Keep values from the beforepost event to the afterpost event - delphi

Question

I am writing this question for Delphi 2007, but I'm pretty sure that this is a common problem in all kind of languages.
So, I have a project where I need to keep informations about the old and new value of certain fields (which are given in the BeforePost event of the dataset I'm working with) and use them in the AfterPost event.
For now, I have been using global variables, but there is already so many of them in the project that this is becoming a real issue when it comes to managing documentation and/or comments.
Basically, I am asking if there is a better way (in Delphi 2007 or in general) to keep the informations from the BeforePost event of a Dataset and get them back in the AfterPost event.

Answer 1

first create a new Custom Data Source
TDataRecord = array of record
FieldName: string;
FieldValue: Variant;
end;
TMyDataSource = class(TDataSource)
private
LastValues: TDataRecord;
procedure MyDataSourceBeforePost(DataSet: TDataSet);
procedure SetDataSet(const Value: TDataSet);
function GetDataSet: TDataSet;
public
constructor Create(AOwner: TComponent); override;
destructor Destroy; override;
function GetLastValue(FieldName: string): Variant;
property MyDataSet: TDataSet read GetDataSet write SetDataSet;
end;
{ TMyDataSource }
constructor TMyDataSource.Create(AOwner: TComponent);
begin
inherited Create(AOwner);
end;
destructor TMyDataSource.Destroy;
begin
SetLength(LastValues, 0);
inherited Destroy;
end;
function TMyDataSource.GetDataSet: TDataSet;
begin
Result := DataSet;
end;
procedure TMyDataSource.SetDataSet(const Value: TDataSet);
begin
DataSet := Value;
DataSet.BeforePost := MyDataSourceBeforePost;
end;
procedure TMyDataSource.MyDataSourceBeforePost(DataSet: TDataSet);
var
i: integer;
begin
SetLength(LastValues, DataSet.FieldCount);
for i:=0 to DataSet.FieldCount-1 do
begin
LastValues[i].FieldName := DataSet.Fields.Fields[i].FieldName;
LastValues[i].FieldValue := DataSet.Fields.Fields[i].OldValue;
end;
end;
function TMyDataSource.GetLastValue(FieldName: string): Variant;
var
i: integer;
begin
Result := Null;
for i:=0 to Length(LastValues)-1 do
if SameText(FieldName, LastValues[i].FieldName) then
begin
Result := LastValues[i].FieldValue;
break;
end;
end;
and after override application Data Source
TForm1 = class(TForm)
private
MyDataSource: TMyDataSource;
end;
procedure TForm1.FormCreate(Sender: TObject);
begin
ADOQuery1.Active := true;
MyDataSource := TMyDataSource.Create(Self);
MyDataSource.MyDataSet := ADOQuery1;
DBGrid1.DataSource := MyDataSource;
end;
procedure TForm1.FormDestroy(Sender: TObject);
begin
MyDataSource.Free;
end;
procedure TForm1.ADOQuery1AfterPost(DataSet: TDataSet);
var
AValue: Variant;
begin
AValue := MyDataSource.GetLastValue('cname');
if not VarIsNull(AValue) then;
end;