Author: Tomas Rutkauskas 

I'm having a problem sending messages to my threads. I can send back to parent form 
very easily with PostMessage, but I have tried to communicate to my threads via 
PostMessage and PostThreadMessage to no avail. I read some cryptic remarks in the 
PostThreadMessage help that seemed to indicate that I would have to induce the API 
into creating a message queue for the thread. Can anyone shed some light?

Answer:

1   type
2     TMyThread = class(TThread)
3       AHwnd: HWND;
4       procedure Execute; override;
5       procedure Terminate;
6       destructor Destroy; override;
7     end;
8   
9   procedure TMyThread.Execute;
10  var
11    msg: TMsg;
12    MyTerminated: Boolean;
13  begin
14    MyTerminated := False;
15    while not MyTerminated do
16    begin
17      WaitMessage;
18      if PeekMessage(Msg, 0, 0, 0, PM_REMOVE) then
19      begin
20        TranslateMessage(Msg);
21        case Msg.message of
22          WM_QUIT: MyTerminated := True;
23          WM_USER: PostMessage(AHwnd, WM_USER, 0, GetTickCount);
24        end;
25      end;
26    end;
27  end;
28  
29  procedure TMyThread.Terminate;
30  begin
31    PostThreadMessage(ThreadID, WM_QUIT, 0, 0);
32    inherited;
33  end;
34  
35  destructor TMyThread.Destroy;
36  begin
37    Terminate;
38    inherited;
39  end;
40  
41  var
42    MyThread: TMyThread;
43  
44  procedure TForm1.WMUser(var msg: TMessage); {message WM_USER;}
45  begin
46    Caption := IntToStr(msg.LParam);
47  end;
48  
49  procedure TForm1.Button1Click(Sender: TObject);
50  begin
51    MyThread := TMyThread.Create(False);
52    MyThread.AHwnd := Handle;
53  end;
54  
55  procedure TForm1.Button2Click(Sender: TObject);
56  begin
57    PostThreadMessage(MyThread.ThreadID, WM_USER, 0, 0);
58  end;
59  
60  procedure TForm1.Button3Click(Sender: TObject);
61  begin
62    MyThread.Free;
63  end;