I have a calculated filed 'Balance' which display the balance amount between two dataset fields and I used the next code to calculate its value:

var
  Balance : Currency;

...

procedure TMyForm.MyQueryCalcFields(DataSet: TDataSet);
begin
  if MyQuery.FieldByName('MyFirst Field').AsCurrency > 0 then    
    Balance := Balance MyQuery.FieldByName('MyFirst Field').AsCurrency;

  if MyQuery.FieldByName('MySecond Field').AsCurrency > 0 then   
    Balance := Balance-MyQuery.FieldByName('MySecond Field').AsCurrency;

  MyQuery.FieldByName('Balance').AsCurrency := Balance;
end; 

and I used TDBGrid to display the result of my query , The dbgrid display the correct values until I start srolling its vertical scrollbars, in this case it shows wrong values.

I tried assigning '0' to the balance but its the same (Wrong values when start scrolling).

How I can make the dbgrid keep showing the right values?

CodePudding user response：

OnCalcField event handler is designed for calculations that rely on the current record values only.

If the dataset's class you're using allows that, I suggest you to calculate the balance field once, as soon as you load the data, for example:

var
    Balance : Currency;
begin
    Balance := 0;

    MyQuery.DisableControls();
    try
        //loading data
        MyQuery.Open;
        
        //calculating balance
        while(not MyQuery.Eof) do
        begin
            MyQuery.Edit;
            Balance := Balance   MyQuery.FieldByName('MyFirst Field').AsCurrency - MyQuery.FieldByName('MySecond Field').AsCurrency;
            MyQuery.FieldByName('Balance').AsCurrency := Balance;
            MyQuery.Post;
            
            MyQuery.Next;
        end;
        MyQuery.First;
    finally
        MyQuery.EnableControls();
    end;
end;