UICollectionView cellForItemAtIndexPath is called but data source is empty - ios

Question

I have a custom tab on top of view which a user can switch two view back and forth. Both view contains UICollectionView dynamically filled with data from a server. In my understanding, if numberOfItemInSection return 0 when [collectionView reloadData] is called, cellForItemAtIndexPath won't be called. In other word, if cellForItemAtIndexPath is being called, it means there is some data in source. Below is some code I used when switch to one view to other view.
- (void)refresh
{
[self.collectionView setContentOffset:CGPointZero];
[self.data removeAllObjects];
[self.refreshControl beginRefreshing];
if (self.collectionView.contentOffset.y == 0) {
[UIView animateWithDuration:0.25 delay:0 options:UIViewAnimationOptionBeginFromCurrentState animations:^(void){
self.collectionView.contentOffset = CGPointMake(0, -MPViewHeight(self.refreshControl));
} completion:^(BOOL finished){
}];
}
[self.collectionView scrollRectToVisible:CGRectMake(0, 0, 1, 1) animated:YES];
[self requestToServerAtIndex:0];
}
- (void) requestToServerAtIndex:(NSInteger)index
{
#weakify(self)
[[self.serverHelper getData:index requestCount:20] subscribeNext:^(id JSON) {
#strongify(self)
NSArray *data = [ParserForJSON parse:JSON];
if(data.count != 20) {
self.moreData = NO;
}
[self.data addObjectsFromArray:data];
[self.collectionView reloadData];
[self.refreshControl endRefreshing];
} error:^(NSError *error) {
}];
}
- (NSInteger)collectionView:(UICollectionView *)collectionView numberOfItemsInSection:(NSInteger)section
{
return self.data.count;
}
- (UICollectionViewCell *)collectionView:(UICollectionView *)collectionView cellForItemAtIndexPath:(NSIndexPath *)indexPath
{
CustomData *data = [self.data objectAtIndex:indexPath.row];
CustomCell *cell = [collectionView dequeueReusableCellWithReuseIdentifier:#"Cell" forIndexPath:indexPath];
///.... some cell configuration
return cell;
}
However, I got random crash on cellForItemAtIndexPath when it tried to access data source which was happened to be empty. Surprisingly, the data source was not empty when I checked on console at that point. So my wild guess is there was some timing issue. I would like to get some in depth explanation on this ambiguous behavior.

Answer 1

When you call refresh method, you remove all data objects, so when your cellForItemAtIndexPath was called, there's no data in data source. That's why when call id object = [data objectAtIndex:indexPath.row] return nil. So make sure after requestToServerAtIndex make the new datas realy then remove the old objects and update the UI.
EDIT:
Try to remove [self.collectionView scrollRectToVisible:CGRectMake(0, 0, 1, 1) animated:YES]; to test, this method should call cellForItemAtIndexPath