docs/doxygen/overviews/thread.h

   1 /////////////////////////////////////////////////////////////////////////////
   2 // Name:        thread.h
   3 // Purpose:     topic overview
   4 // Author:      wxWidgets team
   5 // Licence:     wxWindows licence
   6 /////////////////////////////////////////////////////////////////////////////
   7
   8 /*
   9     NOTE: we explicitly don't name wxMutexGUIEnter() and wxMutexGUILeave()
  10           as they're not safe. See also ticket #10366.
  11 */
  12
  13 /**
  14
  15 @page overview_thread Multithreading Overview
  16
  17 @tableofcontents
  18
  19 wxWidgets provides a complete set of classes encapsulating objects necessary in
  20 multi-threaded (MT) applications: the wxThread class itself and different
  21 synchronization objects: mutexes (see wxMutex) and critical sections (see
  22 wxCriticalSection) with conditions (see wxCondition). The thread API in
  23 wxWidgets resembles to POSIX1.c threads API (a.k.a. pthreads), although several
  24 functions have different names and some features inspired by Win32 thread API
  25 are there as well.
  26
  27 These classes hopefully make writing MT programs easier and they also provide
  28 some extra error checking (compared to the native - be it Win32 or Posix -
  29 thread API), however it is still a non-trivial undertaking especially for large
  30 projects. Before starting an MT application (or starting to add MT features to
  31 an existing one) it is worth asking oneself if there is no easier and safer way
  32 to implement the same functionality. Of course, in some situations threads
  33 really make sense (classical example is a server application which launches a
  34 new thread for each new client), but in others it might be an overkill. On the
  35 other hand, the recent evolution of the computer hardware shows an important
  36 trend towards multi-core systems, which are better exploited using multiple
  37 threads (e.g. you may want to split a long task among as many threads as many
  38 CPU (cores) the system reports; see wxThread::GetCPUCount).
  39
  40 To implement non-blocking operations @e without using multiple threads you have
  41 two possible implementation choices:
  42
  43 - use wxIdleEvent (e.g. to perform a long calculation while updating a progress dialog)
  44 - do everything at once but call wxWindow::Update() or wxApp::YieldFor(wxEVT_CATEGORY_UI)
  45   periodically to update the screen.
  46
  47 If instead you choose to use threads in your application, please read the
  48 following section of this overview.
  49
  50 @see wxThread, wxThreadHelper, wxMutex, wxCriticalSection, wxCondition,
  51      wxSemaphore
  52
  53
  54
  55 @section overview_thread_notes Important Notes for Multi-threaded Applications
  56
  57 When writing a multi-threaded application, it is strongly recommended that
  58 <b>no secondary threads call GUI functions</b>. The design which uses one GUI
  59 thread and several worker threads which communicate with the main one using
  60 @b events is much more robust and will undoubtedly save you countless problems
  61 (example: under Win32 a thread can only access GDI objects such as pens,
  62 brushes, device contexts created by itself and not by the other threads).
  63
  64 For communication between secondary threads and the main thread, you may use
  65 wxEvtHandler::QueueEvent or its short version ::wxQueueEvent. These functions
  66 have a thread-safe implementation so that they can be used as they are for
  67 sending events from one thread to another. However there is no built in method
  68 to send messages to the worker threads and you will need to use the available
  69 synchronization classes to implement the solution which suits your needs
  70 yourself. In particular, please note that it is not enough to derive your class
  71 from wxThread and wxEvtHandler to send messages to it: in fact, this does not
  72 work at all. You're instead encouraged to use wxThreadHelper as it greatly
  73 simplifies the communication and the sharing of resources.
  74
  75 You should also look at the wxThread docs for important notes about secondary
  76 threads and their deletion.
  77
  78 Last, remember that if wxEventLoopBase::YieldFor() is used directly or
  79 indirectly (e.g. through wxProgressDialog) in your code, then you may have both
  80 re-entrancy problems and also problems caused by the processing of events out
  81 of order. To resolve the last problem wxThreadEvent can be used: thanks to its
  82 implementation of the wxThreadEvent::GetEventCategory function wxThreadEvent
  83 classes in fact do not get processed by wxEventLoopBase::YieldFor() unless you
  84 specify the @c wxEVT_CATEGORY_THREAD flag.
  85
  86 See also the @sample{thread} for a sample showing some simple interactions
  87 between the main and secondary threads.
  88
  89 */