iPhone/Objective-C: How to modify UILabel from timer - iphone

Question

I have the following timer:
uploadGPS_timer=[NSTimer scheduledTimerWithTimeInterval:5.0 target:self selector:#selector(uploadGPS_tick:) userInfo:nil repeats:YES];
[self uploadGPS_tick:nil];
And here is the callback: uploadGPS_tick():
-(void)uploadGPS_tick:(NSTimer*)timer{
if(!lat || !lng){
//do nothing
}else{
NSString *urlStr=[[NSString alloc] initWithFormat:#"http://www.example.com/ajax/updateCoords.php"];
NSURL *url=[NSURL URLWithString:urlStr];
__block ASIFormDataRequest *request=[[ASIFormDataRequest alloc ]initWithURL:url];
[request setPostValue:lat forKey:#"lat"];
[request setPostValue:lng forKey:#"lng"];
NSLog(#"EOH: %#",lat);
[request setDelegate:self];
[request setCompletionBlock:^{
NSString *response=[request responseString];
NSLog(#"%#",response);
if([response isEqualToString:#"LO"]){
[self.navigationItem setBackBarButtonItem:nil];
DriverLogin *x= [[DriverLogin alloc] initWithNibName:nil bundle:nil];
[[self navigationController]pushViewController:x animated:NO];
}
SBJsonParser *parser=[[SBJsonParser alloc]init];
NSMutableDictionary *obj=[[NSMutableDictionary alloc]init ];
obj=[[parser objectWithString:[request responseString] error:nil]retain];
credits.text=[[obj objectForKey:#"credits"] stringValue]; //this won't show...
creditsUsed.text=[[obj objectForKey:#"creditsUsed"] stringValue]; //this won't show...
NSInteger timeLeftSecs=[[obj objectForKey:#"creditTimeLeft"] intValue];
NSInteger timeLeftMins=(int)(timeLeftSecs/60);
creditTimeLeft.text=[[NSString alloc]initWithFormat:#"%d",timeLeftMins]; //this won't show...
NSLog(#"xxx:%#",obj);
}];
[request setFailedBlock:^{
NSError *error =[request error];
NSLog(#"%#",error);
//do nothing
}];
[request startAsynchronous];
}
}
As you can see, every five seconds, a JSON object is sent from the server. This JSON object is then parsed and three UILabels are set based on this JSON data.
The trouble I'm having is that the UILabels aren't getting their text set! Even though I can clearly see NSLog(#"xxx:%#",obj); in the debugger. The UILabels are connected properly in the .xib.
Any help greatly appreciated.

Answer 1

You should do it from main thread. Replace the label text assignment with following code:
dispatch_async(dispatch_get_main_queue(), ^{
creditTimeLeft.text=[NSString stringWithFormat:#"%d",timeLeftMins];
});