Calling methods inside block and trying to reload data crash

Question

I use parse.com as backend, and my query was very slow, so I am trying to change it to use blocks.

Basically, my query populates an array with everything I need, and according to if statements, I'm calling methods inside the block, these methods populate the array that I will use in cellForRowAtIndexPath. The problem is that when I try to reloadDatainside block, the app crashes.

Here is the code:

- (void)queryForTable {
    PFQuery *exerciciosQuery = [PFQuery queryWithClassName:@"ExerciciosPeso"];
    [exerciciosQuery whereKey:@"usuario" equalTo:[PFUser currentUser]];
    [exerciciosQuery includeKey:@"exercicio"];

    exerciciosQuery.cachePolicy = kPFCachePolicyCacheElseNetwork;

    [exerciciosQuery findObjectsInBackgroundWithBlock:^(NSArray *objects, NSError *error) {

         [self configurarDatas];

         _seriesArray = objects;

            if (_seriesArray.count > 0) {
                NSPredicate *predIniciante = [NSPredicate predicateWithFormat:@"serie contains [cd] %@", @"Ini"];
                NSArray *arrayIniciante = [_seriesArray filteredArrayUsingPredicate:predIniciante];


                NSArray *arrayInicianteApenasSeries = arrayIniciante;
                NSArray *arrayInicianteApenasSeries2 = [arrayInicianteApenasSeries valueForKey:@"serie"];
                NSSet *setInicianteApenasSeries = [NSSet setWithArray:arrayInicianteApenasSeries2];
                NSArray *arrayInicianteCount = [setInicianteApenasSeries allObjects];

                if (arrayInicianteCount.count > 0) {
                    [self popularSeriesInicianteAB];
                // [self.tableView reloadData];


                }
                else if (arrayInicianteCount.count > 8) {

                    [self popularSeriesInicianteC];
                    //   [self.tableView reloadData];

                }
                else {

                    [self popularSeriesAvancado];
                    //   [self.tableView reloadData];
                    NSLog(@"POPULAR SERIES AVANÇADO");
                }
            }
    }];

}

I have also tried to reloadData using dispatch_async(dispatch_get_main_queue(), ^{ [myDisplayedTable reloadData]; });, but it didn't work as well.

Anyway, I imagine that if I erase my methods and put everything inside the block, it will work, but I don't want to do that, as calling methods using if make my code easier to follow.

UPDATE:

For completeness, here are my DataSource methods:

 - (NSInteger)tableView:(UITableView *)tableView numberOfRowsInSection:(NSInteger)section
     {
     NSLog(@"numberOfRowsInSection %li", (unsigned long)_seriesForDisplay.count);
     return _seriesForDisplay.count;

 }


- (UITableViewCell *)tableView:(UITableView *)tableView cellForRowAtIndexPath:(NSIndexPath *)indexPath object:(PFObject *)object {
    static NSString *CellIdentifier = @"Cell";

    PFTableViewCell *cell = [tableView dequeueReusableCellWithIdentifier:CellIdentifier];
    if (cell == nil) {
        cell = [[PFTableViewCell alloc] initWithStyle:UITableViewCellStyleSubtitle reuseIdentifier:CellIdentifier];
    }





    PFObject *o = _seriesForDisplay[indexPath.row];
    // Texto da célula
    cell.textLabel.text = o[@"serieDisplay"];

    cell.detailTextLabel.text = o[@"grupos"];



    return cell;

}

Answer 1

There are 2 way to implement data in tableview with Parse SDK

1. PFqueryTableViewController: you need implement

   - (PFQuery *)queryForTable {
        return query;
     }
   
   - (PFObject *)objectAtIndex:(NSIndexPath *)indexPath {
     // overridden, since we want to implement sections
     }
   
    - (UITableViewCell *)tableView:(UITableView *)tableView cellForRowAtIndexPath: 
      (NSIndexPath *)indexPath object:(PFObject *)object {
     //get data: object
     }
   
    - (UITableViewCell *)tableView:(UITableView *)tableView cellForNextPageAtIndexPath:
    (NSIndexPath *)indexPath 
   

2. UITableViewController/ UITableview in UIViewController:
   This is normal way, implement uitableview delegate/datasource, you can write a method like (void)queryForTable and call reloadData in here.

Answer 2

I was using PFQueryTableViewController, which is from parse.com backend and has some different methods. I changed my query to void and used findObjectsInBackgroundWithBlock. Therefore, my method - (UITableViewCell *)tableView:(UITableView *)tableView cellForRowAtIndexPath:(NSIndexPath *)indexPath object:(PFObject *)object, was not receiving the PFObjects, that's why I was getting a crash when tried to reloadData.

I changed my table back to UITableViewController.

I'm using dispatch_async(dispatch_get_main_queue(), ^{ [self.tableView reloadData]; }); inside block and it's working now.

Great answers to this post, helped me a lot.

Answer 3

If you can't guarantee what thread a block will be run on (if it can run on a background thread) then you should switch to the main thread before actioning any UI changes. This could be done with performSelectorOnMainThread:withObject:waitUntilDone: or dispatch_async(dispatch_get_main_queue(), ^{.