docs/latex/wx/tthreads.tex

   1 \section{Multithreading overview}\label{wxthreadoverview}
   2
   3 Classes: \helpref{wxThread}{wxthread}, \helpref{wxMutex}{wxmutex},
   4 \helpref{wxCriticalSection}{wxcriticalsection},
   5 \helpref{wxCondition}{wxcondition}
   6
   7 wxWidgets provides a complete set of classes encapsulating objects necessary in
   8 multithreaded (MT) programs: the \helpref{thread}{wxthread} class itself and different
   9 synchronization objects: \helpref{mutexes}{wxmutex} and
  10 \helpref{critical sections}{wxcriticalsection} with
  11 \helpref{conditions}{wxcondition}. The thread API in wxWidgets resembles to
  12 POSIX1.c threads API (a.k.a. pthreads), although several functions have
  13 different names and some features inspired by Win32 thread API are there as
  14 well.
  15
  16 These classes will hopefully make writing MT programs easier and they also
  17 provide some extra error checking (compared to the native (be it Win32 or Posix)
  18 thread API), however it is still an non-trivial undertaking especially for large
  19 projects. Before starting an MT application (or starting to add MT features to
  20 an existing one) it is worth asking oneself if there is no easier and safer way
  21 to implement the same functionality. Of course, in some situations threads
  22 really make sense (classical example is a server application which launches a
  23 new thread for each new client), but in others it might be a very poor choice
  24 (example: launching a separate thread when doing a long computation to show a
  25 progress dialog). Other implementation choices are available: for the progress
  26 dialog example it is far better to do the calculations in the
  27 \helpref{idle handler}{wxidleevent} or even simply do everything at once
  28 but call \helpref{wxWindow::Update()}{wxwindowupdate} periodically to update
  29 the screen.
  30
  31 If you do decide to use threads in your application, it is strongly recommended
  32 that no more than one thread calls GUI functions. The thread sample shows that
  33 it {\it is} possible for many different threads to call GUI functions at once
  34 (all the threads created in the sample access GUI), but it is a very poor design
  35 choice for anything except an example. The design which uses one GUI thread and
  36 several worker threads which communicate with the main one using events is much
  37 more robust and will undoubtedly save you countless problems (example: under
  38 Win32 a thread can only access GDI objects such as pens, brushes, \&c created by
  39 itself and not by the other threads).
  40
  41 For communication between secondar threads and the main thread, use may use
  42 \helpref{wxEvtHandler::AddPendingEvent}{wxevthandleraddpendingevent}
  43 or its short version \helpref{wxPostEvent}{wxpostevent}. These functions
  44 have thread safe implementation so that they can be used as they are for
  45 sending event from one thread to another. However there is no built in method
  46 to send messages to the worker threads and you will need to use the available
  47 synchronization classes to implement the solution which suits your needs
  48 yourself. In particular, please notice that it is \emph{not} enough to derive
  49 your class from \helpref{wxThread}{wxthread} and
  50 \helpref{wxEvtHandler}{wxevthandler} to send messages to it: in fact, this does
  51 \emph{not} work at all.
  52
  53