IPhone : Separate API-class for Parse backend (IOS)

I am new with Parse and Ios-development.

I develop a ios-app that use Parse as backend.

I have got the main-function to work now, but i have a BIG problem.

I want to create a separate class for my API-handling to Parse. As i set it up now i have my parse-code directly in my view-controllers and as far as i know that not that nice coding.

But, the issue is to handle the background-jobs. Let say if i want to do a GET from the server, this can be done in a background-thread, just using "findObjectsInBackgroundWithBlock"

The problem is when i move this method to a separate API-class. Then my ViewController ask my API-class to get all the objects an the API-class will return it as soon its done. It will nor run in the background, i cant return a NSMutableArray with objects to the viewController until the fetch is done.

I have thinking that i maybe can get the data from parse synchronously in my API-class by using [query findObjects:&error] , as long as i figure out how to create my get-method in the API-class to run asynchronously.

I have try to create my API-method as a asynchronously method using blocks but will not run in background on a separate thread. (I am new to blocks an dont evan no if thats the correct way to crate a method that will run in a separate thread when using it)

Here is my API-method (Class: APIClient)

` + (void) GETAllShoppingGroups:(void (^) (NSMutableArray*))completionBlock{

    //Create a mutable array (nil)
    NSMutableArray *shoppingGroupsArray = nil;

    //Create query for class ShoppingGroupe
    PFQuery *query = [ShoppingGroupe query];    

    //Filter - find only the groups the current user is related to
    [query whereKey:@"members" equalTo:[PFUser currentUser]];

    //Sort Decending
    [query orderByDescending:@"createdAt"];

    //Tell Parse to also send the real member-objects and not only id
    [query includeKey:@"members"];

    //Send request of query to Parse with a "error-pointer"and fetch in a     temp-array
    NSError *error = nil;
    NSArray *tempArray = [NSArray arrayWithArray:[query findObjects:&error]];

    //Check for success
    if (!tempArray) {
    NSLog(@"%@", error);
    NSLog(@"ERROR: %@", [error userInfo][@"error"]);
    return completionBlock(shoppingGroupsArray);
    } else {
    //Seccess
    shoppingGroupsArray = tempArray.mutableCopy;
    completionBlock(shoppingGroupsArray);
    }
}`

Here is my ViewController Class (Class: ShoppingGruopViewController)

  `- (void) getAllObjects{

   //Init array if nil
   if (!self.shoppingGroupeArray) {
   self.shoppingGroupeArray = [[NSMutableArray alloc]init];
   }

   //Remove old objects
   [self.shoppingGroupeArray removeAllObjects];

   //Get objects
   [APIClient GETAllShoppingGroups:^(NSMutableArray* completionBlock){
   if (completionBlock) {
   [self.shoppingGroupeArray addObjectsFromArray:completionBlock]; }

   [self.tableView reloadData];
   }];
} `