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

[Lukas Gradl]

Graeme Geldenhuys schrieb:
> 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?

It is - theoretically. I use threads quite a lot and found it a constant 
source of problems, to create a deadlock here and there when using the 
messagequeue or to forget some lock/unlock somewhere. (you will need 
more than PostMessage for your queue! *bg*)

I prefer to implement the messaging in the object it belongs to - in 
this case on fpgApplication. The Queue will not be visible from outside, 
only funcions like PostMessage, GetMessage or DiscardMessage. This way 
you can make shure, the lock/unlock sequence will be used correctly...

Regards
Lukas
(Just seen that Vincent was a bit faster than me, but with quite the 
same thougts...)


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