Graeme Geldenhuys wrote:
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?
Yes and no. Allocating the message from the queue might be thread safe
now, but I strongly suspect that other threads are trying to do
something with the queue (like evaluating or removing messages).
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;
So this piece of code is probably not enough to make the queue thread
safe. IMO it would be best to encapsulate any thread-safety inside the
queue class itself, instead of sprinkling "fpgApplication.{Un}Lock" all
over the source. (I know, this can be inefficient, but when having the
choice between "efficient" or "correct", I'd rather go for "correct".)
Vinzent.
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