canvas does not allow drawing

mohsen24000 · Post by **mohsen24000** » Sat Dec 22, 2012 3:51 pm

Hello dear,
when i use 'chat demo' in TIdUDPServer onRead event:

procedure TfrmMain.UpdateChat(msg: string);
begin
  ParseString(mChat,msg); // from chat demo
  mChat.Invalidate;
try
  mChat.Format;
except
  mChat.FormatTail;
end;
  mChat.Update;
  mChat.DeleteUnusedStyles(True, True, True);
  mChat.VScrollPos:= mChat.VScrollMax;
end;

this error Sometimes appear:
"canvas does not allow drawing"
[/code][/b]

Post by **Sergey Tkachenko** » Sat Dec 22, 2012 4:06 pm

Is this code called in the context of the main thread?

mohsen24000 · Post by **mohsen24000** » Sat Dec 22, 2012 6:52 pm

Sergey Tkachenko wrote:Is this code called in the context of the main thread?

yes.

Post by **Sergey Tkachenko** » Sun Dec 23, 2012 2:14 pm

What's the value of ThreadedEvent property?
(sorry for double checking)

mohsen24000 · Post by **mohsen24000** » Sun Dec 23, 2012 2:27 pm

Oh!
It was True and change it False and all things right.
i think resolved.
Thanks a lot.
--------------------------------
Is it possible to correct this problem with Threads!?

Post by **Sergey Tkachenko** » Wed Dec 26, 2012 12:08 pm

VCL is not thread-safe, any drawing must be made in the context of the main process. This is not only for our components, this is a VCL design.
If you need to initiate redrawing from a thread, use Synchronize.