Development/Tutorials/KCM HowTo

From KDE TechBase

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

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_foo"))

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.

Exporting more than one module

If you need to export more than one module per library, you have to use the old loader. To do so, you must declare C functions named "create_${kcm_name}". For example if your module exposes two to declare 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)

Initializing on startup

If your module needs to initialize on KDE session startup, you must have a construct like: 

    extern "C"
    {
        KCModule *init_foo(QWidget *parent, const char *name)
        {
            // Do initialization here
        };
    }

Don't forget to add a "X-KDE-Init" key to your desktop file. (see below)

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.

Mandatory keys

A KCM desktop file must contains the following keys:

Type

Should be "Service".

X-KDE-ServiceTypes

Should be "KCModule".

Icon

Specifies the icon for the module.

Exec

Should be "kcmshell4 foo".

Name

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

Comment

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.

Categories

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

X-KDE-ParentApp

The application you put in this entry determines in what situations it will show. For example, if the line says "X-KDE-ParentApp=kcontrol" the module will show up in KControl. It is very crucial the selected ParentApp is correct, otherwise the KCM will show up in unnecessary places.

X-KDE-System-Settings-Parent-Category

Defines where the KCM will appear in System Settings. (FIXME: Where is the category list?)

X-KDE-Keywords

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

X-KDE-Library

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

Optional keys

Additionally the KCM desktop file may contains the following keys:

X-KDE-FactoryName

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: 

X-KDE-Library=frog
X-KDE-FactoryName=kermit

and "kcm_quak.desktop" would contain: 

X-KDE-Library=frog
X-KDE-FactoryName=quak

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

X-KDE-RootOnly

If this is set to "true", the module must be executed with root permissions. The module loader will then show the module in greyed-out (disabled) state with a warning until the "modify" button is pressed which allows running the module in an root environment using kdesu and QXEmbed.

X-KDE-Init

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 is "bell", for example, the function "init_bell" is called in the library indicated by X-KDE-Library.

X-KDE-Test-Module

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 is "bell", for example, the function "init_bell" is called in the library indicated by X-KDE-Library.

NoDisplay

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.

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)

include(KDE4Defaults)
include(MacroLibrary)

set(FOO_SRCS
    foo.cpp
    # Other sources go there
)

kde4_add_plugin(kcm_foo ${FOO_SRCS})

target_link_libraries(kcm_foo
    ${KDE4_KDEUI_LIBS}
    # Other necessary libraries go there
)

install(TARGETS kcm_foo
    DESTINATION ${PLUGIN_INSTALL_DIR}
)

install(FILES kcm_foo.desktop
    DESTINATION ${SERVICES_INSTALL_DIR}
)

What else do I need?

There are a number of additional things for convenience.

kcmshell4

Consider you want to run a module standalone. Call "kcmshell4 module". For example, to get the font and the desktop color settings, use: 

   kcmshell4 fonts colors

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 to the behavior and results than starting kcmshell4 in a separate process.

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

KCModuleContainer

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

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.

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 updated to KDE4 by Aurélien Gâteau <[email protected]>.

Original copyright header: 

Copyright (C) 2003 Daniel Molkentin <[email protected]>
Copyright (C) 2004 Frans Englich <[email protected]>

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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