I have a problem with NSArrayController : I have a NSTableView and a NSArrayController that is bound to. There are, for example, 5 values for each NSManagedObject of the NSArrayController. But what I want is to have a 6th column, calculated ,and that depends on the other Objects…

To try to be clear, each object is a financial transaction, with an amount, and I want my NSTableView to print a balance for each transaction, indicating the account balance up to this particular transaction. That is : that balance value depends on the previous transaction and the current transaction amount.

I’ve tried to add a instance variable in the Transaction Class :

• initialize { …

    NSArray *keys = [NSArray arrayWithObjects: @"amount", nil];
    [self setKeys:keys triggerChangeNotificationsForDependentKey:@"balance"];
    ... }
  

• balance { … }

But from the balance method I cannot reach the previous Transaction Object. So I think what I want should be implemented in a NSArrayController subclass.

Does anyone have an idea about it ?

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Using a controller and bindings for some columns in your table does not prevent you from using a traditional data source for your other columns.

I’d recommend that you create a data source object that has a pointer to the array controller, and calculate your value on-demand inside tableView:objectValueForTableColumn:row: by examining the array controller’s arrangedObjects.

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Many thanks, that worked. I linked a delegate to my tableview, and coded the following method in it :

• (id)tableView:(NSTableView *)aTableView objectValueForTableColumn:(NSTableColumn *)aTableColumn row:(int)rowIndex { if (aTableColumn identifier] isEqualToString:@”balance”]) { [[NSNumber *currentAmount = [transactionArrayController arrangedObjects] objectAtIndex:rowIndex] valueForKey:@”amount”]; [[NSNumber *previousBalance; NSNumber *balance;

    if (rowIndex < 0)
        return nil;
  
    // Balance is first amount
    if (rowIndex == 0)
        balance = currentAmount;
    else {
        previousBalance = [transactionArrayController arrangedObjects] objectAtIndex:rowIndex - 1] valueForKey:@"balance"];
        balance = [[[NSNumber numberWithDouble: [previousBalance doubleValue] + [currentAmount doubleValue]];
    }
  
    [[[transactionArrayController arrangedObjects] objectAtIndex:rowIndex] setValue:balance forKey:@"balance"];
  
    return balance;   }
  

  return nil; }