docs/doxygen/overviews/windowdeletion.h

   1 /////////////////////////////////////////////////////////////////////////////
   2 // Name:        windowdeletion
   3 // Purpose:     topic overview
   4 // Author:      wxWidgets team
   5 // RCS-ID:      $Id$
   6 // Licence:     wxWindows license
   7 /////////////////////////////////////////////////////////////////////////////
   8
   9 /*!
  10
  11  @page overview_windowdeletion Window deletion overview
  12
  13  Classes: #wxCloseEvent, #wxWindow
  14  Window deletion can be a confusing subject, so this overview is provided
  15  to help make it clear when and how you delete windows, or respond to user requests
  16  to close windows.
  17  @b What is the sequence of events in a window deletion?
  18  When the user clicks on the system close button or system close command,
  19  in a frame or a dialog, wxWidgets calls wxWindow::Close. This
  20  in turn generates an EVT_CLOSE event: see #wxCloseEvent.
  21  It is the duty of the application to define a suitable event handler, and
  22  decide whether or not to destroy the window.
  23  If the application is for some reason forcing the application to close
  24  (wxCloseEvent::CanVeto returns @false), the window should always be destroyed, otherwise there is the option to
  25  ignore the request, or maybe wait until the user has answered a question
  26  before deciding whether it is safe to close. The handler for EVT_CLOSE should
  27  signal to the calling code if it does not destroy the window, by calling
  28  wxCloseEvent::Veto. Calling this provides useful information
  29  to the calling code.
  30  The wxCloseEvent handler should only call wxWindow::Destroy to
  31  delete the window, and not use the @b delete operator. This is because
  32  for some window classes, wxWidgets delays actual deletion of the window until all events have been processed,
  33  since otherwise there is the danger that events will be sent to a non-existent window.
  34  As reinforced in the next section, calling Close does not guarantee that the window
  35  will be destroyed. Call wxWindow::Destroy if you want to be
  36  certain that the window is destroyed.
  37  @b How can the application close a window itself?
  38  Your application can either use wxWindow::Close event just as
  39  the framework does, or it can call wxWindow::Destroy directly.
  40  If using Close(), you can pass a @true argument to this function to tell the event handler
  41  that we definitely want to delete the frame and it cannot be vetoed.
  42  The advantage of using Close instead of Destroy is that it will call any clean-up code
  43  defined by the EVT_CLOSE handler; for example it may close a document contained in
  44  a window after first asking the user whether the work should be saved. Close can be vetoed
  45  by this process (return @false), whereas Destroy definitely destroys the window.
  46  @b What is the default behaviour?
  47  The default close event handler for wxDialog simulates a Cancel command,
  48  generating a wxID_CANCEL event. Since the handler for this cancel event might
  49  itself call @b Close, there is a check for infinite looping. The default handler
  50  for wxID_CANCEL hides the dialog (if modeless) or calls EndModal(wxID_CANCEL) (if modal).
  51  In other words, by default, the dialog @e is not destroyed (it might have been created
  52  on the stack, so the assumption of dynamic creation cannot be made).
  53  The default close event handler for wxFrame destroys the frame using Destroy().
  54  @b What should I do when the user calls up Exit from a menu?
  55  You can simply call wxWindow::Close on the frame. This
  56  will invoke your own close event handler which may destroy the frame.
  57  You can do checking to see if your application can be safely exited at this point,
  58  either from within your close event handler, or from within your exit menu command
  59  handler. For example, you may wish to check that all files have been saved.
  60  Give the user a chance to save and quit, to not save but quit anyway, or to cancel
  61  the exit command altogether.
  62  @b What should I do to upgrade my 1.xx OnClose to 2.0?
  63  In wxWidgets 1.xx, the @b OnClose function did not actually delete 'this', but signaled
  64  to the calling function (either @b Close, or the wxWidgets framework) to delete
  65  or not delete the window.
  66  To update your code, you should provide an event table entry in your frame or
  67  dialog, using the EVT_CLOSE macro. The event handler function might look like this:
  68
  69  @code
  70  void MyFrame::OnCloseWindow(wxCloseEvent& event)
  71    {
  72      if (MyDataHasBeenModified())
  73      {
  74        wxMessageDialog* dialog = new wxMessageDialog(this,
  75          "Save changed data?", "My app", wxYES_NO|wxCANCEL);
  76
  77        int ans = dialog-ShowModal();
  78        dialog-Destroy();
  79
  80        switch (ans)
  81        {
  82          case wxID_YES:      // Save, then destroy, quitting app
  83            SaveMyData();
  84            this-Destroy();
  85            break;
  86          case wxID_NO:       // Don't save; just destroy, quitting app
  87            this-Destroy();
  88            break;
  89          case wxID_CANCEL:   // Do nothing - so don't quit app.
  90          default:
  91            if (!event.CanVeto()) // Test if we can veto this deletion
  92              this-Destroy();    // If not, destroy the window anyway.
  93            else
  94              event.Veto();     // Notify the calling code that we didn't delete the frame.
  95            break;
  96        }
  97      }
  98    }
  99  @endcode
 100
 101  @b How do I exit the application gracefully?
 102  A wxWidgets application automatically exits when the last top level window
 103  (#wxFrame or #wxDialog), is destroyed. Put
 104  any application-wide cleanup code in wxApp::OnExit (this
 105  is a virtual function, not an event handler).
 106  @b Do child windows get deleted automatically?
 107  Yes, child windows are deleted from within the parent destructor. This includes any children
 108  that are themselves frames or dialogs, so you may wish to close these child frame or dialog windows
 109  explicitly from within the parent close handler.
 110  @b What about other kinds of window?
 111  So far we've been talking about 'managed' windows, i.e. frames and dialogs. Windows
 112  with parents, such as controls, don't have delayed destruction and don't usually have
 113  close event handlers, though you can implement them if you wish. For consistency,
 114  continue to use the wxWindow::Destroy function instead
 115  of the @b delete operator when deleting these kinds of windows explicitly.
 116
 117  */
 118
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