Example 3: Reacting to a checkbox toggle

Uses OnCheckChange to keep a related field in sync whenever a boolean field is toggled from the grid, and to optionally veto the change.

Scenario

An Active boolean field. When a row is switched off, its Priority field (an integer, used for a separate work queue) should be cleared to zero at the same time, so both fields land in the dataset together.

The handler

procedure TfrmOrders.GridCheckChange(Column: TColumn);
begin
  if Column.FieldName <> 'Active' then Exit;
  // OnCheckChange fires BEFORE the field is flipped: Column.Field.AsBoolean
  // is still the OLD value here, so the value about to be set is its
  // negation, not the value already in the field.
  //
  // The dataset is also not yet in edit state at this point (see "Why it
  // works this way" below) - Table.Edit is required before touching
  // Priority, or this raises "Dataset not in edit or insert mode".
  // Calling Edit when already editing is harmless, so this is always safe.
  Table.Edit;
  if Column.Field.AsBoolean then  // about to become False
    Table.FieldByName('Priority').AsInteger := 0;
end;

Why it works this way

• Timing: the event name reads like something that already happened, but it fires before the toggle - Column.Field.AsBoolean above is still the value the checkbox showed before this click, and the dataset is not even in edit state yet at this point. Reason about the checkbox as “about to become not Column.Field.AsBoolean”, not “has just become something”.
• Table.Edit is required here: unlike a normal keystroke edit, the grid only puts the dataset into edit state for the Active field’s own toggle after OnCheckChange returns - not before. Writing to Priority without calling Edit first raises an exception. Both fields still end up posted together afterward, in the grid’s own automatic Post call once the checkbox’s own toggle completes - no separate Table.Post is needed in the handler, only the Edit.
• Vetoing indirectly: there is no boolean “cancel” parameter - the way to stop the toggle is to raise an exception from the handler (for example, after a failed business-rule check). The grid does not catch it, so the exception propagates and the field is never actually flipped.

procedure TfrmOrders.GridCheckChange(Column: TColumn);
begin
  if (Column.FieldName = 'Active') and Column.Field.AsBoolean
    and (Table.FieldByName('OpenTickets').AsInteger > 0) then
    raise Exception.Create('Nao e possivel desactivar: existem tickets em aberto.');
end;

Also available per column

TEgColumn.OnCheckChange is the column-scoped equivalent, firing after the grid-level handler when both are assigned - attach it directly to the Active column instead of checking Column.FieldName in a grid-wide handler, if this logic only ever applies to that one column.

See Also

• OnCheckChange
• CheckBoxes
• TEgDBGridCheckChangeEvent
• Examples