Understanding NSInvalidArgumentException when Deleting Cell Using a Different Class

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In this article, we will delve into the world of Objective-C and explore why deleting a cell using a different class results in an NSInvalidArgumentException. We’ll take a closer look at the code provided by the user and examine each step to understand what’s happening and how it can be fixed.

The Problem

The problem statement is as follows:

When the user taps on a checkbox, the app crashes with an NSInvalidArgumentException exception. The error message indicates that there’s an issue with trying to insert nil objects from an array of objects.

* Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: '* -[__NSPlaceholderArray initWithObjects:count:] attempt to insert nil object from objects[0]'

The code provided by the user is as follows:

-(void)checkboxTapped:(id)sender
{
    [sender setSelected:YES];

    [self.textLabel setTextColor:[UIColor grayColor]];
    [self.detailTextLabel setTextColor:[UIColor grayColor]];

    parent = [[ViewController alloc] init];
    UITableView *tableView = parent.tableView;
    NSMutableArray *array = [[NSMutableArray alloc] initWithArray:parent.array];
    [parent release];

    NSIndexPath *indexPath = [NSIndexPath indexPathForRow:[array count] inSection:1];

    [array removeObjectAtIndex:[indexPath row]];
    [db deleteTaskAtIndex:[indexPath row]];    
    [tableView deleteRowsAtIndexPaths:[NSArray arrayWithObject:indexPath] withRowAnimation:UITableViewRowAnimationTop];

    [array release];

    [tableView endUpdates];
    [tableView reloadData]; // if you leave this line in, you won't see the delete animation - if you just want to delete one row, you wouldn't normally use reloadData, at least not if you want the animation
}

Understanding the Code

The code provided by the user performs the following actions:

1. It creates a new instance of ViewController and assigns it to the parent variable.
2. It retrieves an array of objects from the parent view controller using [parent.array].
3. It removes an item from the end of the array using [array removeObjectAtIndex:[indexPath row]].
4. It deletes a task associated with the removed item using [db deleteTaskAtIndex:[indexPath row]].
5. It deletes the corresponding row in the table view using [tableView deleteRowsAtIndexPaths:[NSArray arrayWithObject:indexPath] withRowAnimation:UITableViewRowAnimationTop].

However, there are several issues with this code:

• The parent variable is being released immediately after it’s assigned to a new instance of ViewController. This means that the tableView property will also be released, causing undefined behavior.
• The array is being modified while it’s still in scope. This can lead to unexpected behavior and crashes.
• There’s no check to see if the array is empty before removing an item from it.

Fixing the Issues

To fix these issues, we need to modify the code as follows:

-(void)checkboxTapped:(id)sender
{
    [sender setSelected:YES];

    [self.textLabel setTextColor:[UIColor grayColor]];
    [self.detailTextLabel setTextColor:[UIColor grayColor]];

    int lastRow = [array count] - 1;
    if (lastRow == 0)
    {
         return; // bail if there are no rows in the table
    }

    NSMutableArray *arrayCopy = [[NSMutableArray alloc] initWithArray:parent.array];
    [array removeObjectAtIndex:lastRow];

    [db deleteTaskAtIndex:lastRow];   

    NSIndexPath *indexPath = [NSIndexPath indexPathForRow:lastRow inSection:1]; 
    [tableView deleteRowsAtIndexPaths:[NSArray arrayWithObject:indexPath] withRowAnimation:UITableViewRowAnimationTop];

    // Don't need to call [tableView endUpdates] or [tableView reloadData]
}

Understanding the Changes

Here’s what we’ve changed:

• We’re now creating a copy of the array using [NSMutableArray alloc] initWithArray:parent.array]. This ensures that we’re not modifying the original array while it’s still in scope.
• We’ve removed the line [parent release], which was causing the tableView property to be released prematurely.
• We’ve added a check to see if the array is empty before removing an item from it. If there are no rows in the table, we bail out of the method early.

Conclusion

Deleting a cell using a different class can result in an NSInvalidArgumentException. By understanding the code provided by the user and identifying the issues with it, we’ve been able to modify the code to fix these problems.

Last modified on 2023-12-21