How to Thread-safe a ClientDataSet on a worker thread? - multithreading

Question

I have a really slow query that always make windows mark my program as Not responding. I decided to create a background worker to perform this query while the main thread show a GIF. I did everything and it works! =D
But... When I close my Form I got a EInvalidPointer Exception, ONLY when I use the worker thread.
Here is my code:
Main thread Call to worker thread
if not TThreadDB.ExecutarThreadDB(cdsSolicitacao,
FConsultaSql,
nil,
tpHigher) then
begin
Exit;
end;
Where:
cdsSolicitacao is a clientDataSet I want to share between the threads,
FConsultaSql string (my query)
My thread Unit
unit UThreadDB;
interface
uses Classes, DBClient, DB, SysUtils, Variants, JvJCLUtils;
type
TParametros = class
private
FTotal: Integer;
FCampo: array of string;
FTipo: array of TFieldType;
FValor: array of Variant;
public
constructor Create(ACampos: string; ATipos: array of TFieldType; AValores: array of Variant); reintroduce;
end;
TThreadDB = class(TThread)
private
FExecutou: Boolean;
FClientDataSet: TClientDataSet;
FConsultaSQL: string;
FParametros: TParametros;
procedure CarregarDados;
protected
procedure Execute; override;
public
constructor Create(ACreateSuspended: Boolean; AClientDataSet: TClientDataSet; AConsultaSQL: string = '';
AParametros: TParametros = nil); reintroduce;
class function ExecutarThreadDB(AClientDataSet: TClientDataSet; AConsultaSQL: string = '';
AParametros: TParametros = nil; APriority: TThreadPriority = tpNormal): Boolean;
class procedure ExecutarThreadDBParalela(AThreadDB: TThreadDB; AClientDataSet: TClientDataSet;
AConsultaSQL: string = ''; AParametros: TParametros = nil; APriority: TThreadPriority = tpNormal);
end;
implementation
uses
BIBLIO;
{ TThreadDB }
class function TThreadDB.ExecutarThreadDB(AClientDataSet: TClientDataSet; AConsultaSQL: string = '';
AParametros: TParametros = nil; APriority: TThreadPriority = tpNormal): Boolean;
var
lThreadDB: TThreadDB;
begin
lThreadDB := TThreadDB.Create(True, AClientDataSet, AConsultaSQL, AParametros);
try
//lThreadDB.FreeOnTerminate := True;
lThreadDB.Priority := APriority;
lThreadDB.Resume;
lThreadDB.WaitFor;
Result := lThreadDB.FExecutou;
finally
lThreadDB.Terminate;
//lThreadDB := nil;
FreeAndNil(lThreadDB);
end;
end;
class procedure TThreadDB.ExecutarThreadDBParalela(AThreadDB: TThreadDB; AClientDataSet: TClientDataSet;
AConsultaSQL: string = ''; AParametros: TParametros = nil; APriority: TThreadPriority = tpNormal);
begin
AThreadDB := TThreadDB.Create(True, AClientDataSet, AConsultaSQL, AParametros);
AThreadDB.FreeOnTerminate := True;
AThreadDB.Priority := APriority;
AThreadDB.Resume;
end;
procedure TThreadDB.CarregarDados;
var
lIndex: Integer;
begin
FClientDataSet.Close;
try
if (FConsultaSQL <> '') then
begin
FClientDataSet.CommandText := FConsultaSQL;
end;
if (FParametros <> nil) then
begin
for lIndex := 0 to (FParametros.FTotal - 1) do
begin
case FParametros.FTipo[lIndex] of
ftInteger : FClientDataSet.Params.ParamByName(FParametros.FCampo[lindex]).AsInteger := FParametros.FValor[lIndex];
ftString : FClientDataSet.Params.ParamByName(FParametros.FCampo[lindex]).AsString := FParametros.FValor[lIndex];
ftDate : FClientDataSet.Params.ParamByName(FParametros.FCampo[lindex]).AsDate := FParametros.FValor[lIndex];
end;
end;
end;
FClientDataSet.Open;
FExecutou := True;
except
on E: Exception do
begin
Erro('Não foi possível carregar os dados solicitados!' + #13 +
'Classe do erro: ' + E.ClassName + #13 +
'Mensagem: ' + E.Message);
end;
end;
if (FParametros <> nil) then
begin
FreeAndNil(FParametros);
end;
end;
constructor TThreadDB.Create(ACreateSuspended: Boolean; AClientDataSet: TClientDataSet; AConsultaSQL: string = '';
AParametros: TParametros = nil);
begin
inherited Create(ACreateSuspended);
FClientDataSet := AClientDataSet;
FConsultaSQL := AConsultaSQL;
FParametros := AParametros;
FExecutou := False;
end;
procedure TThreadDB.Execute;
begin
CarregarDados;
end;
{ TParametros }
constructor TParametros.Create(ACampos: string; ATipos: array of TFieldType; AValores: array of Variant);
var
lIndex: Integer;
begin
inherited Create;
FTotal := ContaCaracteres(ACampos, ';') + 1;
SetLength(FCampo, FTotal);
SetLength(FTipo, FTotal);
SetLength(FValor, FTotal);
for lIndex := 0 to FTotal - 1 do
begin
FCampo[lIndex] := ExtractDelimited(lIndex + 1, ACampos , [';']);
end;
for lIndex := 0 to FTotal - 1 do
begin
FTipo[lIndex] := ATipos[lIndex];
end;
for lIndex := 0 to FTotal - 1 do
begin
FValor[lIndex] := AValores[lIndex];
end;
end;
end.
Any ideas of what I'm missing?

Answer 1

You haven't mentioned that in your question, but I guess the problem you describe may be caused by modifying dataset object, which is at the same time consumed by the main thread (displayed in a grid, for example). In other words, that you're passing to a worker thread dataset, which is linked to some controls in your main thread. Or, yet another way described, your cdsSolicitacao dataset object is linked through data source object to some control(s) on your main form.
Do note that main thread cannot ever work with an object that is being modified by a worker thread.
And even modal animation won't stop main thread from consuming that just modified dataset. For example DB grid can be requested for repaint which needs access to its underlying dataset which is at the same time being modified by a worker thread.
If that is your case, and you don't want to create a new dataset instance that you then replace with the consumed one when the worker thread finishes,
you'll need to (ideally) disconnect that dataset object from every linked control before passing to the worker thread and reconnect when finishes.