Notions about Gnome to develop extensions in GJS (from https://gjs.guide/extensions/)

Gnome-notions.md

D-Bus

• Interprocess communication (IPC) system for apps.
• Runs as a service.
• Two buses:
  • System bus: e.g. to notify about hardware connect/disconnect. Its service: systemctl status dbus.
  • Session bus: Specific to user sessions, e.g. communication between Gnome components. Its service: systemctl --user status dbus.
• dbus-run-session <cmd>: Starts a new session bus and run cmd in that session (to have access to the session dbus for IPC):
  • e.g gnome-shell --nested: Start Gnome within an existing session (for dev. & testing) without affecting the primary UI.

Extensions

Create a new extension: gnome-extension create --interactive

Files

• Two mandatory files: metadata.json, and extension.js
• prefs.json: Preferences for extension.
  • Uses GTK4 and Adwaita.
  • Adwaita: Toolkit that offers widgets to build Gnome apps (extends GTK base widgets).
  • Runs in a separate process from Gnome-shell.
• extension.js:
  • Uses Clutter and St toolkits (latter builds on Clutter to provide more Widgets & CSS).

Settings

• GSettings: System for Gnome apps to manage their settings.
• dconf: underlying backend database for GSettings, with parameters stored in key-value format.

GVariant

• Variant type (like C's union and C++'s std::variant) that may contain:
  • Simple types: int, bool.
  • Complex types: array of dict.
• Usage: data to be serialized:
  • Data sent over D-Bus.
  • Saved settings with GSettings.