More doxygen overview updates.

[wxWidgets.git] / docs / doxygen / overviews / app.h

/////////////////////////////////////////////////////////////////////////////
// Name:        app.h
// Purpose:     topic overview
// Author:      wxWidgets team
// RCS-ID:      $Id$
// Licence:     wxWindows license
/////////////////////////////////////////////////////////////////////////////

/*!

 @page overview_app wxApp overview

 Classes: wxApp


 @li @ref overview_app_shutdown


 <hr>


 A wxWidgets application does not have a @e main procedure; the equivalent is the
 wxApp::OnInit member defined for a class derived from wxApp.

 @e OnInit will usually create a top window as a bare minimum.
 Unlike in earlier versions of wxWidgets, OnInit does not return a frame. Instead it
 returns a boolean value which indicates whether processing should continue (@true) 
 or not (@false). You call wxApp::SetTopWindow to let wxWidgets know about the top window.

 Note that the program's command line arguments, represented by @e argc
 and @e argv, are available from within wxApp member functions.

 An application closes by destroying all windows. Because all frames must
 be destroyed for the application to exit, it is advisable to use parent
 frames wherever possible when creating new frames, so that deleting the
 top level frame will automatically delete child frames. The alternative
 is to explicitly delete child frames in the top-level frame's wxCloseEvent
 handler.

 In emergencies the wxExit function can be called to kill the
 application however normally the application shuts down automatically,
 see @ref overview_app_shutdown.

 An example of defining an application follows:

 @code
 class DerivedApp : public wxApp
 {
 public:
   virtual bool OnInit();
 };

 IMPLEMENT_APP(DerivedApp)

 bool DerivedApp::OnInit()
 {
   wxFrame *the_frame = new wxFrame(@NULL, ID_MYFRAME, argv[0]);
   ...
   the_frame-Show(@true);
   SetTopWindow(the_frame);

   return @true;
 }
 @endcode

 Note the use of IMPLEMENT_APP(appClass), which allows wxWidgets to dynamically create 
 an instance of the application object at the appropriate point in wxWidgets initialization.
 Previous versions of wxWidgets used to rely on the creation of a global application object,
 but this is no longer recommended, because required global initialization may not have 
 been performed at application object construction time.

 You can also use DECLARE_APP(appClass) in a header file to declare the wxGetApp function 
 which returns a reference to the application object. Otherwise you can only use the global
 @c wxTheApp pointer which is of type @c wxApp *.



 @section overview_app_shutdown Application shutdown

 The application normally shuts down when the last of its top level windows is
 closed. This is normally the expected behaviour and means that it is enough to
 call wxWindow::Close() in response to the @c "Exit" menu command if your program has a
 single top level window. If this behaviour is not desirable wxApp::SetExitOnFrameDelete
 can be called to change it.

 Note that such logic doesn't apply for the windows shown before the program enters the 
 main loop: in other words, you can safely show a dialog from wxApp::OnInit and not be 
 afraid that your application terminates when this dialog -- which is the last top level 
 window for the moment -- is closed.

 Another aspect of the application shutdown is wxApp::OnExit
 which is called when the application exits but @e before wxWidgets cleans up
 its internal structures. You should delete all wxWidgets object that you
 created by the time OnExit finishes. 

 In particular, do @b not destroy them from application class' destructor!
 For example, this code may crash:

 @code
 class MyApp : public wxApp
 {
  public:
     wxCHMHelpController m_helpCtrl;
     ...
 };
 @endcode

 The reason for that is that @c m_helpCtrl is a member object and is
 thus destroyed from MyApp destructor. But MyApp object is deleted after
 wxWidgets structures that wxCHMHelpController depends on were
 uninitialized! The solution is to destroy HelpCtrl in @e OnExit:

 @code
 class MyApp : public wxApp
 {
  public:
     wxCHMHelpController *m_helpCtrl;
     ...
 };

 bool MyApp::OnInit()
 {
   ...
   m_helpCtrl = new wxCHMHelpController;
   ...
 }

 int MyApp::OnExit()
 {
   delete m_helpCtrl;
   return 0;
 }
 @endcode

*/