ios - One task with multiple threading and different viewcontrollers -

i'm still facing problems multithreading on ios. want fetch json in mainviewcontroller (when app launches) , populate left menu (swrevealviewcontroller) data.

but let's consider situation low internet speed, when user reveals menu , data didn't come mainviewcontroller. i'm fetching json both controllers. don't know how make sidebar wait data until mainviewcontroller finishes fetching?

call mainviewcontroller

[[categoriesstore sharedstore] fetchdatawithcallback:^(nsarray *allcategories, nserror* error) {  }]; 

call sidebarviewcontroller inside async blocks

[[categoriesstore sharedstore] fetchdatawithcallback:^(nsarray *allcategories, nserror* error) {     if (error) {     } else {         //[self.tableview beginupdates];         blocksafe.alldirectories = allcategories;         [blocksafe.tableview reloadsections:[nsindexset indexsetwithindex:1] withrowanimation:uitableviewrowanimationfade];      } }]; 

fetching json categoriesstore (singleton model)

+(instancetype)sharedstore { static categoriesstore *sharedstore = nil; static dispatch_once_t oncetoken;  dispatch_once(&oncetoken, ^{     sharedstore = [[self alloc] initprivate];   });  return sharedstore;  }  - (instancetype)initprivate { self = [super init];  if (self) {     [self sessionconf];     _allcategories = nil; }  return self; }  - (void)fetchdatawithcallback:(void(^)(nsarray *allcategories, nserror* error))callback { nslog(@"categoriesstore - %@",self.description); @synchronized(self) {     if (!_allcategories) {         nsurlsessiondatatask *getcategories =         [self.session datataskwithurl:categoriesurl                     completionhandler:^(nsdata *data,                                         nsurlresponse *response,                                         nserror *error) {                         if (error) {                             nslog(@"error - %@",error.localizeddescription);                             callback(nil, error);                             return;                         }                          nserror *jsonerror = nil;                          nsarray *json =                         [nsjsonserialization jsonobjectwithdata:data                                                         options:nsjsonreadingmutablecontainers                                                           error:&jsonerror];                         if (!jsonerror) {                             _allcategories = json;                             nslog(@"categories fetched");                             callback(_allcategories, nil);                         } else {                             callback(nil, jsonerror);                         }                     }];          [getcategories resume];     } else {         callback(_allcategories,nil);     } } } 

thanks , advices!

you can this: dispatch_group_t group_t = dispatch_group_create();

    dispatch_group_async(group_t, dispatch_get_global_queue(dispatch_queue_priority_default, 0), ^{         //do first thing     });      dispatch_group_async(group_t, dispatch_get_global_queue(dispatch_queue_priority_default, 0), ^{         //do second thing     });      dispatch_group_notify(group_t, dispatch_get_main_queue(), ^{         //update ui     });