Question

I’m getting a warning in the following method:

- (void)tableView:(NSTableView *)aTableView
 setObjectValue:(id)anObject
 forTableColumn:(NSTableColumn *)aTableColumn
            row:(int)rowIndex
{
    NSString *identifier = [aTableColumn identifier];
    Person *person = [employees objectAtIndex:rowIndex];

    NSUndoManager *undo = [self undoManager];
    [[undo prepareWithInvocationTarget:self] tableView:aTableView setObjectValue:[person valueForKey:identifier] forTableColumn:aTableColumn row:rowIndex];
    if(![undo isUndoing])
    {
        [undo setActionName:@"Edit Person"];
    }
    [person setValue:anObject forKey:identifier];
    [tableView reloadData];
}

I’m trying to implement undo so I figure I would call the same method and just pass the old value. The error I’m getting is “warning: multiple methods named ‘-tableView:setObjectValue:forTableColumn:row:’ found”. Any idea why?

Answer 1

The method on NSTableViewDataSource is declared as:

- (void)tableView:(NSTableView *)aTableView setObjectValue:(id)anObject forTableColumn:(NSTableColumn *)aTableColumn row:(NSInteger)rowIndex

Your method uses int as the type of the rowIndex variable. Change that to NSInteger and all will be fine.

The underlying issue is that Objective-C does not support co-variance or contra-variance in method argumentation, leading to the warning.

---

Note also that you are colluding data model with view layer. Undo in this fashion is going to be very very tricky; if you don’t also manage the undo stack in the context of every sort of the table and/or addition/removal of rows, undo is going to apply the value to the wrong row.

You would be far better off reworking your app such that you have a proper separation of model view and controller.