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API Documentation
Library references can be found in the releases:
These libraries are often also installed by distributions in /usr/share/gtk-doc/html/.
Additionally, Xfce uses the GTK+/Glib family of libraries which can be found at the following:
Tutorials for getting started with GTK+ are available at:
If you're looking to write a panel plugin, the xfce4-sample-plugin provides a base to get started.