How to update GUI from Thread using Delphi

Question

I am using Delphi anonymous thread to execute code. In the middle of the thread, a couple of GUI updates have to take place, a couple of labels changing etc.

If I do

Answer 1

Use TThread.Synchronize and pass another anonymous function to it. Then you can call updateui in the anonymous function:

TThread.CreateAnonymousThread(
  procedure
  begin
    // do whatever you want

    TThread.Synchronize(nil,
      procedure
      begin
        updateui();
      end);

   // do something more if you want
  end
).Start();

Synchronizations are generally expensive (regarding performance). Only do them when they are really neccessary. You can increase the performance if you extend the updateui-method to reduce paint-operations.

This is possible to a call to SendMessage with WM_SETREDRAW:

procedure StopDrawing(const Handle: HWND);
const
  cnStopDrawing = 0;
begin
  SendMessage(Handle, WM_SETREDRAW, cnStopDrawing, 0);
end;

procedure ContinueDrawing(const Handle: HWND);
const
  cnStartDrawing = 1;
begin
  SendMessage(Handle, WM_SETREDRAW, cnStartDrawing, 0);

  // manually trigger the first draw of the window
  RedrawWindow(Handle, nil, 0,
    RDW_ERASE or RDW_FRAME or RDW_INVALIDATE or RDW_ALLCHILDREN);
end;

Add a call to StopDrawing() at the top of updateui() and a call to ContinueDrawing() at the end of updateui(). The call to ContinueDrawing() should be in a finally-block. This will ensure that the window will be painted even after an exception has occured during the execution of updateui.

Example:

procedure TMain.updateui;
begin
  try
    StopDrawing(Handle);

    Panel3.Show;

    Label57.Caption := 'Change 1';
    Label59.Caption := 'Change 2';

    // ...
  finally
    // Code under finally gets executed even if there was an error
    ContinueDrawing(Handle);
  end;
end;