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