3a5677401f
git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@59002 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775
91 lines
4.4 KiB
C
91 lines
4.4 KiB
C
/////////////////////////////////////////////////////////////////////////////
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// Name: thread.h
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// Purpose: topic overview
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// Author: wxWidgets team
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// RCS-ID: $Id$
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// Licence: wxWindows license
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/////////////////////////////////////////////////////////////////////////////
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/*
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NOTE: we explicitely don't name wxMutexGUIEnter() and wxMutexGUILeave()
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as they're not safe. See also ticket #10366.
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*/
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/**
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@page overview_thread Multithreading Overview
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Classes: wxThread, wxThreadHelper, wxMutex, wxCriticalSection, wxCondition, wxSemaphore
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@section overview_thread_intro When to use multiple threads
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wxWidgets provides a complete set of classes encapsulating objects necessary in
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multithreaded (MT) programs: the wxThread class itself and different
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synchronization objects: mutexes (see wxMutex) and critical sections (see
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wxCriticalSection) with conditions (see wxCondition). The thread API in
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wxWidgets resembles to POSIX1.c threads API (a.k.a. pthreads), although several
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functions have different names and some features inspired by Win32 thread API
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are there as well.
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These classes hopefully make writing MT programs easier and they also
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provide some extra error checking (compared to the native - be it Win32 or
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Posix - thread API), however it is still a non-trivial undertaking especially
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for large projects.
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Before starting an MT application (or starting to add MT
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features to an existing one) it is worth asking oneself if there is no easier
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and safer way to implement the same functionality.
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Of course, in some situations threads really make sense (classical example is a
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server application which launches a new thread for each new client), but in others
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it might be an overkill.
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On the other hand, the recent evolution of the computer hardware shows
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an important trend towards multi-core systems, which are better exploited using
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multiple threads (e.g. you may want to split a long task among as many threads
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as many CPU (cores) the system reports; see wxThread::GetCPUCount).
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To implement non-blocking operations @e without using multiple threads you have
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two possible implementation choices:
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- use wxIdleEvent (e.g. to perform a long calculation while updating a progress dialog)
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- do everything at once but call wxWindow::Update() or wxApp::YieldFor(wxEVT_CATEGORY_UI)
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periodically to update the screen.
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If instead you choose to use threads in your application, please read also
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the following sections of this overview.
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@section overview_thread_notes Important notes for multithreaded applications
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When writing a multi-threaded application, it is strongly recommended
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that <b>no secondary threads call GUI functions</b>.
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The design which uses one GUI thread and several worker threads which communicate
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with the main one using @b events is much more robust and will undoubtedly save you
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countless problems (example: under Win32 a thread can only access GDI objects such
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as pens, brushes, device contexts created by itself and not by the other threads).
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For communication between secondary threads and the main thread, you may use
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wxEvtHandler::QueueEvent or its short version ::wxQueueEvent. These functions
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have a thread-safe implementation so that they can be used as they are for
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sending events from one thread to another. However there is no built in method
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to send messages to the worker threads and you will need to use the available
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synchronization classes to implement the solution which suits your needs
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yourself. In particular, please note that it is not enough to derive
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your class from wxThread and wxEvtHandler to send messages to it: in fact, this
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does not work at all.
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You're instead encouraged to use wxThreadHelper as it greatly simplifies the
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communication and the sharing of resources.
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You should also look at the wxThread docs for important notes about secondary
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threads and their deletion.
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Last, remember that if wxEventLoopBase::YieldFor() is used directly or indirectly
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(e.g. through wxProgressDialog) in your code, then you may have both re-entrancy
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problems and also problems caused by the processing of events out of order.
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To resolve the last problem wxThreadEvent can be used: thanks to its implementation
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of the wxThreadEvent::GetEventCategory function wxThreadEvent classes in fact
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do not get processed by wxEventLoopBase::YieldFor() unless you specify the
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@c wxEVT_CATEGORY_THREAD flag.
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See also the @sample{thread} for a sample showing some simple interactions
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between the main and secondary threads.
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*/
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