X-Git-Url: https://git.saurik.com/wxWidgets.git/blobdiff_plain/d230488b29e48d130a8883215db18fbd92daf48c..a25a17732c2d4d34d5d35a5318053dd8c6e29722:/docs/doxygen/overviews/thread.h?ds=sidebyside diff --git a/docs/doxygen/overviews/thread.h b/docs/doxygen/overviews/thread.h index 9dae705ae1..4880efbeea 100644 --- a/docs/doxygen/overviews/thread.h +++ b/docs/doxygen/overviews/thread.h @@ -6,52 +6,85 @@ // Licence: wxWindows license ///////////////////////////////////////////////////////////////////////////// -/*! - - @page overview_thread Multithreading overview - - 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 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 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 @ref 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::AddPendingEvent or its short version #wxPostEvent. 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. +/* + NOTE: we explicitely 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 no secondary threads call GUI functions. +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. */