Смотрите книгу "Developing Custom Delphi Components" от Рэя Конопки.
Здесь немного исправленный пример из этой книги -
function TMyClass.CreateCalcField( const AFieldName:
string; AFieldClass: TFieldClass; ASize: Word ): TField;
begin
Result := FDataSet.FindField( AFieldName ); // Field may
already exists!
if Result<>nil then Exit;
if AFieldClass = nil then
begin
DBErrorFmt( SUnknownFieldType, [AFieldName] );
end;
Result := FieldClass.Create( Owner );
with Result do
try
FieldName := AFieldName;
if ( Result is TStringField ) or ( Result is TBCDField )
or ( Result is TBlobField ) or ( Result is TBytesField ) or (
Result is TVarBytesField ) then
begin
Size := ASize;
end;
Calculated := True;
DataSet := FDataset;
Name := FDataSet.Name + AFieldName;
except
Free; // We must release allocated memory on error!
raise;
end;
end;
Alex Konshin
(2:5030/217.217)