load objects from parse and display on map - ios

Question

Working on an App that allow user to see what local people have posted (businesses, schools, or just people). I am able to post the information and save it to parse. I am also able to download user data near current location, but it issue is that my helper method does not do it right away when the app first launches.
I have tried to call the helper method below in the viewDidLoad,viewWillAppear and viewDidAppear and no luck. The postArray is always null when the user initially opens the application and goes to the map. If I move to another screen and come back to the mapVC the data is there...(I NSlog the postArray) and all the posts near the current location print out. I would like to have this data initially right when the map presents the user's location.
Questions.
Where should I call my helper method? When the app launches I want to
have the data so i can display it on the map.
Is there another method that I need to write?
Is there something wrong with the current method.
- (void)loadLocalPosts {
NSLog(#"Querying for Local Posts");
[activityIndicator startAnimating];
PFQuery *query = [PFQuery queryWithClassName:#"Post"];
[PFGeoPoint geoPointForCurrentLocationInBackground:^(PFGeoPoint *geoPoint, NSError
*error) {
geopoint = geoPoint;
[query whereKey:#"location" nearGeoPoint:geoPoint];
[query setLimit:50];
[query addDescendingOrder:#"createdAt"];
[query findObjectsInBackgroundWithBlock:^(NSArray *objects, NSError *error) {
postArray = objects;
}];
}];
}

Answer 1

The problem here is that postArray is populated in a background thread and your application initializes faster than it can populate the array. This is common in asynchronous programming. The way to fix this, is by asking the map to refresh in the main thread.
- (void)loadLocalPosts {
NSLog(#"Querying for Local Posts");
[activityIndicator startAnimating];
PFQuery *query = [PFQuery queryWithClassName:#"Post"];
[PFGeoPoint geoPointForCurrentLocationInBackground:^(PFGeoPoint *geoPoint, NSError
*error) {
geopoint = geoPoint;
[query whereKey:#"location" nearGeoPoint:geoPoint];
[query setLimit:50];
[query addDescendingOrder:#"createdAt"];
[query findObjectsInBackgroundWithBlock:^(NSArray *objects, NSError *error) {
postArray = objects;
dispatch_async(dispatch_get_main_queue(),^
{
// update view properties, refresh etc.
});
}];
}];
}
Note: All view related modifications should always happen in the main thread. Hence the dispatch_get_main_queue(). Now the main loop could however, be doing view related operations, hence a synchronous call will crash the application. Hence dispatch_async is used. This will add the block to the next run loop to be executed after the current one.