Question

Below just a simple race between 2 button in 2 threads,

and this will freeze other components on the form.

procedure moveButton1();
var
  I: Integer;
begin
  for I := 0 to 6000 do
    Form1.Button1.Left := Form1.Button1.Left - 1;
  Form1.Caption := 'Button1 won!';
  EndThread(0);
end;


procedure moveButton2();
var
  I: Integer;
begin
  for I := 0 to 6000 do
    Form1.Button2.Left := Form1.Button2.Left - 1;
  Form1.Caption := 'Button2 won!';
  EndThread(0);
end;

procedure TForm1.Button3Click(Sender: TObject);
var
  thread1, thread2,tick : Integer;
  id1, id2 : LongWord;
begin
  thread1 := BeginThread(nil,
                         0,
                         Addr(moveButton1),
                         nil,
                         0,
                         id1);

thread2 := BeginThread(nil,
                         0,
                         Addr(moveButton2),
                         nil,
                         0,
                         id2);
  CloseHandle(thread1);
  CloseHandle(thread2);
end;

Answer 1

The VCL (and parts of the RTL) is not thread-safe, so you cannot move components around from a thread. You have several options:

• Use a TTimer component. You don’t need a thread and the timer’s event handler will be executed in the context of the main thread. The timer was designed exactly for things like that.
• Synchronize all VCL related stuff in the thread. TThread provides a static method Synchronize that does exactly that.
• Send messages from the thread to the form using SendMessage or PostMessage and handle this message in the form.

You might also consider to use the TThread wrapper class instead of using BeginThread and EndThreadexplicitly when working with threads.