Difference between revisions of "Development/Tutorials/KCM HowTo"

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Please note that your library name should only contain letters (upper and lowercase), numbers and underscores ("[A-Z][a-z][0-9]_"), because this name will be used internally as a dbus object. So "kcm_foo" is fine while "kcm-foo" is not.
==What else do I need?==
==What else do I need?==

Latest revision as of 14:43, 2 July 2013

Tutorial Series   Plugins and KParts
What's Next  
Further Reading   KCModule Class Reference


[edit] Introduction

This howto describes how to write KConfig Modules (KCMs from now on). These KCM can appear in System Settings or in the configuration dialog of individual applications.

A KCM is made of two elements:

  • A shared library
  • A desktop file

[edit] The shared library

Implementing a KCM is done by:

  1. Subclassing the KCModule class. In this documentation we assume the class inheriting from KCModule is named FooKcm.
  2. Exporting the module so that System Settings or kcmshell4 can find it.

The prefered way to export the module is through KPluginFactory. Here is how to do it:

    #include <KPluginFactory>
    K_PLUGIN_FACTORY(FooKcmFactory, registerPlugin<FooKcm>();)
    K_EXPORT_PLUGIN(FooKcmFactory("kcm_foo" /* kcm name */, "kcm_foo" /* catalog name */))

If you get errors, make sure the constructor signature of your derived class matches with the signature of KCModule constructor (the QVariantList argument matters).

If the name of your module is "foo", the name of the library should be "kcm_foo.so". It should be installed into $KDEDIR/lib/kde4.

[edit] Exporting more than one module

If you need to export more than one module per library, you have to declare them another way: you must create C functions named "create_${kcm_name}". For example if your module exposes two KCMs named Foo1 and Foo2, the "create_" functions would look like this:

    extern "C"
        KCModule *create_foo1(QWidget *parent, const char *name)
            return new Foo1Kcm(parent, name);
        KCModule *create_foo2(QWidget *parent, const char *name)
            return new Foo2Kcm(parent, name);

Additionally, you will need to add "X-KDE-FactoryName" keys to your desktop file. (see below)

[edit] Initializing on startup

It is possible to get code from the KCM to be run at startup by kcminit.

To do so you must declare a function named "kcminit_${module}", like this:

    extern "C"
        KDEEXPORT void kcminit_foo()
            // Do initialization here

You must also add a "X-KDE-Init-Symbol" key to your desktop file. (see below)

[edit] The desktop file

To declare a KCModule's existence a desktop file must be installed in the proper place.

Desktop files are defined in the desktop file specification.

[edit] Mandatory keys

A KCM desktop file must contains the following keys:


Should be "Service".


Should be "KCModule" for most KCM. If you want to use the kcminit feature it should be "KCModule,KCModuleInit".


Specifies the icon for the module.


Should be "kcmshell4 foo".


This will be used by System Settings as your KCM label.


This text shows up in the title area in System Settings. If the module is not grouped with other modules it will also be used as a tooltip in the KCM list view.


Should contain at least "Qt;KDE;X-KDE-settings-system;".


Set this to "kcontrol" if you want your KCM to show up in System Settings.

The application you put in this key determines in what situations your KCM will show. It is crucial to select a correct ParentApp, otherwise the KCM will show up in unnecessary places.


Defines where the KCM will appear in System Settings.

Possible values at the time of this writing are:

  • settings-lost-and-found
  • settings-hardware
  • settings-network-and-connectivity
  • settings-system-administration
  • settings-application-appearance-and-behavior
  • settings-workspace-appearance-and-behavior

You can get a list of possible categories with this command, which lists all toplevel categories:

ktraderclient --servicetype SystemSettingsCategory --constraint "[X-KDE-System-Settings-Parent-Category] == ''" | grep DesktopEntryName

Just do not use the "settings-lost-and-found" entry.


A comma-separated list containing words the search functionality should trigger on.


This is the name of the library, without the "kcm_" prefix. In our example, it should be "foo".

[edit] Optional keys

Additionally the KCM desktop file may contains the following keys:


This entry can be used to set the name of the factory function in the library. If you only have one KCModule in a library this key is not needed. If you have several KCMs in one library you will need a desktop file for each KCM.

For example, if you have a library named: kcm_frog.so with two modules, named "kermit" and "quak", kcm_kermit.desktop would contain:


and "kcm_quak.desktop" would contain:


The module loader would then call the "create_kermit" and "create_quak" functions respectively.


If the module has to perform some action at system startup, use this entry to build the name of a function to call. if X-KDE-Init-Symbol is "bell", for example, the function "kcminit_bell" is called in the library indicated by X-KDE-Library.


If this is set to true the module will not show up in System Settings or when viewed with kcmshell4. This is useful when you need to do something at startup using X-KDE-Init but don't want the module to show up in System Settings.

You can also make the value of this key depend on the output of a program, using the "[$e]" key suffix. For example:

Hidden[$e]=$(if test -e /dev/js*; then echo "false"; else echo "true"; fi)

This example executes the given code in a shell and uses the stdout output for the Hidden value (so it's either Hidden=true or Hidden=false).

[edit] Example CMakeLists.txt

Here is a minimal CMakeLists.txt which builds and installs the shared library and the desktop files at the right places:

find_package(KDE4 REQUIRED)
    # Other sources go there
kde4_add_plugin(kcm_foo ${FOO_SRCS})
    # Other necessary libraries go there
install(TARGETS kcm_foo
install(FILES kcm_foo.desktop

Please note that your library name should only contain letters (upper and lowercase), numbers and underscores ("[A-Z][a-z][0-9]_"), because this name will be used internally as a dbus object. So "kcm_foo" is fine while "kcm-foo" is not.

[edit] What else do I need?

There are a number of additional things for convenience.

[edit] kcmshell4

You can run one or several modules independently with "kcmshell4 [module_name]". For example, to get the font and the desktop color settings, use "kcmshell4 fonts colors".

To get a list of the available modules, use "kcmshell4 --list".

[edit] KCMultiDialog

Sometimes, you may want to reuse your KCModule inside an application. There are two ways to accomplish this:

The first option is to simply fork and call "kcmshell4 foo".

The second option is to use KCMultiDialog. This is a simple dialog which can show an arbitrary number of modules in a normal KDialog. This approach gives you finer control than starting kcmshell4 in a separate process.

Since your module is a simple library, you can just link to it anyway.

[edit] KCModuleContainer

The class KCModuleContainer allows great flexibility to handle modules. The API docs explains its usage the best.

[edit] Debugging your module

You can attach gdb, valgrind or whatever to "kcmshell4 [yourmodule]" to track down leaks or crashes. If you need to trace it down inside System Settings, make sure you pass --nofork to System Settings on startup.

You really want to use kcmshell4 for debugging as long as your debugging does not involve debugging bad interaction with the System Settings framework itself.

[edit] About this howto

This howto has been imported from http://websvn.kde.org/trunk/www/sites/developer/documentation/other/kcm_howto.html?view=markup and refreshed for KDE4 by Aurélien Gâteau <agateau@kde.org>.

Original copyright header:

Copyright (C) 2003 Daniel Molkentin <molketin@kde.org>
Copyright (C) 2004 Frans Englich <frans.englich@telia.com>

Permission is granted to copy, distribute and/or modify this document under the
terms of the GNU Free Documentation License, Version 1.2 or any later version
published by the Free Software Foundation; with no Invariant Sections, no
Front-Cover Texts, and no Back-Cover Texts. A copy of the license is included
in the section entitled "GNU Free Documentation License".

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