If you subclass NSOutlineView, you can override menuForEvent:
to return a menu only if the user clicked on the correct row. Here's an example:
- (NSMenu *)menuForEvent:(NSEvent *)event;
{
//The event has the mouse location in window space; convert it to our (the outline view's) space so we can find which row the user clicked on.
NSPoint point = [self convertPoint:[event locationInWindow] fromView:nil];
NSInteger row = [self rowAtPoint:point];
//If the user did not click on a row, or is not exactly one level down from the top level of hierarchy, return nil—that is, no menu.
if ( row == -1 || [self levelForRow:row] != 1 )
return nil;
//Create and populate a menu.
NSMenu *menu = [[NSMenu alloc] init];
NSMenuItem *delete = [menu addItemWithTitle:NSLocalizedString( @"Delete", @"" ) action:@selector(delete:) keyEquivalent:@""];
[self selectRowIndexes:[NSIndexSet indexSetWithIndex:row] byExtendingSelection:NO];
//Set the Delete menu item's represented object to the clicked-on item. If the user chooses this item, we'll retrieve its represented object so we know what to delete.
[delete setRepresentedObject:[self itemAtRow:row]];
return menu;
}
This assumes we're compiling with ARC, so you don't need to autorelease the menu object being created.