Delphi XE2 DataSnap - Download File via TStream Wi

Question

I've written a DataSnap server method that returns a TStream object to transfer a file. The client application calls the method and reads the stream fine. My issue is that the method call takes a while to complete before the TStream object is available to read, but on the server side I can see that the method call only takes a second to create the object to return. I was hoping the stream object would be returned immediately so that I can read the stream and display a progress bar for the download progress. Is there another way I can do this?

The server method is very simple :

function TServerMethods.DespatchDocument(sCompanyID, sDocOurRef: string): TStream;
var
  sSourceFilePath: string;
  strFileStream: TFileStream;
begin
  sSourceFilePath := GetDocumentPDFFilePath(sCompanyID, sDocOurRef);

  strFileStream := TFileStream.Create(sSourceFilePath, fmOpenRead);
  Result := strFileStream;
end;

Answer 1

This is how I did it a while back. I used XE and haven't had a chance to clean it up.

//Server side:

function TServerMethods1.DownloadFile(out Size: Int64): TStream;
begin
    Result := TFileStream.Create('upload.fil', fmOpenRead or fmShareDenyNone);
    Size := Result.Size;

    Result.Position := 0;
end;

//Client side:

procedure TfMain.DownloadFile(Sender: TObject);
var
    RetStream: TStream;
    Buffer: PByte;
    Mem: TMemoryStream;
    BytesRead: Integer;
    DocumentId: Int64;
    Size: Int64;
    filename: WideString;
    BufSize: Integer;
begin
    BufSize := 1024;

    try
      Mem := TMemoryStream.Create;
      GetMem( Buffer, BufSize );

      try
        RetStream := FDownloadDS.DownloadFile(Size);
        RetStream.Position := 0;

        if ( Size <> 0 ) then
        begin
          filename := 'download.fil';

          repeat
            BytesRead := RetStream.Read( Pointer( Buffer )^, BufSize );

            if ( BytesRead > 0 ) then
            begin
              Mem.WriteBuffer( Pointer( Buffer )^, BytesRead );
            end;

            lStatus.Caption := IntToStr( Mem.Size ) + '/' + IntToStr( Size );
            Application.ProcessMessages;

          until ( BytesRead < BufSize );

          if ( Size <> Mem.Size ) then
          begin
            raise Exception.Create( 'Error downloading file...' );
          end;
        end
        else
        begin
          lStatus.Caption := '';
        end;
      finally
        FreeMem( Buffer, BufSize );
        FreeAndNIl(Mem);
      end;
    except
      on E: Exception do
      begin
        lErrorMessage.Caption := PChar( E.ClassName + ': ' + E.Message );
      end;
    end;
end;

You can adjust BufSize however you like. I was having trouble getting the size of the stream until I did it this way. I experimented with XE2 and didn't seem to have the same problem but I was uploading. There is probably a better way to retrieve the size of the stream. If I get the answer soon I'll let you know....

On another note - I haven't figured out how to display a progress bar on the server side. I'm still trying to figure this out too.

I hope this helps! Let me know if you have any questions!

Answer 2

Glad you have some luck! This is the other fix I had to do. You can refer to this link https://forums.embarcadero.com/thread.jspa?threadID=66490&tstart=0

After diving in the code I found in "Data.DBXJSONReflect.pas"

procedure TJSONPopulationCustomizer.PrePopulate(Data: TObject; rttiContext: TRttiContext); ...

3473: rttiField.GetValue(Data).AsObject.Free;
3474: rttiField.SetValue(Data, TValue.Empty);

...

I think it should be this way:

3473: rttiField.SetValue(Data, TValue.Empty);
3474: rttiField.GetValue(Data).AsObject.Free;