IOS 7 - Update an UILabel with value computed in background using delegation - xcode

Question

I am having a problem trying to update UILabel.text using a value calculate in background using delegation (which seems to be slow).
#interface BTAViewController : UIViewController <ExchangeFetcherManagerDelegate>
#property (weak, nonatomic) IBOutlet UILabel *bidPriceLabel;
#property (strong, nonatomic) FetcherManager *fetcherManager;
#end
my BTAViewController.m
- (void)viewDidLoad
{
[super viewDidLoad];
self.fetcherManager = [[FetcherManager alloc]init];
double calculateValue = [self.fetcherManager fetchPricesFor];
self.bidPriceLabel.contentMode = UIViewContentModeRedraw;
[self.bidPriceLabel setNeedsDisplay];
self.bidPriceLabel.text = [[NSString alloc] initWithFormat:#"%f", calculateValue];
}];
The label has been correctly initialised in a method not shown here and it shows the correct default value at startup. Once I execute this code, the label will get the value 0.00000000, because my calculateValue has not been calculate yet.
How can I wait for this value to be calculated and then display it has text of the label ?
I can't post images because I don't have enough reputation...
Thanks

Answer 1

Are you sure the value calculated is not purely 0 or null?
What I would do is to add a delegate callback function in BTAViewController.m from FetchManager
Something like this:
BTAViewController.h
//add delegate callback function under #interface
-(void)valueReturned:(float)someValue;
BTAViewController.m
-(void)valueReturned:(float)someValue{
//updates value when the delegate returns the value
self.bidPriceLabel.text = [[NSString alloc] initWithFormat:#"%f", calculateValue];
}
- (void)viewDidLoad
{
[super viewDidLoad];
self.fetcherManager = [[FetcherManager alloc]init];
self.fetchManager.delegate = (id)self;
[self.fetchManager getPricesFor];
// double calculateValue = [self.fetcherManager fetchPricesFor];
self.bidPriceLabel.contentMode = UIViewContentModeRedraw;
[self.bidPriceLabel setNeedsDisplay];
[self.view addSubview:self.bidPriceLabel];
}];
In FetchManager.h and .m
FetcherManager.h
#property (retain,nonatomic)id<ExchangeFetcherManagerDeelgate>delegate;
-(void)getPricesFor;
FetcherManager.m
#synthesize delegate;
-(void)getPricesFor{
float price;
//calculate price
[self.delegate valueReturned:price];
}

Answer 2

Add the label to the view, set it to be hidden (bidPriceLabel.hidden = YES). Then, create and add a method for the delegate (BTAViewController) which when fired, checks the hidden property of the UILabel, and if it is TRUE (is hidden), set bidPriceLabel.hidden = NO which will then display the UILabel as you intended to.
Call this method in FetchManager once you are done retrieving the information.