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Creating your first Plasmoid
Tutorial Series   Plasma Tutorial
Previous   C++, Qt, KDE4 development environment
What's Next   Development/Tutorials/Plasma/GettingStarted..Some_More
Further Reading   CMake


This tutorial needs KDE 4.2 (trunk) to build. We are going to create a simple plasmoid in this tutorial. To keep things simple, we will only create a static plasmoid which will contain the following items:

  • An SVG Image
  • Icon
  • Some nice text

The Code

The .desktop file

Every Plasmoid needs a .desktop file to tell plasma how it should be started and what name it carries.


[Desktop Entry]
Name=Tutorial 1
Comment=Plasma Tutorial 1
X-KDE-PluginInfo-Author=Bas Grolleman
The most important bits are the '''X-KDE-Library''' and '''X-KDE-PluginInfo-Name''', they are the "glue" between your class and plasma, without it, nothing will start.  For '''X-KDE-PluginInfo-Category''', refer to the [[Projects/Plasma/PIG | PIG]].
=== The header file ===
This is the example header file. Comments have been added in the code for clarity.
<syntaxhighlight lang="cpp-qt">
// Here we avoid loading the header multiple times
#ifndef Tutorial1_HEADER
#define Tutorial1_HEADER
// We need the Plasma Applet headers
#include <KIcon>
#include <Plasma/Applet>
#include <Plasma/Svg>
class QSizeF;
// Define our plasma Applet
class PlasmaTutorial1 : public Plasma::Applet
        // Basic Create/Destroy
        PlasmaTutorial1(QObject *parent, const QVariantList &args);
        // The paintInterface procedure paints the applet to screen
        void paintInterface(QPainter *p,
                const QStyleOptionGraphicsItem *option,
                const QRect& contentsRect);
	void init();
        Plasma::Svg m_svg;
        KIcon m_icon;
==== void paintInterface(QRectF contentsRect) ====
This can be considered the main function since it paints the plasmoid on to the screen. Here, you define how you want your plasmoid to look.
You should only paint in the boundaries defined by contentsRect and avoid using geometry(). When a plasmoid does not have a standard background, e.g. when it's disabled with a setBackgroundHints() call or it's in the panel, geometry() and boundingRect() behave the same; however, when the standard background is enabled (the usual case), the appplet will have a margin where it should not be painted
=== The actual work file ===
Here is the body of the function, again with a lot of comments in between. 
<syntaxhighlight lang="cpp-qt">
#include "plasma-tutorial1.h"
#include <QPainter>
#include <QFontMetrics>
#include <QSizeF>
#include <plasma/svg.h>
#include <plasma/theme.h>
PlasmaTutorial1::PlasmaTutorial1(QObject *parent, const QVariantList &args)
    : Plasma::Applet(parent, args),
    // this will get us the standard applet background, for free!
    resize(200, 200);
    if (hasFailedToLaunch()) {
        // Do some cleanup here
    } else {
        // Save settings
void PlasmaTutorial1::init()
    // A small demonstration of the setFailedToLaunch function
    if (m_icon.isNull()) {
        setFailedToLaunch(true, "No world to say hello");
void PlasmaTutorial1::paintInterface(QPainter *p,
        const QStyleOptionGraphicsItem *option, const QRect &contentsRect)
    // Now we draw the applet, starting with our svg
    m_svg.resize((int)contentsRect.width(), (int)contentsRect.height());
    m_svg.paint(p, (int)contentsRect.left(), (int);
    // We place the icon and text
    p->drawPixmap(7, 0, m_icon.pixmap((int)contentsRect.width(),(int)contentsRect.width()-14));
                Qt::AlignBottom | Qt::AlignHCenter,
                "Hello Plasmoid!");
// This is the command that links your applet to the .desktop file
K_EXPORT_PLASMA_APPLET(tutorial1, PlasmaTutorial1)
#include "plasma-tutorial1.moc"
==== K_EXPORT_PLASMA_APPLET ( <name>, <class> ) ====
This is a small but very important part that links your classname to the applet name in the .desktop file. If your applet doesn't seem to get loaded, there may be a difference between this declaration and your .desktop file
The K_EXPORT_PLASMA_APPLET adds "plasma_applet_", please pay attention to this when setting up your .desktop file to avoid a name difference
==== Plasma/Svg ====
As you can see in the example code we are using the {{class|Plasma::Svg}} object, there are some important things to note here.
First we're using a relative path '''widgets/background''' which causes {{class|Plasma::Svg}} to use {{class|Plasma::Theme}} to locate the SVG data. While {{class|Plasma::Svg}} does support loading arbitrary files when passed an absolute path, use relative paths from the theme as often as possible as it makes Plasma skinable and the individual plasmoids look like a combined whole instead of a group of separate unrelated applications. You can see a list of available image components on the [[Projects/Plasma/Theme|Plasma Theme page]].
In either mode, {{class|Plasma::Svg}} can be used to draw a subset of the SVG file by passing it an element id that appears in the SVG document. As a good example, if you open the clock.svg file that ships with the default theme, you will see that it has a background, 3 needles (hour, minute and seconds) and a foreground (the glass). Due to the ability to put all the elements in one file the SVG file shows a clock. This is much nicer for artists compared to editing 5 separate files that they have to imagine on top of each other, and much nicer for performance as only one SVG renderer and one file read from disk is necessary.
==== setBackgroundHints(DefaultBackground) ====
Since drawing a background is a common function there is a fast and easier way of doing it. By adding <tt>setBackgroundHints(DefaultBackground)</tt> to the code, the default Plasma background gets drawn behind your plasmoid. This not only saves you time and code, but creates a more consistent presentation for the user.
==== The init() method ====
In the constructor you only tell plasma about the background and configuration file if any. You also set the start size in the constructor. After that, plasma will take care of any resizing and you never have to worry about size. In the <tt>init()</tt> method you initialize everything that needs to be initialized such as reading config data for example.
==== hasFailedToLaunch() ====
If for some reason, the applet fails to get up on its feet (the library couldn't be loaded, necessary hardware support wasn't found, etc..) this method returns true. Using this function gives your application a chance to cleanup before quiting. 
==== setFailedToLaunch(bool, QString) ====
When your application is unable to start, this function allows you to inform Plasma and give an optional reason why. Plasma will then draw a standardized error interface to inform the user of the situation and your applet will not be called upon to do any drawing on its own from that point forward. If your plasmoid becomes more complex and depends on multiple factors this is the nicest way to cleanup.
==== dataUpdated(const QString &source, const Plasma::DataEngine::Data &data) ====
If you would like to connect to any of Plasma's DataEngines, you can implement the dataUpdated method in your Plasmoid. When a DataEngine is connected directly to your Applet subclass, dataUpdated will be called when the DataEngine sends you updated data.
==== Determine the applet size and geometry: geometry() and contentsRect() ====
If you need to know, in your applet code, what the applet size and geometry is, call contentsRect() and contentsRect().size(). Avoid calling geometry() and size() because they don't take into account the margin's size set by the applets default background.
Also avoid using absolute numbers to position items in the applet like QPoint(0, 0) to indicate the top-left point of your applet, instead use contentsRect().topLeft().
=== Building it all, the CMakeLists.txt ===
Finally, to put everything together you need to build everything. To tell cmake what needs to go where there is the CMakeLists.txt file. 
For more details on CMake please read [[Development/Tutorials/CMake]]
<code bash>
# Project Needs a name ofcourse
# Find the required Libaries
find_package(KDE4 REQUIRED)
add_definitions (${QT_DEFINITIONS} ${KDE4_DEFINITIONS})
# We add our source code here
set(tutorial1_SRCS plasma-tutorial1.cpp)
# Now make sure all files get to the right place
kde4_add_plugin(plasma_applet_tutorial1 ${tutorial1_SRCS})
                      ${KDE4_PLASMA_LIBS} ${KDE4_KDEUI_LIBS})
install(TARGETS plasma_applet_tutorial1
install(FILES plasma-applet-tutorial1.desktop
== Testing the Applet ==
If your current Development Environment differs from the Test Installation, you have to run cmake with -DCMAKE_INSTALL_PREFIX=`kde-config --prefix`. Then run make. If succesfull the applet can be installed by running sudo make install
* cp ./lib/ $KDEDIR/lib/kde4 
* cp ./plasma-applet-tutorial1.desktop $KDEDIR/share/kde4/services/
and run kbuildsycoca4 (so that KDE apps will know about the new desktop files).
In order to test your Applet you can use the '''plasmoidviewer''' program:
<code bash>
kbuildsycoca4 #Needed once to let KDE know there is a new plugin
plasmoidviewer applet_name
You can even view your Applet in a small desktop using plasmoidviewer:
<code bash>
plasmoidviewer -c desktop applet_name
Where '''applet_name''' is the value specified into .desktop for the '''X-KDE-PluginInfo-Name''' key.
Otherwise you can restart plasma, so the Applet will be displayed in the Applet Browser:
<code bash>
kquitapp plasma-desktop
If that doesn't work you will have to restart your KDE session by logging out and back in. Or try to set export KDEDIRS=/usr/local:'kde4-config --prefix' and run kbuildsycoca4 again
== Wow that was fun! ==
Now that you made your first C++ Plasmoid, if you would like to continue down the path of enlightenment, check out: [[Development/Tutorials/Plasma/GettingStarted..Some_More]]

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