Merge in from trunk r68684 - r69046

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

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

/*
    NOTE: we explicitly don't name wxMutexGUIEnter() and wxMutexGUILeave()
          as they're not safe. See also ticket #10366.
*/

/**

@page overview_thread Multithreading Overview

Classes: wxThread, wxThreadHelper, wxMutex, wxCriticalSection, wxCondition, wxSemaphore


@section overview_thread_intro When to use multiple threads

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 in
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 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 an overkill.
On the other hand, the recent evolution of the computer hardware shows
an important trend towards multi-core systems, which are better exploited using
multiple threads (e.g. you may want to split a long task among as many threads
as many CPU (cores) the system reports; see wxThread::GetCPUCount).

To implement non-blocking operations @e without using multiple threads you have
two possible implementation choices:
- use wxIdleEvent (e.g. to perform a long calculation while updating a progress dialog)
- do everything at once but call wxWindow::Update() or wxApp::YieldFor(wxEVT_CATEGORY_UI)
  periodically to update the screen.
If instead you choose to use threads in your application, please read also
the following sections of this overview.

@section overview_thread_notes Important notes for multithreaded applications

When writing a multi-threaded application, it is strongly recommended
that <b>no secondary threads call GUI functions</b>.
The design which uses one GUI thread and several worker threads which communicate
with the main one using @b 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, device contexts 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.
You're instead encouraged to use wxThreadHelper as it greatly simplifies the
communication and the sharing of resources.

You should also look at the wxThread docs for important notes about secondary
threads and their deletion.

Last, remember that if wxEventLoopBase::YieldFor() is used directly or indirectly
(e.g. through wxProgressDialog) in your code, then you may have both re-entrancy
problems and also problems caused by the processing of events out of order.
To resolve the last problem wxThreadEvent can be used: thanks to its implementation
of the wxThreadEvent::GetEventCategory function wxThreadEvent classes in fact
do not get processed by wxEventLoopBase::YieldFor() unless you specify the
@c wxEVT_CATEGORY_THREAD flag.

See also the @sample{thread} for a sample showing some simple interactions
between the main and secondary threads.

*/