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Development/Tutorials/Session Management

Session Management
Tutorial Series   Basics
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What's Next  
Further Reading   Session Management in the Qt documentation; KMainWindow; KApplication
This page needs a review and probably holds information that needs to be fixed.

Parts to be reviewed:

Move away from KApplication and kdelibs4support

About KDE and X11 session management

Since KDE 1.0, KDE supports the legacy X11R4 and ICCCM session management protocols. It will restart with the specified command all legacy applications that define the WM_COMMAND property or support the WM_SAVE_YOURSELF protocol. It will also restore the window geometries on a best effort basis.

Since KDE 2.0, KDE also supports the standard X11R6 session management protocol XSMP and uses it. You can download the official documentation of the standard from the X Consortium's FTP server ftp.x.org. Unlike the legacy protocols, the new X11R6 session management gives a chance to save application dependent settings when you log out. A text editor, for instance, would save the names of the loaded files and would reload them when you log in again. Another major advantage of the new protocol is the support for a clean and safe logout procedure even if the users decides not to restore the session next time. The protocol gives applications the possibility to interact with the user if they are in danger to lose some data, and to cancel the shutdown process if necessary.

Further Reading

An introductory overview of session management functionality and the Qt API for it is available from doc.trolltech.com.

In KDE, the classes KApplication and KMainWindow hide all the ugly details from the programmer. Basically, a KApplication manages a KConfig configuration object (available trough KApplication::sessionConfig()) for you, that your application can use to store session specific data.

Please read the class documentation, especially the one of KMainWindow, for a detailed interface description. With the advanced functionality in KMainWindow, it's really just a matter of a few lines to get even a multi-window application to retains its state between different user sessions.

Implementing session management in your application

Here's just a brief overview how you can add session management to your application. Again, see the class documentation for details.

Implementing session management in KDE is easy. If your main window inherits from KMainWindow, you have just 2 things to do:

  • Reimplement some virtual functions of KMainWindow.
  • Add session management support to your main() function.

That's all.

Reimplement some virtual functions of KMainWindow

KMainWindow will automatically save its position, geometry and positions of toolbars and menu bar on logout - and restore it on the next startup.

For every other data that your application needs to restore a session, you have to write the code for saving and restoring yourself. However, KMainWindow makes this task easy. You can just re-implement the following functions:

  • saveProperties(): This function is called only when the session manager closes the application (and not when the user closes the application). To save the data that you need to restore the session, reimplement this function. (For a text editor that would be the list of loaded files) Note that you may not do any user interaction in this function! (For example, you may not display any dialog!)
  • readProperties(): To read the data again on next login, reimplement this function.

Furthermore, the function queryClose() could be interesting for you. This function is called always before the window is closed, either by the user or indirectly by the session manager. (Note that this is not the case for a call of KApplication::quit() because this function will exit the event loop without causing a close event for the main windows. It will even not destroy them.) Typically, here you can warn the user that the application or some windows have unsaved data on close or logout (example: show a dialog with the buttons "Save changes" and "Discard changes"). However, for session management it isn't nice to need a user interaction before closing, so you better avoid this. Note that it is not determined if saveProperties() is called before or after queryClose()!

To save your application-wide properties (data that is only needed once per application, and not for each main window instance) reimplement saveGlobalProperties() and it's counterpart readGlobalProperties. Normally, you don't need these functions.

Add session management support to your main() function

While KMainWindow::saveProperties() (and KMainWindow::queryClose()) will be called automatically, KMainWindow::readProperties() will not. You have to add some code to your main() function to add session restoring.

Imagine you have an application with a main window MyWindow inherited from KMainWindow (or from KXmlGuiWindow, which inherits from KMainWindow). In your main() function, you would then create/restore the application windows with something like this:

KApplication app;
if ( app.isSessionRestored() ) {
  kRestoreMainWindows< MyWindow >();
} else {
  // create default application as usual
  // example:
  MyWindow * window = new MyWindow();
  // The function will replace '#' with numbers that are
  // unique within the application:
return app.exec();

kRestoreMainWindows<>() will create (on the heap) as many instances of your main windows as have existed in the last session and call KMainWindow::restore() with the correct arguments. Note that also QWidget::show() is called implicitly.

About setObjectName("MyWindow#"): For session management and window management to work properly, all main windows in the application should have a different name. If you don't do it, KMainWindow will create a unique name, but it's recommended to explicitly pass a window name that will also describe the type of the window. If there can be several windows of the same type, append '#' (hash) to the name, and KMainWindow will replace it with numbers to make the names unique. For example, for a mail client which has one main window showing the mails and folders, and which can also have one or more windows for composing mails, the name for the folders window should be e.g. "mainwindow" and for the composer windows "composer#".

With this you can easily restore all top-level windows of your application.

It is also possible to restore different types of top-level windows (each derived from KMainWindow, of course) within one application. Imagine you have three classes of main windows: childMW1, childMW2 and childMW3:

KApplication app;
if ( app.isSessionRestored() ) {
  kRestoreMainWindows< childMW1, childMW2, childMW3 >();
} else {
  // create default application as usual
  // example:
  childMW1* window1 = new childMW1();
  childMW2* window2 = new childMW2();
  childMW3* window3 = new childMW3();
  // The function will replace '#' with numbers that are
  // unique within the application:
return app.exec();

Currently, kdelibs provides the kRestoreMainWindows<>() template functions for up to three template arguments.

Appendix: Architecture of the KDE session manager

The name of the session management server in KDE is ksmserver and it is part of the kdebase package. The server interacts with the KDE window manager kwin to save and restore the window geometries and to perform legacy session management. To make session management work, ksmserver has to be started as last process of the X login procedure. This happens automatically at the end of the startkde script.

Initial Author: Matthias Ettrich

This page was last edited on 4 October 2020, at 09:36. Content is available under Creative Commons License SA 4.0 unless otherwise noted.