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 A wxWidgets application does not have a @e main procedure; the equivalent is
  16 the wxApp::OnInit member defined for a class derived from wxApp.
  17
  18 @e OnInit will usually create a top window as a bare minimum. Unlike in earlier
  19 versions of wxWidgets, OnInit does not return a frame. Instead it returns a
  20 boolean value which indicates whether processing should continue (@true) or not
  21 (@false).
  22
  23 Note that the program's command line arguments, represented by @e argc and
  24 @e argv, are available from within wxApp member functions.
  25
  26 An application closes by destroying all windows. Because all frames must be
  27 destroyed for the application to exit, it is advisable to use parent frames
  28 wherever possible when creating new frames, so that deleting the top level
  29 frame will automatically delete child frames. The alternative is to explicitly
  30 delete child frames in the top-level frame's wxCloseEvent handler.
  31
  32 In emergencies the wxExit function can be called to kill the application
  33 however normally the application shuts down automatically, see
  34 @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
  53     return true;
  54 }
  55 @endcode
  56
  57 Note the use of IMPLEMENT_APP(appClass), which allows wxWidgets to dynamically
  58 create an instance of the application object at the appropriate point in
  59 wxWidgets initialization. Previous versions of wxWidgets used to rely on the
  60 creation of a global application object, but this is no longer recommended,
  61 because required global initialization may not have been performed at
  62 application object construction time.
  63
  64 You can also use DECLARE_APP(appClass) in a header file to declare the wxGetApp
  65 function which returns a reference to the application object. Otherwise you can
  66 only use the global @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 wxWindow::Close() in response to the "Exit" menu command if your program
  75 has a single top level window. If this behaviour is not desirable
  76 wxApp::SetExitOnFrameDelete can be called to change it.
  77
  78 Note that such logic doesn't apply for the windows shown before the program
  79 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
  81 dialog -- which is the last top level window for the moment -- is closed.
  82
  83 Another aspect of the application shutdown is wxApp::OnExit which is called
  84 when the application exits but @e before wxWidgets cleans up its internal
  85 structures. You should delete all wxWidgets object that you created by the time
  86 OnExit finishes.
  87
  88 In particular, do @b not destroy them from application class' destructor! For
  89 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 thus
 101 destroyed from MyApp destructor. But MyApp object is deleted after wxWidgets
 102 structures that wxCHMHelpController depends on were uninitialized! The solution
 103 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 */