Delphi: Stop a download using IdHttp and Thread

Question

I have a thread defined THttpThread for downloading of a file. I would like to stop the download if modal form is closed or if Cancel button is pressed.

In the example from bellow I got Access Violation probably because of the way how I reuse the thread.

procedure Tform_update.button_downloadClick(Sender: TObject);
var
  HttpThread: THttpThread;
begin
  //download
  if button_download.Tag = 0 then
    begin
      HttpThread:= THttpThread.Create(True);
      //...
      HttpThread.Start;
    end
  //cancel download
  else
    begin
      HttpThread.StopDownload:= True;
    end;
end;

I sow the answers from How stop (cancel) a download using TIdHTTP and some many others but I still don't get it how to update a property of a running thread.

Answer 1

I will give the answer found, using also the hints from users comments.

The access violation comes from the fact HttpThread was not assigned during cancellation. Reason why HttpThread: THttpThread must be defined under his form like:

Tform_update = class(TForm)
//...
private
  HttpThread: THttpThread;

and then the code should be:

procedure Tform_update.button_downloadClick(Sender: TObject);
begin
  //download
  if button_download.Tag = 0 then
    begin
      HttpThread:= THttpThread.Create(True);
      //...
      HttpThread.Start
    end
  //cancel download
  else
    begin
      if Assigned(HttpThread) then HttpThread.StopDownload:= True;
    end;
end;

Same for form close

procedure Tform_update.FormClose(Sender: TObject; var Action: TCloseAction);
begin
  if Assigned(HttpThread) then HttpThread.StopDownload:= True;
end;

As some users ask for in some comments, there is no need for the code of thread.