add wxCANCEL_DEFAULT support

[wxWidgets.git] / docs / doxygen / overviews / thread.h

/////////////////////////////////////////////////////////////////////////////
// Name:        thread.h
// Purpose:     topic overview
// Author:      wxWidgets team
// RCS-ID:      $Id$
// Licence:     wxWindows license
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/**

@page overview_thread Multithreading

Classes: wxThread, wxMutex, wxCriticalSection, wxCondition

wxWidgets provides a complete set of classes encapsulating objects necessary in
multithreaded (MT) programs: the wxThread class itself and different
synchronization objects: mutexes (see wxMutex) and critical sections (see
wxCriticalSection)  with conditions (see wxCondition). The thread API i
wxWidgets resembles to POSIX1.c threads API (a.k.a. pthreads), although several
functions have different names and some features inspired by Win32 thread API
are there as well.

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 a non-trivial 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 idle handler (see wxIdleEvent) or even simply do everything
at once but call wxWindow::Update() 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 @e 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).

For communication between secondary threads and the main thread, you may use
wxEvtHandler::QueueEvent or its short version ::wxQueueEvent. These functions
have a thread-safe implementation so that they can be used as they are for
sending events from one thread to another. However there is no built in method
to send messages to the worker threads and you will need to use the available
synchronization classes to implement the solution which suits your needs
yourself. In particular, please note that it is not enough to derive
your class from wxThread and wxEvtHandler to send messages to it: in fact, this
does not work at all.

*/