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type
  TQueryThread = class(TThread)
  private
    FConnection: TIBCConnection;
    FTransaction : TIBCTransaction;
    FQuery: TLFQuery;
  public
    procedure Execute ; override;
    function CancelOpening : Boolean;
    constructor Create;
    destructor Destroy; Override;
  end;

var
  Form1: TForm1;
  QueryThread: TQueryThread;

implementation

{$R *.dfm}

constructor TQueryThread.Create;
begin
  inherited Create(False);

  FreeOnTerminate := True;

  FConnection := TIBCConnection.Create(Nil);
  FConnection.Database := 'D:\Banco de Dados\Casual\Banco Antigo\DATA-EXPERT.DAE';
  FConnection.Username := 'SYSDBA';
  FConnection.Password := 'masterkey';
  FConnection.Connected := True;

  FTransaction := TIBCTransaction.Create(Nil);
  FTransaction.DefaultConnection := FConnection;
  FTransaction.Active := True;

  FQuery := TLFQuery.Create(nil);
  FQuery.Connection := FConnection;
  FQuery.Transaction := FTransaction;
  FQuery.SQL.Text := Form1.Memo1.Text;

end;

destructor TQueryThread.Destroy;
begin
  FQuery.Free;
  FConnection.Free;

  inherited;
end;

function TQueryThread.CancelOpening : boolean;
begin
  Result := not FQuery.Fetched;
  if Result then FQuery.BreakExec; // Error invalid database handle (no active connection)
end;

procedure TQueryThread.Execute;
begin
 inherited;

 FQuery.Open;

end;

procedure TForm1.Btn_OpenClick(Sender: TObject);
begin
 QueryThread := TQueryThread.Create;

end;

procedure TForm1.Btn_CancelClick(Sender: TObject);
begin
 if QueryThread.CancelOpening then
  Showmessage('Cancelou')
 Else
  Showmessage('Executando');

end;