docs/doxygen/overviews/windowdeletion.h

   1 /////////////////////////////////////////////////////////////////////////////
   2 // Name:        windowdeletion.h
   3 // Purpose:     topic overview
   4 // Author:      wxWidgets team
   5 // Licence:     wxWindows licence
   6 /////////////////////////////////////////////////////////////////////////////
   7
   8 /**
   9
  10 @page overview_windowdeletion Window Deletion
  11
  12 @tableofcontents
  13
  14 Window deletion can be a confusing subject, so this overview is provided to
  15 help make it clear when and how you delete windows, or respond to user requests
  16 to close windows.
  17
  18 @see wxCloseEvent, wxWindow
  19
  20
  21
  22 @section overview_windowdeletion_sequence Sequence of Events During Window Deletion
  23
  24 When the user clicks on the system close button or system close command, in a
  25 frame or a dialog, wxWidgets calls wxWindow::Close. This in turn generates an
  26 EVT_CLOSE event: see wxCloseEvent.
  27
  28 It is the duty of the application to define a suitable event handler, and
  29 decide whether or not to destroy the window. If the application is for some
  30 reason forcing the application to close (wxCloseEvent::CanVeto returns @false),
  31 the window should always be destroyed, otherwise there is the option to ignore
  32 the request, or maybe wait until the user has answered a question before
  33 deciding whether it is safe to close. The handler for EVT_CLOSE should signal
  34 to the calling code if it does not destroy the window, by calling
  35 wxCloseEvent::Veto. Calling this provides useful information to the calling
  36 code.
  37
  38 The wxCloseEvent handler should only call wxWindow::Destroy to delete the
  39 window, and not use the @c delete operator. This is because for some window
  40 classes, wxWidgets delays actual deletion of the window until all events have
  41 been processed, since otherwise there is the danger that events will be sent to
  42 a non-existent window.
  43
  44 As reinforced in the next section, calling Close does not guarantee that the window
  45 will be destroyed. Call wxWindow::Destroy if you want to be
  46 certain that the window is destroyed.
  47
  48
  49 @section overview_windowdeletion_close Closing Windows
  50
  51 Your application can either use wxWindow::Close event just as the framework
  52 does, or it can call wxWindow::Destroy directly. If using Close(), you can pass
  53 a @true argument to this function to tell the event handler that we definitely
  54 want to delete the frame and it cannot be vetoed.
  55
  56 The advantage of using Close instead of Destroy is that it will call any
  57 clean-up code defined by the EVT_CLOSE handler; for example it may close a
  58 document contained in a window after first asking the user whether the work
  59 should be saved. Close can be vetoed by this process (return @false), whereas
  60 Destroy definitely destroys the window.
  61
  62
  63 @section overview_windowdeletion_default Default Window Close Behaviour
  64
  65 The default close event handler for wxDialog simulates a Cancel command,
  66 generating a wxID_CANCEL event. Since the handler for this cancel event might
  67 itself call Close, there is a check for infinite looping. The default handler
  68 for wxID_CANCEL hides the dialog (if modeless) or calls EndModal(wxID_CANCEL)
  69 (if modal). In other words, by default, the dialog @e is not destroyed (it
  70 might have been created on the stack, so the assumption of dynamic creation
  71 cannot be made).
  72
  73 The default close event handler for wxFrame destroys the frame using Destroy().
  74
  75
  76 @section overview_windowdeletion_menuexit User Calls to Exit From a Menu
  77
  78 What should I do when the user calls up Exit from a menu? You can simply call
  79 wxWindow::Close on the frame. This will invoke your own close event handler
  80 which may destroy the frame.
  81
  82 You can do checking to see if your application can be safely exited at this
  83 point, either from within your close event handler, or from within your exit
  84 menu command handler. For example, you may wish to check that all files have
  85 been saved. Give the user a chance to save and quit, to not save but quit
  86 anyway, or to cancel the exit command altogether.
  87
  88
  89 @section overview_windowdeletion_exitapp Exiting the Application Gracefully
  90
  91 A wxWidgets application automatically exits when the last top level window
  92 (wxFrame or wxDialog), is destroyed. Put any application-wide cleanup code in
  93 wxApp::OnExit (this is a virtual function, not an event handler).
  94
  95
  96 @section overview_windowdeletion_deletion Automatic Deletion of Child Windows
  97
  98 Child windows are deleted from within the parent destructor. This includes any
  99 children that are themselves frames or dialogs, so you may wish to close these
 100 child frame or dialog windows explicitly from within the parent close handler.
 101
 102
 103 @section overview_windowdeletion_windowkinds Other Kinds of Windows
 104
 105 So far we've been talking about 'managed' windows, i.e. frames and dialogs.
 106 Windows with parents, such as controls, don't have delayed destruction and
 107 don't usually have close event handlers, though you can implement them if you
 108 wish. For consistency, continue to use the wxWindow::Destroy function instead
 109 of the @c delete operator when deleting these kinds of windows explicitly.
 110
 111 */