My app has two CollectionViewControllers. Only one is visible at a given time.
I have created the following structure on storyboard: two container views on top of each other. Every container view has a CollectionViewController embedded. The visibility of a particular container view determines which collectionViewController is visible.
This is the problem. Both CollectionViewControllers are receiving data in parallel but iOS has a bug that will make the app crash if one CollectionViewController tries to execute an insert using performBatchUpdates while it is invisible.
Trying to prevent that, I have created a BOOL flag on both CollectionViewControllers so they can know if they are visible and execute or not the performBatchUpdates. Something like:
if (self.isThisCollectionViewVisible == NO) return;
[self.collectionView performBatchUpdates:^{
// bla bla... perform insert,m remove...
This solves part of the problem. But the app continues to crash on the following condition: if I tap the button to switch to the invisible CollectionViewController making it visible while it is receiving updates.
I mean this: lets call A the first CollectionViewController and B the second one. A is visible and B is invisible at this point. B starts receiving data and is trying to do a performBatchUpdates but as it is invisible, the if (self.isThisCollectionViewVisible == NO) return; is preventing performBatchUpdates to run, what is fine. Now I make A invisible and B visible. At this point the flag self.isThisCollectionViewVisible is set to YES and performBatchUpdates makes the app crash with this error:
* Assertion failure in -[CollectionView _endItemAnimationsWithInvalidationContext:tentativelyForReordering:], /BuildRoot/Library/Caches/com.apple.xbs/Sources/UIKit/UIKit-3512.60.7/UICollectionView.m:4625
* Terminating app due to uncaught exception 'NSInternalInconsistencyException', reason: 'Invalid update: invalid
number of items in section 0. The number of items contained in an
existing section after the update (76) must be equal to the number of
items contained in that section before the update (70), plus or minus
the number of items inserted or deleted from that section (5 inserted,
2 deleted) and plus or minus the number of items moved into or out of
that section (0 moved in, 0 moved out).'
I think the CollectionViewController is really not yet ready and updated to be able to do a performBatchUpdates... and this is not a matter of not updating the data source previously because it is being updated.
What checks can I do to prevent that from happening?
NOTE: I noticed something strange about this crash in particular. It says that 5 elements are being inserted and 2 deleted but in fact 3 elements are being inserted, 0 deleted and 2 changed when the crashes happen.
For me adding self.collectionView.numberOfItemsInSection(0) fixed the crash.
The collectionView has issues while inserting items when it is not visible.
Seems like I'm not alone with my solution: http://www.openradar.me/15262692
This crash told you that you didn't updated your datasource for collection. You need to update your dataSource (array or dictionary) and reload collection view data after you perform performBatchUpdates.
Invalid update: invalid number of items in section 0. The number of items contained in an existing section after the update (76) must be equal to the number of items contained in that section before the update (70), plus or minus the number of items inserted or deleted from that section (5 inserted, 2 deleted) and plus or minus the number of items moved into or out of that section (0 moved in, 0 moved out).'
As written in apple docs
Deletes are processed before inserts in batch operations. This means
the indexes for the deletions are processed relative to the indexes of
the collection view’s state before the batch operation, and the
indexes for the insertions are processed relative to the indexes of
the state after all the deletions in the batch operation.
So, move the changes before the inserts and it will the trick!
Encountered the same error today, for me, in performBatchUpdates block replace this:
NSArray *selectedItemsIndexPaths = [self.collectionView indexPathsForSelectedItems];
with this:
NSIndexPath *selectedIndexPath = [NSIndexPath indexPathForRow:self.selectIndex inSection:0];
NSArray *selectedItemsIndexPaths = #[selectedIndexPath];
Maintain the index by myself, it's OK now. The error should not be associated with data source, if you have had update the data source. It maybe related to the reuse of cells.
Related
I have problem with my collection view and reloading/inserting items. I have my items array in view controller which contains UICollectionView. I change content of items array in many places. Sometimes I insert new items and sometimes I must reload items again from database.
Not all the time but sometimes I get "classic" error when inserting new items:
Invalid update: invalid number of items in section 0. The number of
items contained in an existing section after the update (16) must be
equal to the number of items contained in that section before the
update (16), plus or minus the number of items inserted or deleted from
that section (1 inserted, 0 deleted) and plus or minus the number of
items moved into or out of that section (0 moved in, 0 moved out).
I know meaning of this error but It's really hard to fix this because as I wrote I can get notifications and then I reload data immediately (get items from database and reload collection view) and it could happen when inserting new item and similar conflicts could happen.
So I am not sure what is best solution for this. Should I lock array and make it thread safe (but I think I edit it mostly in main thread) or should I somehow "lock" collection and when there is insert operation then there couldn't be changes to items or collection view couldn't be reloaded? Or is there other solution?
Bonus: Is it possible that app wouldn't crash when error above happens and just full reload collection view? Propably not but it could help a lot.
I'm allowing users to select/deselect cells in my collection view and then hit a save/delete button.
When I'm selecting rows, I add them to a dictionary
var selectedIndexes = Dictionary<IndexPath, String> ()
and when I deselect the row I set the selectedIndexes[indexPath] = nil
When the user hits delete I run
for index in selectedIndexes.keys {
indexesToDelete.append(index)
}
collectionView.deleteItems(at: indexesToDelete)
This goes into the selectedIndexes dictionary, grabs the indexes, adds those indexes into an array of indexes "indexesToDelete", and then after the forloop is over, I'm deleting that array of indexes.
When I run this I get:
Terminating app due to uncaught exception 'NSInternalInconsistencyException', reason: 'Invalid update: invalid number of items in section 0. The number of items contained in an existing section after the update (5) must be equal to the number of items contained in that section before the update (5), plus or minus the number of items inserted or deleted from that section (0 inserted, 2 deleted) and plus or minus the number of items moved into or out of that section (0 moved in, 0 moved out).'
I've printed everything out and the indexesToDelete are the correct indexes for what I'm trying to delete. I don't fully understand what the error message is saying.
What it is saying is that your collection view is no longer in sync with your data source. So you also need to update whatever you are using as your dataSource to reflect your changes and then call deleteCells.
for index in selectedIndexes.keys {
indexesToDelete.append(index)
}
yourCollectionViewDataSourceArray.remove(at: index)
collectionView.deleteItems(at: indexesToDelete)
if your collectionView data source is yourCollectionViewDataSourceArray, both are synced together. Every time collectionView reloads data it uses the the function with argument numberOfItemsInSection. When it finds out that your array.count is different than the total of items after deletion it gives you this error. Take it as a rule every time you want to update your collectionView by code, update the data array before updating it. Hope that helps
I reload, insert and delete table view's rows and sections in a single begin/end update block. I do not call moveSection:toSection: nor moveRowAtIndexPath:toIndexPath: method.
I am aware that deletion and reloading of existing cells are being executed first, regardless of the order of method calls in update block. I do not insert rows for the new section that is inserted. Also, I am updating data source before table view update block.
Deletion and reloading work just fine for me, but I have a problem with inserting new sections. I got error messages of type:
Terminating app due to uncaught exception 'NSInternalInconsistencyException', reason: 'Invalid update: invalid number of rows in section 5. The number of rows contained in an existing section after the update (1) must be equal to the number of rows contained in that section before the update (3), plus or minus the number of rows inserted or deleted from that section (0 inserted, 0 deleted) and plus or minus the number of rows moved into or out of that section (0 moved in, 0 moved out).'
It is interesting that I do not delete or reload any table view's rows and sections in the corresponding table view update block. Also, there is 1 row in all sections that come before section 5. New section was inserted at index 0. So it seems to me that I might be missing moveSection:toSection: method call..?
I couldn't find more complex examples of simultaneous inserting, deleting and reloading rows and sections in a single update block on Internet, so I would really appreciate any suggestions and links to the examples that could guide me how to handle more complex table view updates.
I'm trying to use this method to implement an infinitely scrolling UITableView
The core logic of the solution is:
To increase the tableview content by a factor of 3, so that we make the 3 copies of the content laid one after another vertically.
Whenever the top end of the scroll is reached, move the the scroll offset back to start of the 2nd copy
When the bottom end of the scroll is reached, we move the scroll offset back to the start of the 2nd copy minus the height of the tableview, so that we end up showing the same content as we are now.
This means that a single cell insert or delete actually results in three inserts or deletes. Because my table's datasource is populated by an NSFetchedResultsController it causes an Assertion Failure.
The number of rows contained in an existing section after the update (12) must be equal to the number of rows contained in that section before the update (15), plus or minus the number of rows inserted or deleted from that section (0 inserted, 1 deleted) and plus or minus the number of rows moved into or out of that section (0 moved in, 0 moved out).
Is there some way to stop the program from crashing in these cases? I'd really appreciate any help/pointers. Thanks.
This can be kind of tricky in iOS especially when you're first starting out. The key is to update the datasource that is backing your UITableView before you inform the tableview of changes.
For instance, if you have an NSArray backing your tableview, then you will want to remove or add items to it, prior to calling reloadSection: on the tableview.
Although it is the least optimized solution, while your testing feel free to simply call reloadData which will ignore what the tableview has cached and force it to recalculate based on whatever is backing the tableview.
You can use NSFetchedResultsControllerDelegate method controller:didChangeObject:atIndexPath:forChangeType:newIndexPath:. To insert\delete 3 objects.
To keep it simple you could return 3 for number of sections in your table view (this will handle getting 3 copies of objects). And then in your NSFetchedResultsControllerDelegate update objects in all sections of your UITableView (insert, delete or move). You should use row number from the NSFetchedResultsControllerDelegate and apply all changes 3 times each for different section. Remember to keep it consistent with UITableView data source method tableView:numberOfRowsInSection:.
I have a table view with each rows having image and some text. The images are loaded asyc. As soon as image is fetched from server, the delegate methods gets called. The delegate method contains the index path which initiated the image fetching, so that I can reload only those cells. I have extra check to make sure that once the image is fetched, the data source contains enough data so that index path doesn't go out of bounds. Despite setting this condition the app crashes.
There is a chance that my table data gets updated before the image is fetched. I know this is cause of the issue, but I am not sure why this is making a crash despite adding a check before
reloadRowsAtIndexPaths?
The error is:
Fatal Exception NSInternalInconsistencyException Invalid update: invalid number of rows in section 2. The number of rows contained in an existing section after the update (4) must be equal to the number of rows contained in that section before the update (6), plus or minus the number of rows inserted or deleted from that section (1 inserted, 1 deleted) and plus or minus the number of rows moved into or out of that section (0 moved in, 0 moved out).
Can some one share any inputs on this?
Code snippet that reloads the table
// this method will be called on main thread
- (void)loadImageAtIndexPath:(NSIndexPath *)inIndexPath {
if(inIndexPath.row < dataSource.count) {
[self.listView reloadRowsAtIndexPaths:[NSArray arrayWithObject:inIndexPath]
withRowAnimation:UITableViewRowAnimationNone];
}
}