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