I’m trying to implement a GtkSourceView in Gnome-Builder ( using Python as the language ) and I’m totally unable to get it working.

I have written a small program that works perfectly as a proof of concept, but I’m unable to get it working with Builder.

import gi
gi.require_version("Gtk", "4.0")
gi.require_version("GtkSource", "5")
from gi.repository import GObject, Gtk, GtkSource

def on_activate(app):
    
    # Define Window Properties
    win = Gtk.ApplicationWindow(application=app)
    win.set_title("test")
    win.set_default_size( 800, 600 )
    
    # Create scrollable Container
    scroll = Gtk.ScrolledWindow()
    
    
    # Create and configure codeview with it's settings
    codeview = GtkSource.View()                     # See https://gnome.pages.gitlab.gnome.org/gtksourceview/gtksourceview5/class.View.html
    codeview.set_wrap_mode( Gtk.WrapMode.WORD_CHAR )# See https://docs.gtk.org/gtk4/enum.WrapMode.html
    codeview.set_monospace( True )                  # See https://docs.gtk.org/gtk4/method.TextView.set_monospace.html
    codeview.set_smart_home_end( 1 )                # See https://gnome.pages.gitlab.gnome.org/gtksourceview/gtksourceview5/enum.SmartHomeEndType.html
    codeview.set_tab_width( 4 )                     # See https://gnome.pages.gitlab.gnome.org/gtksourceview/gtksourceview5/method.View.set_tab_width.html
    codeview.set_show_line_marks( True )            # See https://gnome.pages.gitlab.gnome.org/gtksourceview/gtksourceview5/method.View.set_show_line_marks.html
    codeview.set_show_line_numbers( True )          # See https://gnome.pages.gitlab.gnome.org/gtksourceview/gtksourceview5/method.View.set_show_line_numbers.html
    
    # Prepare a colorscheme
    manager = GtkSource.StyleSchemeManager().get_default()
    scheme = manager.get_scheme("oblivion")
    
    # Prepare the textbuffer
    textbuffer = GtkSource.Buffer()
    textbuffer.set_text("teststring", -1)
    textbuffer.set_style_scheme(scheme)

    # Link all widgets
    codeview.set_buffer(textbuffer)
    scroll.set_child( codeview )

    # Add widgets to window and show it.
    win.set_child( scroll )
    win.present()

# Define app
app = Gtk.Application(application_id='org.gtk.Example')
# Connect methods
app.connect('activate', on_activate)
# Run the app
app.run(None)

This works, but whenever I try to use GtkSourceView in the UI template in Builder it complains about not knowing the type:

Invalid object type ‘GtkSourceView’

Can somebody please give me a hint where to start and how to implement this? The documentation is very very sparse and I’m having a hard time finding documentation about the Gtk4 Libraries with examples in Python.

Best regards