On Fri, 8 Oct 2010, Graeme Geldenhuys wrote:
Hi,
[Just as well the Lazarus IDE didn't get the thread update mentioned a
few days ago... where a thread was doing work, and the main thread was
waiting for that thread to finish... it wouldn't have worked under
Linux it seems.]
I just stumbled across this issue in fpGUI, and I see MSEgui and
LCL-GTK2 has the
exact same problem. See attached project. I haven't tried LCL-QT, but I presume
that same issue will persist.
A simple program (just for illustrative purposes), where a thread
updates the progress bar (the thread could be doing any long running process).
Click the button, it creates a thread and
starts it off. If then waits for the thread to finish, then displays
that it has finished in the GUI, and free's the thread manually.
If you use the MyThread.WaitFor to wait for the thread to finish, then
the application is instantly frozen (even the thread), nothing gets
updated and you have to kill the process.
This is normal, since WaitFor() is a blocking call.
You should never use WaitFor() in a GUI app, instead, you should set a
callback on the thread Terminate, and run the message queue while you're
waiting. It's not different from showing a modal form with ShowModal().
Procedure MyForm.OnThreadDone(Sender : TObject);
begin
FThreadDone:=true;
end;
Procedure MyForm.DoSomethingInThread;
begin
T:=TMythread.Create(True);
T.OnTerminate:=OnThreadDone;
DoSomethingElse;
While not FThreadDone do
begin
CheckSynchronize(10); // normally called in ProcessMessages.
Application.ProcessMessages;
end;
// Now thread is finished.
end;
We could alleviate this by allowing to hook into WaitForThreadTerminate and
with this provide 'run message queue' callbacks.
Michael.
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