docs/doxygen/overviews/app.h

   1 /////////////////////////////////////////////////////////////////////////////
   2 // Name:        app.h
   3 // Purpose:     topic overview
   4 // Author:      wxWidgets team
   5 // RCS-ID:      $Id$
   6 // Licence:     wxWindows license
   7 /////////////////////////////////////////////////////////////////////////////
   8
   9 /*!
  10
  11  @page overview_app wxApp overview
  12
  13  Classes: wxApp
  14
  15
  16  @li @ref overview_app_shutdown
  17
  18
  19  <hr>
  20
  21
  22  A wxWidgets application does not have a @e main procedure; the equivalent is the
  23  wxApp::OnInit member defined for a class derived from wxApp.
  24
  25  @e OnInit will usually create a top window as a bare minimum.
  26  Unlike in earlier versions of wxWidgets, OnInit does not return a frame. Instead it
  27  returns a boolean value which indicates whether processing should continue (@true)
  28  or not (@false). You call wxApp::SetTopWindow to let wxWidgets know about the top window.
  29
  30  Note that the program's command line arguments, represented by @e argc
  31  and @e argv, are available from within wxApp member functions.
  32
  33  An application closes by destroying all windows. Because all frames must
  34  be destroyed for the application to exit, it is advisable to use parent
  35  frames wherever possible when creating new frames, so that deleting the
  36  top level frame will automatically delete child frames. The alternative
  37  is to explicitly delete child frames in the top-level frame's wxCloseEvent
  38  handler.
  39
  40  In emergencies the wxExit function can be called to kill the
  41  application however normally the application shuts down automatically,
  42  see @ref overview_app_shutdown.
  43
  44  An example of defining an application follows:
  45
  46  @code
  47  class DerivedApp : public wxApp
  48  {
  49  public:
  50    virtual bool OnInit();
  51  };
  52
  53  IMPLEMENT_APP(DerivedApp)
  54
  55  bool DerivedApp::OnInit()
  56  {
  57    wxFrame *the_frame = new wxFrame(@NULL, ID_MYFRAME, argv[0]);
  58    ...
  59    the_frame-Show(@true);
  60    SetTopWindow(the_frame);
  61
  62    return @true;
  63  }
  64  @endcode
  65
  66  Note the use of IMPLEMENT_APP(appClass), which allows wxWidgets to dynamically create
  67  an instance of the application object at the appropriate point in wxWidgets initialization.
  68  Previous versions of wxWidgets used to rely on the creation of a global application object,
  69  but this is no longer recommended, because required global initialization may not have
  70  been performed at application object construction time.
  71
  72  You can also use DECLARE_APP(appClass) in a header file to declare the wxGetApp function
  73  which returns a reference to the application object. Otherwise you can only use the global
  74  @c wxTheApp pointer which is of type @c wxApp *.
  75
  76
  77
  78  @section overview_app_shutdown Application shutdown
  79
  80  The application normally shuts down when the last of its top level windows is
  81  closed. This is normally the expected behaviour and means that it is enough to
  82  call wxWindow::Close() in response to the @c "Exit" menu command if your program has a
  83  single top level window. If this behaviour is not desirable wxApp::SetExitOnFrameDelete
  84  can be called to change it.
  85
  86  Note that such logic doesn't apply for the windows shown before the program enters the
  87  main loop: in other words, you can safely show a dialog from wxApp::OnInit and not be
  88  afraid that your application terminates when this dialog -- which is the last top level
  89  window for the moment -- is closed.
  90
  91  Another aspect of the application shutdown is wxApp::OnExit
  92  which is called when the application exits but @e before wxWidgets cleans up
  93  its internal structures. You should delete all wxWidgets object that you
  94  created by the time OnExit finishes.
  95
  96  In particular, do @b not destroy them from application class' destructor!
  97  For example, this code may crash:
  98
  99  @code
 100  class MyApp : public wxApp
 101  {
 102   public:
 103      wxCHMHelpController m_helpCtrl;
 104      ...
 105  };
 106  @endcode
 107
 108  The reason for that is that @c m_helpCtrl is a member object and is
 109  thus destroyed from MyApp destructor. But MyApp object is deleted after
 110  wxWidgets structures that wxCHMHelpController depends on were
 111  uninitialized! The solution is to destroy HelpCtrl in @e OnExit:
 112
 113  @code
 114  class MyApp : public wxApp
 115  {
 116   public:
 117      wxCHMHelpController *m_helpCtrl;
 118      ...
 119  };
 120
 121  bool MyApp::OnInit()
 122  {
 123    ...
 124    m_helpCtrl = new wxCHMHelpController;
 125    ...
 126  }
 127
 128  int MyApp::OnExit()
 129  {
 130    delete m_helpCtrl;
 131    return 0;
 132  }
 133  @endcode
 134
 135 */
 136