Calling methods inside block and trying to reload data crash - ios

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
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
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(), ^{.