Technical Q&A QA1503

Preventing column reordering in NSTableView

Q: How do I prevent a particular column from being reordered or moved in my NSTableView?

A: You can prevent the moving of a column by implementing the NSTableView delegate method tableView:mouseDownInHeaderOfTableColumn: and responding to NSTableViewColumnDidMoveNotification. The method tableView:mouseDownInHeaderOfTableColumn: will prevent the first column from being dragged, and tableViewColumnDidMove: can be written to restore the original order of the columns if the user attempts to move any column to the left of column 0.

For example, assume the first NSTableColumn (column 0) identifier is set to "FirstName".

Listing 1: Block the NSTableView from dragging the first column using the NSTableView delegate method.

-(void)tableView: (NSTableView*)inTableView mouseDownInHeaderOfTableColumn:(NSTableColumn*)tableColumn
{
    if ([[tableColumn identifier] isEqualToString:@"FirstName"])
    {
        [inTableView setAllowsColumnReordering:NO];
    }
    else
    {
        [inTableView setAllowsColumnReordering:YES];
    }
}

In your awakeFromNib or init method use the following:

[[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(tableViewColumnDidMove:) name:NSTableViewColumnDidMoveNotification object:nil];

Listing 2: Reorder the columns after a drag reorder by responding to NSTableViewColumnDidMoveNotification.

- (void)tableViewColumnDidMove:(NSNotification*)aNotification
{
    NSDictionary* userInfo = [aNotification userInfo];

    // if the user tries to move the first column out, move it back
    if ([[userInfo objectForKey:@"NSOldColumn"] intValue] == 0)
    {
         // temporarily stop listening to column moves to prevent recursion
         [[NSNotificationCenter defaultCenter] removeObserver:self
                name:NSTableViewColumnDidMoveNotification object:nil];

         // move the column
         [tableView moveColumn:[[userInfo objectForKey:@"NSNewColumn"] intValue] toColumn:0];

         // listen again for column moves
         [[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(tableViewColumnDidMove:)
                name:NSTableViewColumnDidMoveNotification object:nil];
    }
    // if the user tries to move a column in front of the first column, move it back
    else if ([[userInfo objectForKey:@"NSNewColumn"] intValue] == 0)
    {
         // temporarily stop listening to column moves to prevent recursion
         [[NSNotificationCenter defaultCenter] removeObserver:self
                name:NSTableViewColumnDidMoveNotification object:nil];

         // move the column
         [tableView moveColumn:0 toColumn:[[userInfo objectForKey:@"NSOldColumn"] intValue]];

         // listen again for column moves
         [[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(tableViewColumnDidMove:)
                name:NSTableViewColumnDidMoveNotification object:nil];
    }
}

Document Revision History

Posted: 2007-01-24