Added a generic wxProgressDialog and made wxPostscriptPrinter use it.

[wxWidgets.git] / docs / latex / wx / tthreads.tex

\section{Multithreading overview}\label{wxthreadoverview}

Classes: \helpref{wxThread}{wxthread}, \helpref{wxMutex}{wxmutex}, 
\helpref{wxCriticalSection}{wxcriticalsection}, 
\helpref{wxCondition}{wxcondition}

wxWindows provides a complete set of classes encapsulating objects necessary in
multithreaded (MT) programs: the \helpref{thread}{wxthread} class itself and different
synchronization objects: \helpref{mutexes}{wxmutex} and 
\helpref{critical sections}{wxcriticalsection} with 
\helpref{conditions}{wxcondition}.

These classes will hopefully make writing MT programs easier and they also
provide some extra error checking (compared to the native (be it Win32 or Posix)
thread API), however it is still an untrivial undertaking especially for large
projects. Before starting an MT application (or starting to add MT features to
an existing one) it is worth asking oneself if there is no easier and safer way
to implement the same functionality. Of course, in some situations threads
really make sense (classical example is a server application which launches a
new thread for each new client), but in others it might be a very poor choice
(example: launching a separate thread when doing a long computation to show a
progress dialog). Other implementation choices are available: for the progress
dialog example it is far better to do the calculations in the 
\helpref{idle handler}{wxidleevent} or call \helpref{wxYield()}{wxyield}
periodically to update the screen.

If you do decide to use threads in your application, it is strongly recommended
that no more than one thread calls GUI functions. The thread sample shows that
it {\it is} possible for many different threads to call GUI functions at once
(all the threads created in the sample access GUI), but it is a very poor design
choice for anything except an example. The design which uses one GUI thread and
several worker threads which communicate with the main one using events is much
more robust and will undoubtedly save you countless problems (example: under
Win32 a thread can only access GDI objects such as pens, brushes, \&c created by
itself and not by the other threads).

Final note: in the current release of wxWindows, there are no specific
facilities for communicating between the threads. However, the usual 
\helpref{ProcessEvent()}{wxevthandlerprocessevent} function may be used for
thread communication too - but you should provide your own synchronisation
mechanism if you use it (e.g. just use a critical section before sending a
message) because there is no built-in synchronisation.