source: trunk/doc/src/howtos/session.qdoc@ 846

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/*!
    \page session.html
    \title Session Management
    \brief How to do session management with Qt.
    \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
*/