[fpc-pascal] PostMessage() from a thread (in a thread save manner)

[Graeme Geldenhuys]

Some developers prefer to pass in a GUI component to a thread
instance, so the thread knows what widget to update. Other users
prefer to post a "gui update" message to the application event queue.

I'm very new to threads and might know just enough to make in
dangerous. :-)  In fpGUI the application's "custom" event queue is a
linked-list structure.  If threads post messages to that queue, I
obviously need to make the queue manipulation code thread safe.

fpgApplication is a global singleton of the TfpgApplication class.
fpgPostMessage() is a stand-alone function (not a member of
TfpgApplication class).  To make fpgPostMessage() thread save, I
instantiated a TCriticalSection instance in the TfpgApplication class
- in the constructor. I added two member functions .Lock and .Unlock
which calls FCritSect.Enter and FCritSect.Leave respectively.  In
fpgPostMessage, I call fpgApplication.Lock, then manipulate the
message queue and then call fpgApplication.Unlock.

Is this the correct way to make my event queue thread save?

eg:

procedure fpgPostMessage(Sender, Dest: TObject; MsgCode: integer;
    var aparams: TfpgMessageParams); overload;
var
  p: PfpgMessageRec;
begin
  if Dest = nil then
    Exit; //==>
  fpgApplication.Lock;   // Enter the critical section
  try
    p := fpgAllocateMessage;
    if p <> nil then
    begin
      p^.MsgCode := MsgCode;
      p^.Sender  := Sender;
      p^.Dest    := Dest;
      p^.Params  := aparams;
    end
    else
      DebugLn('THE MESSAGE QUEUE IS FULL.');
  finally
    fpgApplication.Unlock;  // leave the critical section
  end;
end;


Regards,
  - Graeme -


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fpGUI - a cross-platform Free Pascal GUI toolkit
http://opensoft.homeip.net/fpgui/