Thread message loop for a thread with a hidden window?

Question

I have a Delphi 6 application that has a thread dedicated to communicating with a foreign application that uses SendMessage() and WM_COPYDATA messages to interface with external programs. Therefore, I create a hidden window with AllocateHWND() to service that need since a thread message queue won't work due to the SendMessage() function only accepting window handles, not thread IDs. What I'm not sure about is what to put in the thread Execute() method.

I assume that if I use a GetMessage() loop or a create a loop with a WaitFor*() function call in it that the thread will block and therefore the thread's WndProc() will never process the SendMessage() messages from the foreign program right? If so, what is the correct code to put in an Execute() loop that will not consume CPU cycles unnecessarily but will exit once a WM_QUIT message is received? I can always do a loop with a Sleep() if necessary but I'm wondering if there is a better way.

Answer 1

AllocateHWnd() (more specifically, MakeObjectInstance()) is not thread-safe, so you have to be careful with it. Better to use CreatWindow/Ex() directly instead (or a thread-safe version of AllocateHWnd(), like DSiAllocateHwnd().

In any case, an HWND is tied to the thread context that creates it, so you have to create and destroy the HWND inside your Execute() method, not in the thread's constructor/destructor. Also, even though SendMessage() is being used to send the messages to you, they are coming from another process, so they will not be processed by your HWND until its owning thread performs message retrieval operations, so the thread needs its own message loop.

Your Execute() method should look something like this:

procedure TMyThread.Execute;
var
  Message: TMsg;
begin
  FWnd := ...; // create the HWND and tie it to WndProc()...
  try
    while not Terminated do
    begin
      if MsgWaitForMultipleObjects(0, nil^, False, 1000, QS_ALLINPUT) = WAIT_OBJECT_0 then
      begin
        while PeekMessage(Message, 0, 0, 0, PM_REMOVE) do
        begin
          TranslateMessage(Message);
          DispatchMessage(Message);
        end;
      end;
    end;
  finally
    // destroy FWnd...
  end;
end;

procedure TMyThread.WndProc(var Message: TMessage);
begin
  if Message.Msg = WM_COPYDATA then
  begin
    ...
    Message.Result := ...;
  end else
    Message.Result := DefWindowProc(FWnd, Message.Msg, Message.WParam, Message.LParam);
end;

Alternatively:

// In Delphi XE2, a virtual TerminatedSet() method was added to TThread,
// which is called when TThread.Terminate() is called.  In earlier versions,
// use a custom method instead...

type
  TMyThread = class(TThread)
  private
    procedure Execute; override;
    {$IF RTLVersion >= 23}
    procedure TerminatedSet; override;
    {$IFEND}
  public
    {$IF RTLVersion < 23}
    procedure Terminate; reintroduce;
    {$IFEND}
  end;

procedure TMyThread.Execute;
var
  Message: TMsg;
begin
  FWnd := ...; // create the HWND and tie it to WndProc()...
  try
    while not Terminated do
    begin
      if WaitMessage then
      begin
        while PeekMessage(Message, 0, 0, 0, PM_REMOVE) do
        begin
          if Message.Msg = WM_QUIT then Break;
          TranslateMessage(Message);
          DispatchMessage(Message);
        end;
      end;
    end;
  finally
    // destroy FWnd...
  end;
end;

{$IF RTLVersion < 23}
procedure TMyThread.Terminate;
begin
  inherited Terminate;
  PostThreadMessage(ThreadID, WM_QUIT, 0, 0);
end;
{$ELSE}
procedure TMyThread.TerminatedSet;
begin
  PostThreadMessage(ThreadID, WM_QUIT, 0, 0);
end;
{$IFEND}