Invalid tableview update when rotating device - ios

Question

I am using what I believe is a common pattern for adding items to a table view -
main controller creates modal controller and registers itself as delegate
modal view controller is presented
user provides some data and hits save button in the modal's navigation bar
modal view controller send its delegate a message containing the details entered
original controller receives the message and dismisses the modal
original controller updates the data model and inserts a new row into its tableview
This is working well except in one specific scenario.
If the device is rotated while the modal is presented, the app crashes after dismissing the modal. The new row is inserted correctly but then immediately afterwards fails:
*** Assertion failure in -[UITableView _endCellAnimationsWithContext:],
/SourceCache/UIKit_Sim/UIKit-2380.17/UITableView.m:1070
2013-07-28 17:28:36.404 NoHitterAlerts[36541:c07] *** Terminating app due to uncaught exception 'NSInternalInconsistencyException', reason: 'Invalid update: invalid number of rows in section 0. The number of rows contained in an existing section after the update (8) must be equal to the number of rows contained in that section before the update (8), plus or minus the number of rows inserted or deleted from that section (1 inserted, 0 deleted) and plus or minus the number of rows moved into or out of that section (0 moved in, 0 moved out).'
I can't for the life of my figure out why this occurs. Test cases that work consistently fail if I rotate with the modal presented. I've noticed that simply reloading the tableview instead of inserting the row with animation works fine.
Here is a bare bones project that demonstrates the same issue:
demo project
Run the project in the iPhone sim
Add an item to the list - works fine
Back on first screen, rotate to landscape
Run the same test again. Still works.
Back on first screen, launch modal. Rotate simulator while modal is still presented. Hit 'Add Item'. Crashes.
Any ideas on what might be happening here?

Answer 1

I see what your problem is. In your MainController's -modalController:didAddItem: method, you are first adding the object to the self.arrayOfStrings, and not inserting the row into the tableView until after the -dismissViewControllerAnimated method has completed.
This appears to work when the orientation is not changed when the modalViewController is open, however if you do change the orientation, the mainController's orientation isn't changed until it is being closed. Once this happens, it seems that the tableView's data is automatically reloaded due to the frame being changed.
Thus, because the arrayOfStrings is having the object added before the animation starts and the -insertRowsAtIndexPaths:withRowAnimation: is not called until after the animation is completed, the table view thinks it has already gotten the rows by the time the insert method is reached.
In order to fix this, all you have to do is move your method that adds the string to the array into the completion block right before you call the insertRows method on your tableView.
So your method would end up looking something like the following with whatever changes you need for your actual project:
- (void)modalController:(ModalController *)controller didAddItem:(NSString *)string
{
//dismiss the modal and add a row at the correct location
[self dismissViewControllerAnimated:YES completion:^{
[self.arrayOfStrings addObject:string];
[self.tableView insertRowsAtIndexPaths:#[[NSIndexPath indexPathForRow:self.arrayOfStrings.count - 1 inSection:0]] withRowAnimation:UITableViewRowAnimationAutomatic];
}];
}