button.enable not refreshing IOS - ios

Question

I have a UIViewController loading data from web, the process takes about 15 secs, So I put my long running process on 2nd thread, when the 2nd thread is completed, i will set the button enable or disable. but the button is not refreshing when the process is done.
#import "ViewController.h"
#import "FunctionNSObject.h"
#interface ViewController ()
#end
#implementation ViewController
NSMutableArray *schoolsAvaliable;
NSDictionary *dict;
NSString *schoolNameCh, *schoolNameEn;
int schoolYear, schoolID;
- (void)viewDidLoad
{
[super viewDidLoad];
}
-(void)viewWillAppear:(BOOL)animated
{
//initial set the button disable
self.button.enabled = NO;
//2nd thread
dispatch_queue_t downloadQueue = dispatch_queue_create("loadSchool", NULL);
dispatch_async(downloadQueue, ^{
//get avalible school info from JSON
schoolsAvaliable = [FunctionNSObject loadDataFromWeb:#"http://some web service"];
//get school year
schoolYear = [FunctionNSObject getSchoolYear];
if (schoolsAvaliable.count != 0)
{
//select the first row from array
dict = schoolsAvaliable[0];
//get the value from dictionary of that row
schoolID = (int)[[dict objectForKey:#"SchoolId"] integerValue];
schoolNameCh = [dict objectForKey:#"SchoolName"];
schoolNameEn = [dict objectForKey:#"SchoolNameEn"];
self.button.enabled = YES;
[self.button setNeedsDisplay];
}
else
{
self.button.enabled = NO;
[self.button setNeedsDisplay];
}
//2nd thread end then
dispatch_async(dispatch_get_main_queue(), ^{
//[self.pickerSchool reloadAllComponents];
self.labelSchool.text = [NSString stringWithFormat:#"%d - %d",schoolYear,schoolYear+1];
NSLog(#"%d",self.button.enabled);
});
});
}
#end

Answer 1

You call related to UI methods on non-main thread. Usually it results to unpredictable behaviour.
Try to call methods related to UI on main thread like this:
dispatch_async(dispatch_get_main_queue(), ^{
self.button.enabled = YES;
});
As #David noticed you needn't call [set.button setNeedsDisplay] because call of setEnabled: method results to call of this method.