]];
NSString *distanceInFeet = [[NSString alloc] initWithData:distanceURLResult
encoding:NSUTF8StringEncoding];
if ([distanceInFeet isEqualToString:#"1"]) {
UIBarButtonItem *btnGo = [[UIBarButtonItem alloc] initWithTitle:#"Button A"
style:UIBarButtonItemStyleBordered target:self action:#selector(actionA)];
self.navigationItem.rightBarButtonItem = btnGo;
[self.navigationItem.rightBarButtonItem setTintColor:[UIColor
colorWithRed:44.0/255.0 green:160.0/255.0 blue:65.0/255.0 alpha:1.0]];
UIBarButtonItem *btnGoTwo = [[UIBarButtonItem alloc] initWithTitle:#"Button B"
style:UIBarButtonItemStyleBordered target:self action:#selector(actionB)];
self.navigationItem.rightBarButtonItem = btnGoTwo;
self.navigationItem.rightBarButtonItems = [NSArray arrayWithObjects:btnGo,
btnGoTwo, nil];
}
if ([distanceInFeet isEqualToString:#"0"]) {
UIBarButtonItem *btnGo = [[UIBarButtonItem alloc] initWithTitle:#"Button C"
style:UIBarButtonItemStyleBordered target:self action:#selector(actionC)];
self.navigationItem.rightBarButtonItem = btnGo;
[self.navigationItem.rightBarButtonItem setTintColor:[UIColor
colorWithRed:44.0/255.0 green:160.0/255.0 blue:65.0/255.0 alpha:1.0]];
UIBarButtonItem *btnGoTwo = [[UIBarButtonItem alloc] initWithTitle:#"Button B"
style:UIBarButtonItemStyleBordered target:self action:#selector(actionB)];
self.navigationItem.rightBarButtonItem = btnGoTwo;
self.navigationItem.rightBarButtonItems = [NSArray arrayWithObjects:btnGo,
btnGoTwo, nil];
}
}

Answer 1

UI Update is always done on the main thread. So keep your UI update code out of the despatch queue
Keep [self buttonUpdate]; out of the dispatch_async block
-(void)preButtonUpdate {
dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT,0), ^{
[self buttonUpdate];
});
}
Unblocking Your User Interface - Apple Developer
EDIT:
Put the if condtion code just outside the dispatch_async block