docs/doxygen/overviews/app.h

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