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/****************************************************************************
**
** Copyright (C) 2012 Digia Plc and/or its subsidiary(-ies).
** Contact: http://www.qt-project.org/legal
**
** This file is part of the documentation of the Qt Toolkit.
**
** $QT_BEGIN_LICENSE:LGPL$
** Commercial License Usage
** Licensees holding valid commercial Qt licenses may use this file in
** accordance with the commercial license agreement provided with the
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** a written agreement between you and Digia. For licensing terms and
** conditions see http://qt.digia.com/licensing. For further information
** use the contact form at http://qt.digia.com/contact-us.
**
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** Alternatively, this file may be used under the terms of the GNU Lesser
** General Public License version 2.1 as published by the Free Software
** Foundation and appearing in the file LICENSE.LGPL included in the
** packaging of this file. Please review the following information to
** ensure the GNU Lesser General Public License version 2.1 requirements
** will be met: http://www.gnu.org/licenses/old-licenses/lgpl-2.1.html.
**
** In addition, as a special exception, Digia gives you certain additional
** rights. These rights are described in the Digia Qt LGPL Exception
** version 1.1, included in the file LGPL_EXCEPTION.txt in this package.
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** GNU General Public License Usage
** Alternatively, this file may be used under the terms of the GNU
** General Public License version 3.0 as published by the Free Software
** Foundation and appearing in the file LICENSE.GPL included in the
** packaging of this file. Please review the following information to
** ensure the GNU General Public License version 3.0 requirements will be
** met: http://www.gnu.org/copyleft/gpl.html.
**
**
** $QT_END_LICENSE$
**
****************************************************************************/
/*!
\page session.html
\title Session Management
\ingroup best-practices
A \e session is a group of running applications, each of which has a
particular state. The session is controlled by a service called the \e
session \e manager. The applications participating in the session are
called \e{session clients}.
The session manager issues commands to its clients on behalf of the
user. These commands may cause clients to commit unsaved changes (for
example by saving open files), to preserve their state for future
sessions, or to terminate gracefully. The set of these operations is
called \e session \e management.
In the common case, a session consists of all applications that a
user runs on their desktop at a time. Under Unix/X11, however, a
session may include applications running on different computers and
may span multiple displays.
\section1 Shutting a Session Down
A session is shut down by the session manager, usually on behalf of
the user when they want to log out. A system might also perform an
automatic shutdown in an emergency situation, for example, if power is
about to be lost. Clearly there is a significant difference between
these types of shutdown. During the first, the user may want to
interact with the application, specifying exactly which files should
be saved and which should be discarded. In the latter case, there's no
time for interaction. There may not even be a user sitting in front of
the machine!
\section1 Protocols and Support on Different Platforms
On Mac OS X, and Microsoft Windows versions prior to Windows 2000,
there is nothing like complete session management for applications
yet, i.e. no restoring of previous sessions. (Windows 2000 and XP
provide "hibernation" where the entire memory is saved to disk and
restored when the machine is restarted.) They do support graceful
logouts where applications have the opportunity to cancel the process
after getting confirmation from the user. This is the functionality
that corresponds to the QApplication::commitData() method.
X11 has supported complete session management since X11R6.
\section1 Getting Session Management to Work with Qt
Start by reimplementing QApplication::commitData() to
enable your application to take part in the graceful logout process. If
you are only targeting the Microsoft Windows platform, this is all you can
and must provide. Ideally, your application should provide a shutdown
dialog similar to the following:
\img session.png A typical dialog on shutdown
Example code for this dialog can be found in the documentation of
QSessionManager::allowsInteraction().
For complete session management (only supported on X11R6 at present),
you must also take care of saving the application's state, and
potentially of restoring the state in the next life cycle of the
session. This saving is done by reimplementing
QApplication::saveState(). All state data you are saving in this
function, should be marked with the session identifier
QApplication::sessionId(). This application specific identifier is
globally unique, so no clashes will occur. (See QSessionManager for
information on saving/restoring the state of a particular Qt
application.)
Restoration is usually done in the application's main()
function. Check if QApplication::isSessionRestored() is \c true. If
that's the case, use the session identifier
QApplication::sessionId() again to access your state data and restore
the state of the application.
\bold{Important:} In order to allow the window manager to
restore window attributes such as stacking order or geometry
information, you must identify your top level widgets with
unique application-wide object names (see QObject::setObjectName()). When
restoring the application, you must ensure that all restored
top level widgets are given the same unique names they had before.
\section1 Testing and Debugging Session Management
Session management support on Mac OS X and Windows is fairly limited
due to the lack of this functionality in the operating system
itself. Simply shut the session down and verify that your application
behaves as expected. It may be useful to launch another application,
usually the integrated development environment, before starting your
application. This other application will get the shutdown message
afterwards, thus permitting you to cancel the shutdown. Otherwise you
would have to log in again after each test run, which is not a problem
per se, but is time consuming.
On Unix you can either use a desktop environment that supports
standard X11R6 session management or, the recommended method, use the
session manager reference implementation provided by the X Consortium.
This sample manager is called \c xsm and is part of a standard X11R6
installation. As always with X11, a useful and informative manual page
is provided. Using \c xsm is straightforward (apart from the clumsy
Athena-based user interface). Here's a simple approach:
\list
\i Run X11R6.
\i Create a dot file \c .xsmstartup in your home directory which
contains the single line
\snippet doc/src/snippets/code/doc_src_session.qdoc 0
This tells \c xsm that the default/failsafe session is just an xterm
and nothing else. Otherwise \c xsm would try to invoke lots of
clients including the windowmanager \c twm, which isn't very helpful.
\i Now launch \c xsm from another terminal window. Both a session
manager window and the xterm will appear. The xterm has a nice
property that sets it apart from all the other shells you are
currently running: within its shell, the \c SESSION_MANAGER
environment variable points to the session manager you just started.
\i Launch your application from the new xterm window. It will connect
itself automatically to the session manager. You can check with the \e
ClientList push button whether the connect was successful.
\bold{Note:} Never keep the \e ClientList open when you
start or end session managed clients! Otherwise \c xsm is likely to
crash.
\i Use the session manager's \e Checkpoint and \e Shutdown buttons
with different settings and see how your application behaves. The save
type \e local means that the clients should save their state. It
corresponds to the QApplication::saveState() function. The \e
global save type asks applications to save their unsaved changes in
permanent, globally accessible storage. It invokes
QApplication::commitData().
\i Whenever something crashes, blame \c xsm and not Qt. \c xsm is far
from being a usable session manager on a user's desktop. It is,
however, stable and useful enough to serve as testing environment.
\endlist
*/
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