Objective-C-iOS UITableView底部行在删除时崩溃

Question

I am extending the learn iOS programming today tutorial to include delete functionality.

I have modified the tableView didSelectRowAtIndexPath: method thusly:

- (void)tableView:(UITableView *)tableView didSelectRowAtIndexPath:(NSIndexPath *)indexPath {

    [tableView deselectRowAtIndexPath:indexPath animated:NO];
    ToDoItem *tappedItem = [self.toDoItems objectAtIndex:indexPath.row];
    if (tappedItem.completed) {

        [tableView beginUpdates];
        [self.toDoItems removeObjectAtIndex:indexPath.row];

        [tableView deleteRowsAtIndexPaths:[NSArray arrayWithObject:indexPath] withRowAnimation:UITableViewRowAnimationBottom];
        [tableView endUpdates];

    } else {
        tappedItem.completed = YES;
    }
    [tableView reloadRowsAtIndexPaths:@[indexPath] withRowAnimation:UITableViewRowAnimationNone];
}

In a section with three rows, it works as expected. Tapping produces a checkmark, tapping a check marked item deletes it. But if I tap the bottom row, it crashes with 'attempt to delete row 2 from section 0 which only contains 2 rows before the update' . Note this is happening when the other two rows are still there (my searches found numerous posts where there was a crash when someone was deleting the last remaining row--not the case here). The bottom row will mark itself completed just fine.

Also note, moving the array changing call out of the beginUpdates block changes the error from row 2 to row 3 ... contains 3 rows.

TIA for any assistance.

Edit:

I have fixed the problem by moving [tableView reloadRowsAtIndexPaths:@[indexPath] withRowAnimation:UITableViewRowAnimationNone]; inside of the else block. Can someone explain why?

Answer 1

If you use deleteRowsAtIndexPaths , there is no point in trying to reload the row that you deleted. And, obviously, if you try to reload a cell for an indexPath that is no longer valid, then you will have the sort of problem you describe.

Let's say you have 10 rows, you don't want to say, effectively, "delete the tenth row; now reload the tenth row in a table that now only has nine rows." You can easily imagine why that is problematic.

In this case, you should remove the call to reloadRowsAtIndexPaths altogether. You only have to call reload... if the contents of some of the remaining cells change. If you're just inserting or deleting rows, then just do that, and no call to reloadRowsAtIndexPaths is needed.