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  A wxWidgets application does not have a @e main procedure; the equivalent is the
  15  #OnInit member defined for a class derived from wxApp.
  16
  17  @e OnInit will usually create a top window as a bare minimum.
  18  Unlike in earlier versions of wxWidgets, OnInit does not return a frame. Instead it
  19  returns a boolean value which indicates whether processing should continue (@true)
  20  or not (@false). You call wxApp::SetTopWindow to let wxWidgets know about the top window.
  21
  22  Note that the program's command line arguments, represented by @e argc
  23  and @e argv, are available from within wxApp member functions.
  24
  25  An application closes by destroying all windows. Because all frames must
  26  be destroyed for the application to exit, it is advisable to use parent
  27  frames wherever possible when creating new frames, so that deleting the
  28  top level frame will automatically delete child frames. The alternative
  29  is to explicitly delete child frames in the top-level frame's #wxCloseEvent
  30  handler.
  31
  32  In emergencies the #wxExit function can be called to kill the
  33  application however normally the application shuts down automatically,
  34  see @ref overview_app_shutdown.
  35
  36  An example of defining an application follows:
  37
  38  @code
  39  class DerivedApp : public wxApp
  40  {
  41  public:
  42    virtual bool OnInit();
  43  };
  44
  45  IMPLEMENT_APP(DerivedApp)
  46
  47  bool DerivedApp::OnInit()
  48  {
  49    wxFrame *the_frame = new wxFrame(@NULL, ID_MYFRAME, argv[0]);
  50    ...
  51    the_frame-Show(@true);
  52    SetTopWindow(the_frame);
  53
  54    return @true;
  55  }
  56  @endcode
  57
  58  Note the use of IMPLEMENT_APP(appClass), which allows wxWidgets to dynamically create
  59  an instance of the application object at the appropriate point in wxWidgets initialization.
  60  Previous versions of wxWidgets used to rely on the creation of a global application object,
  61  but this is no longer recommended, because required global initialization may not have
  62  been performed at application object construction time.
  63
  64  You can also use DECLARE_APP(appClass) in a header file to declare the wxGetApp function
  65  which returns a reference to the application object. Otherwise you can only use the global
  66  @c wxTheApp pointer which is of type @c wxApp *.
  67
  68
  69
  70  @section overview_app_shutdown Application shutdown
  71
  72  The application normally shuts down when the last of its top level windows is
  73  closed. This is normally the expected behaviour and means that it is enough to
  74  call #Close() in response to the @c "Exit" menu command if your program has a single
  75  top level window. If this behaviour is not desirable wxApp::SetExitOnFrameDelete can
  76  be called to change it.
  77
  78  Note that starting from wxWidgets 2.3.3 such logic doesn't apply for the windows shown
  79  before the program enters the main loop: in other words, you can safely show a dialog from
  80  wxApp::OnInit and not be afraid that your application terminates when this dialog --
  81  which is the last top level window for the moment -- is closed.
  82
  83  Another aspect of the application shutdown is #OnExit
  84  which is called when the application exits but @e before wxWidgets cleans up
  85  its internal structures. You should delete all wxWidgets object that you
  86  created by the time OnExit finishes.
  87
  88  In particular, do @b not destroy them from application class' destructor!
  89  For example, this code may crash:
  90
  91  @code
  92  class MyApp : public wxApp
  93  {
  94   public:
  95      wxCHMHelpController m_helpCtrl;
  96      ...
  97  };
  98  @endcode
  99
 100  The reason for that is that @c m_helpCtrl is a member object and is
 101  thus destroyed from MyApp destructor. But MyApp object is deleted after
 102  wxWidgets structures that wxCHMHelpController depends on were
 103  uninitialized! The solution is to destroy HelpCtrl in @e OnExit:
 104
 105  @code
 106  class MyApp : public wxApp
 107  {
 108   public:
 109      wxCHMHelpController *m_helpCtrl;
 110      ...
 111  };
 112
 113  bool MyApp::OnInit()
 114  {
 115    ...
 116    m_helpCtrl = new wxCHMHelpController;
 117    ...
 118  }
 119
 120  int MyApp::OnExit()
 121  {
 122    delete m_helpCtrl;
 123    return 0;
 124  }
 125  @endcode
 126
 127 */
 128