/////////////////////////////////////////////////////////////////////////////
// Name:        docs/doxygen/overviews/persistence.h
// Purpose:     overview of persistent objects
// Author:      Vadim Zeitlin
// Created:     2009-01-23
// RCS-ID:      $Id$
// Copyright:   (c) 2009 Vadim Zeitlin <vadim@wxwidgets.org>
// Licence:     wxWindows license
/////////////////////////////////////////////////////////////////////////////

/**

@page overview_persistence Persistent Objects Overview

Persistent objects are simply the objects which automatically save their state
when they are destroyed and restore it when they are recreated, even during
another program invocation.

Most often, persistent objects are, in fact, persistent windows as it is
especially convenient to automatically restore the UI state when the program is
restarted but an object of any class can be made persistent. Moreover,
persistence is implemented in a non-intrusive way so that the original object
class doesn't need to be modified at all in order to add support for saving and
restoring its properties.

The persistence framework involves
 - wxPersistenceManager which all persistent objects register themselves with.
   This class handles actual saving and restoring of persistent data as well as
   various global aspects of persistence, e.g. it can be used to disable
   restoring the saved data.
 - wxPersistentObject is the base class for all persistent objects or, rather,
   adaptors for the persistent objects as this class main purpose is to provide
   the bridge between the original class -- which has no special persistence
   support -- and wxPersistenceManager,
 - wxPersistentWindow<> which derives from wxPersistentObject and implements some
   of its methods using wxWindow-specific functionality. Notably,
   wxPersistenceManager handles the destruction of persistent windows
   automatically implicitly while it has to be done explicitly for the
   arbitrary persistent objects.
 - wxCreatePersistentObject() function which is used to create the
   appropriate persistence adapter for the object.


@section persistence_using Using Persistent Windows

wxWidgets has built-in support for a (constantly growing) number of controls.
Currently the following classes are supported:
    - wxTopLevelWindow (and hence wxFrame and wxDialog)
    - wxBookCtrlBase, i.e. wxNotebook, wxListbook, wxToolbook and wxChoicebook
    - wxTreebook

To automatically save and restore the properties of the windows of classes
listed above you need to

    -# Set a unique name for the window using wxWindow::SetName(): this step is
    important as the name is used in the configuration file and so must be
    unique among all windows of the same class.
    -# Call wxPersistenceManager::Register() at any moment after creating the
    window and then wxPersistenceManager::Restore() when the settings may be
    restored (which can't be always done immediately, e.g. often the window
    needs to be populated first). If settings can be restored immediately after
    the window creation, as is often the case for wxTopLevelWindow, for
    example, then wxPersistenceManager::RegisterAndRestore() can be used to do
    both at once.
    -# If you do not want the settings for the window to be saved (for example
    the changes to the dialog size are usually not saved if the dialog was
    cancelled), you need to call wxPersistenceManager::Unregister() manually.
    Otherwise the settings will be automatically saved when the control itself
    is destroyed.

Example of using a notebook control which automatically remembers the last open
page:
@code
    wxNotebook *book = new wxNotebook(parent, wxID_ANY);
    book->SetName("MyBook"); // do not use the default name
    book->AddPage(...);
    book->AddPage(...);
    book->AddPage(...);
    if ( !wxPersistenceManager::RegisterAndRestore(book) )
    {
        // nothing was restored, so choose the default page ourselves
        book->SetSelection(0);
    }
@endcode


@section persistence_defining Defining Custom Persistent Windows

User-defined classes can be easily integrated with wxPersistenceManager. To add
support for your custom class @c MyWidget you just need to:

    -# Define a new @c MyPersistentWidget class inheriting from
    wxPersistentWindow<MyWidget>.
    -# Implement its pure virtual GetKind() method returning a unique string
    identifying all @c MyWidget objects, typically something like @c "widget"
    -# Implement its pure virtual Save() and Restore() methods to actually save
    and restore the widget settings using wxPersistentObject::SaveValue() and
    wxPersistentObject::RestoreValue() methods.
    -# Define wxCreatePersistentObject() overload taking @c MyWidget * and
    returning a new @c MyPersistentWidget object.

If you want to add persistence support for a class not deriving from wxWindow,
you need to derive @c MyPersistentWidget directly from wxPersistentObject and
so implement its pure virtual wxPersistentObject::GetName() method too.
Additionally, you must ensure that wxPersistenceManager::SaveAndUnregister() is
called when your object is destroyed as this can be only done automatically for
windows.

 */