Dynamically changing data source causing deleteRowsAtIndexPaths:indexes to crash - ios

Question

Tearing my hair out trying to get this to work. I want to perform [self.tableView deleteRowsAtIndexPaths:indexes withRowAnimation:UITableViewRowAnimationLeft];,
More detailed code of how I delete:
int index = (int)[self.messages indexOfObject:self.messageToDelete];
[self.messages removeObject:self.messageToDelete];
NSIndexPath *indexPath = [NSIndexPath indexPathForRow:index inSection:0];
NSArray *indexes = [[NSArray alloc] initWithObjects:indexPath, nil];
[self.tableView deleteRowsAtIndexPaths:indexes withRowAnimation:UITableViewRowAnimationLeft];
This works fine however if I get a push notification (i.e a new message received) whilst deleting the app will crash and display an error like:
Assertion failure in -[UITableView _endCellAnimationsWithContext:],
/SourceCache/UIKit/UIKit-3347.44/UITableView.m:1327
2015-07-04 19:12:48.623 myapp[319:24083] *** Terminating app due to uncaught exception 'NSInternalInconsistencyException', reason:
'attempt to delete row 1 from section 0 which only contains 1 rows
before the update'
I suspect this is because my data source is changing, the size of the array that
-(NSInteger)tableView:(UITableView *)tableView numberOfRowsInSection:(NSInteger)section
references while deleting won't be consistent because it was incremented by one when the push notification triggered a refresh. Is there any way I can work around this? Am I correct that deleteRowsAtIndexPaths uses the numberOfRowsInSection method?

Answer 1

So, in order to solve your problem you need to ensure that your data source will not change while some table view animations are in place. I would propose to do the following.
First, create two arrays: messagesToDelete and messagesToInsert. These will hold information about which messages you want to delete/insert.
Second, add a Boolean property updatingTable to your table view data source.
Third, add the following functions:
-(void)updateTableIfPossible {
if (!updatingTable) {
updatingTable = [self updateTableViewWithNewUpdates];
}
}
-(BOOL)updateTableViewWithNewUpdates {
if ((messagesToDelete.count == 0)&&(messagesToInsert.count==0)) {
return false;
}
NSMutableArray *indexPathsForMessagesThatNeedDelete = [[NSMutableArray alloc] init];
NSMutableArray *indexPathsForMessagesThatNeedInsert = [[NSMutableArray alloc] init];
// for deletion you need to use original messages to ensure
// that you get correct index paths if there are multiple rows to delete
NSMutableArray *oldMessages = [self.messages copy];
for (id message in messagesToDelete) {
int index = (int)[self.oldMessages indexOfObject:message];
[self.messages removeObject:message];
NSIndexPath *indexPath = [NSIndexPath indexPathForRow:index inSection:0];
[indexPathsForMessagesThatNeedDelete addObject:indexPath];
}
for (id message in messagesToInsert) {
[self.messages insertObject:message atIndex:0];
NSIndexPath *indexPath = [NSIndexPath indexPathForRow:0 inSection:0];
[indexPathsForMessagesThatNeedInsert addObject:indexPath];
}
[messagesToDelete removeAllObjects];
[messagesToInsert removeAllObjects];
// at this point your messages array contains
// all messages which should be displayed at CURRENT time
// now do the following
[CATransaction begin];
[CATransaction setCompletionBlock:^{
updatingTable = NO;
[self updateTableIfPossible];
}];
[tableView beginUpdates];
[tableView deleteRowsAtIndexPaths:indexPathsForMessagesThatNeedDelete withRowAnimation:UITableViewRowAnimationLeft];
[tableView insertRowsAtIndexPaths:indexPathsForMessagesThatNeedInsert withRowAnimation:UITableViewRowAnimationLeft];
[tableView endUpdates];
[CATransaction commit];
return true;
}
Lastly, you need to have the following code in all functions which want to add/delete rows.
To add message
[self.messagesToInsert addObject:message];
[self updateTableIfPossible];
To delete message
[self.messagesToDelete addObject:message];
[self updateTableIfPossible];
What this code does is ensures stability of your data source. Whenever there is a change you add the messages that need to be inserted/deleted into arrays (messagesToDelete and messagesToDelete). You then call a function updateTableIfPossible which will update the table view's data source (and will animate the change) provided that there is no current animation in progress. If there is an animation in progress it will do nothing at this stage.
However, because we have added a completion
[CATransaction setCompletionBlock:^{
updatingTable = NO;
[self updateTableIfPossible];
}];
at the end of the animations our data source will check if there are any new changes that need to be applied to the table view and, if so, it will update the animation.
This is a much safer way of updating your data source. Please let me know if it works for you.

Answer 2

Delete a Row
int index = (int)[self.messages indexOfObject:self.messageToDelete];
[self.messages removeObject:self.messageToDelete];
NSIndexPath *indexPath = [NSIndexPath indexPathForRow:index inSection:0];
[tableView beginUpdates];
[tableView deleteRowsAtIndexPaths:#[indexPath] withRowAnimation:UITableViewRowAnimationLeft];
[tableView endUpdates];
Delete a Section
Note : if your TableView have multiple section than you have to delete whole section when section contain only one row instead of deleting row
int index = (int)[self.messages indexOfObject:self.messageToDelete];
[self.messages removeObject:self.messageToDelete];
NSIndexPath *indexPath = [NSIndexSet indexSetWithIndex:0];
[tableView beginUpdates];
[tableView deleteSections:#[indexPath] withRowAnimation:UITableViewRowAnimationLeft];
[tableView endUpdates];

Answer 3

I have a few test about your code above. And you cannot do that at the same time the least we can do is something like:
//This will wait for `deleteRowsAtIndexPaths:indexes` before the `setCompletionBlock `
//
[CATransaction begin];
[CATransaction setCompletionBlock:^{
[tableView reloadData];
}];
[tableView beginUpdates];
[tableView deleteRowsAtIndexPaths:indexes withRowAnimation:UITableViewRowAnimationLeft];
[tableView endUpdates];
[CATransaction commit];
In your code whe have:
/*
int index = (int)[self.messages indexOfObject:self.messageToDelete];
[self.messages removeObject:self.messageToDelete];
NSIndexPath *indexPath = [NSIndexPath indexPathForRow:index inSection:0];
NSArray *indexes = [[NSArray alloc] initWithObjects:indexPath, nil];
[self.tableView deleteRowsAtIndexPaths:indexes withRowAnimation:UITableViewRowAnimationLeft];
*/
This is not safe because if if the notification is triggered and for some reason [self.tableView reloadData]; called right before deleteRowsAtIndexPaths that will cause an crash, because the tableView is currently updating the datas then interrupt by the deleteRowsAtIndexPaths:, try this sequence to check:
/*
...
[self.tableView reloadData];
[self.tableView deleteRowsAtIndexPaths:indexes withRowAnimation:UITableViewRowAnimationLeft];
This will cause a crash...
*/
Hmm.. Back to your code, Let's have a simulation that could cause the crash.. THIS is just an assumption though so this is not 100% sure (99%) only. :)
Let assume self.messageToDelete is equal to nil;
int index = (int)[self.messages indexOfObject: nil];
// since you casted this to (int) with would be 0, so `index = 0`
[self.messages removeObject: nil];
// self.messages will remove nil object which is non existing therefore
// self.messages is not updated/altered/changed
NSIndexPath *indexPath = [NSIndexPath indexPathForRow:index inSection:0];
// indexPath.row == 0
NSArray *indexes = [[NSArray alloc] initWithObjects:indexPath, nil];
[self.tableView deleteRowsAtIndexPaths:indexes withRowAnimation:UITableViewRowAnimationLeft];
// then you attempted to delete index indexPath at row 0 in section 0
// but the datasource is not updated meaning this will crash..
//
// Same thing will happen if `self.messageToDelete` is not existing in your `self.messages `
My suggestion is check the self.messageToDelete first:
if ([self.messages containsObject:self.messageToDelete])
{
// your delete code...
}
Hope this is helpful, Cheers! ;)

Answer 4

'attempt to delete row 1 from section 0 which only contains 1 rows before the update'
This says your index path, during deletion, references row 1. However row 1 doesn't exist. Only row 0 exists (like the section, they are zero-based).
So how are you getting an indexPath one greater than the number of elements?
Can we see your notification handler that inserts a row? You could do some hack where if the animation is in process, you performSelector:withDelay: during the notification processing to allow time for the animation to complete.

Answer 5

In you code, the reason for crash is : you are updating array but not update data source of UITableView. so your dataSource also needs to reflect the changes by the time endUpdates is called.
So as per Apple Documentation.
ManageInsertDeleteRow
To animate a batch insertion, deletion, and reloading of rows and sections, call the corresponding methods within an animation block defined by successive calls to beginUpdates and endUpdates. If you don’t call the insertion, deletion, and reloading methods within this block, row and section indexes may be invalid. Calls to beginUpdates and endUpdates can be nested; all indexes are treated as if there were only the outer update block.
At the conclusion of a block—that is, after endUpdates returns—the table view queries its data source and delegate as usual for row and section data. Thus the collection objects backing the table view should be updated to reflect the new or removed rows or sections.
Example:
[tableView beginUpdates];
[tableView deleteRowsAtIndexPaths:deleteIndexPaths withRowAnimation:UITableViewRowAnimationFade];
[tableView endUpdates];
Hope this help you.

Answer 6

I think the answer is alluded to in many of the comments here, but I'll try and tease it out a bit.
The first problem is that it doesn't look like you are bracketing your deletion method call in the beginUpdates/endUpdates methods. This is the first thing I would fix, otherwise as warned in Apple's documentation things can go wrong. (That doesn't mean they will go wrong, you're just safer doing it this way.)
Once you've done that, the important thing is that calling endUpdates checks the data source to make sure that any insertions or deletions are accounted for by calling numberOfRows. For example, if you delete a row (as you are doing), when you call endUpdates you best be sure that you've also deleted an item from the data source. It looks like you are doing that part right because you are removing an item from self.messages, which I assume is your data source.
(Incidentally you are correct that calling deleteRows... on its own without bracketing in beginUpdates/endUpdates also calls numberOfRows on the data source, which you can easily check by putting a break point in it. But again, don't do this, always use beginUpdates/endUpdates.)
Now it is possible that between calling deleteRows... and endUpdates someone else is modifying the self.messages array by adding an object to it, but I don't know if I really believe that because it would have to be extremely unfortunately timed. It's far more likely that when you receive a push message you aren't handling the insertion of the row properly, again by not using beginUpdates/endUpdates, or by doing something else wrong. It would be helpful if you could post the part of the code that handles the insertion.
If that part looks OK then it does look like you are very unfortunately making changes to the self.messages array on a different thread while the deletion code is being called. You can check this by adding a log line where your insertion code adds a message to the array:
NSLog(#"Running on %# thread", [NSThread currentThread]);
or just putting a break point in it and seeing which thread you end up on. If that is indeed the problem, you can dispatch the code that modifies the array and inserts the row onto the main thread (where it should be anyway since it's a UI operation) by doing something like below:
dispatch_async(dispatch_get_main_queue(), ^{
[self.messages addObject:newMessage];
[self.tableView beginUpdates];
[self.tableView insertRows...]
[self.tableView endUpdates];
});
That will ensure that the messages array won't get mutated by another thread while the main thread is busy deleting the row.