Classes: \helpref{wxCloseEvent}{wxcloseevent}, \helpref{wxWindow}{wxwindow}
Window deletion can be a confusing subject, so this overview is provided
-to make it clear when and how you delete windows, or respond to user requests
+to help make it clear when and how you delete windows, or respond to user requests
to close windows.
\wxheading{What is the sequence of events in a window deletion?}
When the user clicks on the system close button or system close command,
-in a frame or a dialog, wxWindows calls \helpref{wxWindow::Close}{wxwindowclose}.
+in a frame or a dialog, wxWindows calls \helpref{wxWindow::Close}{wxwindowclose}. This
+in turn generates an EVT\_CLOSE event: see \helpref{wxCloseEvent}{wxcloseevent}.
-This function then generates a \helpref{wxCloseEvent}{wxcloseevent} event which
-can be handled by the application (by using an EVT\_CLOSE event table entry). It is the duty of the application to
-define a suitable event handler, and decide whether or not to destroy the window.
-If the application is for some reason forcing the application to close,
-the window should always be destroyed, otherwise there is the option to
+It is the duty of the application to define a suitable event handler, and
+decide whether or not to destroy the window.
+If the application is for some reason forcing the application to close
+(\helpref{wxCloseEvent::CanVeto}{wxcloseeventcanveto} returns FALSE), the window should always be destroyed, otherwise there is the option to
ignore the request, or maybe wait until the user has answered a question
-before deciding whether it's safe to close.
+before deciding whether it is safe to close. The handler for EVT\_CLOSE should
+signal to the calling code if it does not destroy the window, by calling
+\helpref{wxCloseEvent::Veto}{wxcloseeventveto}. Calling this provides useful information
+to the calling code.
The wxCloseEvent handler should only call \helpref{wxWindow::Destroy}{wxwindowdestroy} to
delete the window, and not use the {\bf delete} operator. This is because
for some window classes, wxWindows delays actual deletion of the window until all events have been processed,
since otherwise there is the danger that events will be sent to a non-existent window.
+As reinforced in the next section, calling Close does not guarantee that the window
+will be destroyed. Call \helpref{wxWindow::Destroy}{wxwindowdestroy} if you want to be
+certain that the window is destroyed.
+
\wxheading{How can the application close a window itself?}
-Your application can use the \helpref{wxWindow::Close}{wxwindowclose} event just as
-the framework does. Pass a TRUE argument to this function to tell the event handler
-that we definitely want to delete the frame.
+Your application can either use \helpref{wxWindow::Close}{wxwindowclose} event just as
+the framework does, or it can call \helpref{wxWindow::Destroy}{wxwindowdestroy} directly.
+If using Close(), you can pass a TRUE argument to this function to tell the event handler
+that we definitely want to delete the frame and it cannot be vetoed.
-If for some reason you don't wish to use the {\bf Close} function to delete a window, at least use
-the {\bf Destroy} function so that wxWindows can decide when it's safe to delete the window.
+The advantage of using Close instead of Destroy is that it will call any clean-up code
+defined by the EVT\_CLOSE handler; for example it may close a document contained in
+a window after first asking the user whether the work should be saved. Close can be vetoed
+by this process (return FALSE), whereas Destroy definitely destroys the window.
\wxheading{What is the default behaviour?}
-By default, the close event handlers for wxFrame and wxDialog
-both call the old \helpref{wxWindow::OnClose}{wxwindowonclose} handler
-for backward compatibility. So you can still use the old form if you wish.
-
-In addition, the default close event handler for wxDialog simulates a Cancel command,
+The default close event handler for wxDialog simulates a Cancel command,
generating a wxID\_CANCEL event. Since the handler for this cancel event might
-itself call {\bf Close}, there is a check for infinite looping.
+itself call {\bf Close}, there is a check for infinite looping. The default handler
+for wxID\_CANCEL hides the dialog (if modeless) or calls EndModal(wxID\_CANCEL) (if modal).
+In other words, by default, the dialog {\it is not destroyed} (it might have been created
+on the stack, so the assumption of dynamic creation cannot be made).
-Under Windows, wxDialog also defines a handler for \helpref{wxWindow::OnCharHook}{wxwindowoncharhook} that
-generates a Cancel event if the Escape key has been pressed.
+The default close event handler for wxFrame destroys the frame using Destroy().
+%
+%Under Windows, wxDialog defines a handler for \helpref{wxWindow::OnCharHook}{wxwindowoncharhook} that
+%generates a Cancel event if the Escape key has been pressed.
\wxheading{What should I do when the user calls up Exit from a menu?}
\wxheading{What should I do to upgrade my 1.xx OnClose to 2.0?}
-In wxWindows 1.xx, the {\bf OnClose} function did not actually delete 'this', but signalled
+In wxWindows 1.xx, the {\bf OnClose} function did not actually delete 'this', but signaled
to the calling function (either {\bf Close}, or the wxWindows framework) to delete
or not delete the window.
-You can still use this function unchanged in 2.0, but it's worth upgrading to
-the new method in case future versions of wxWindows does not support the old one.
-
To update your code, you should provide an event table entry in your frame or
dialog, using the EVT\_CLOSE macro. The event handler function might look like this:
\begin{verbatim}
void MyFrame::OnCloseWindow(wxCloseEvent& event)
{
- // If the application forces the deletion,
- // obey without question.
- if (event.GetForce())
- {
- this->Destroy();
- return;
- }
-
- // Otherwise...
if (MyDataHasBeenModified())
{
wxMessageDialog* dialog = new wxMessageDialog(this,
"Save changed data?", "My app", wxYES_NO|wxCANCEL);
int ans = dialog->ShowModal();
- dialog->Close(TRUE);
+ dialog->Destroy();
switch (ans)
{
break;
case wxID_CANCEL: // Do nothing - so don't quit app.
default:
+ if (!event.CanVeto()) // Test if we can veto this deletion
+ this->Destroy(); // If not, destroy the window anyway.
+ else
+ event.Veto(); // Notify the calling code that we didn't delete the frame.
break;
}
}
\wxheading{How do I exit the application gracefully?}
A wxWindows application automatically exits when the designated top window, or the
-last frame or dialog, is destroyed.
+last frame or dialog, is destroyed. Put any application-wide cleanup code in \helpref{wxApp::OnExit}{wxapponexit} (this
+is a virtual function, not an event handler).
\wxheading{Do child windows get deleted automatically?}
projects / wxWidgets.git / blobdiff