// Purpose: topic overview
// Author: wxWidgets team
// RCS-ID: $Id$
-// Licence: wxWindows license
+// 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
+@page overview_thread Multithreading Overview
+
+Classes: wxThread, wxThreadHelper, wxMutex, wxCriticalSection, wxCondition, wxSemaphore
-Classes: wxThread, wxMutex, wxCriticalSection, wxCondition
+
+@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 i
+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 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
+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 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).
+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
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.
*/
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