gergelypolonkai-web-jekyll/_drafts/writing-a-gnome-shell-extension.md

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post	Writing a GNOME Shell extension

I could not find a good tutorial on how to write a GNOME Shell extension. There is a so called step by step instruction list on how to do it, but it has its flaws, including grammar and clearance. As I wanted to create an extension for my SWE GLib library to display the current position of some planets, I dug into existing (and working) extensions’ source code and made up something. Comments welcome!

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GNOME Shell extensions are written in JavaScript and are interpreted by GJS. Using introspected libraries from JavaScript is not a problem for me (see SWE GLib’s Javascript example; it’s not beautiful, but it’s working), but wrapping your head around the Shell’s concept can take some time.

The Shell is a Clutter stage, and all the buttons (including the top-right “Activities” button) are actors on this stage. You can add practically anything to the Shell panel that you can add to a Clutter stage.

The other thing to remember is the lifecycle of a Shell extension. After calling init(), there are two ways forward: you either use a so called extension controller, or plain old JavaScript functions enable() and disable(); I will go on with the former method for reasons discussed later.

If you are fine with the enable()/disable() function version, you can ease your job with the following command:

gnome-shell-extension-tool --create-extension

This will ask you a few parameters and create the necessary files for you. On what these parameters should look like, please come with me to the next section.

Placement and naming

Extensions reside under $HOME/.local/share/gnome-shell/extensions, where each of them have its own directory. The directory name has to be unique, of course; to achieve this, they are usually the same as the UUID of the extension.

The UUID is a string of alphanumeric characters, with some extras added. Generally, it should match this regular expression: ^[-a-zA-Z0-9@._]+$. The convention is to use the form extension-name@author-id, e.g. Planets@gergely.polonkai.eu. Please see this link for some more information about this.

Anatomy of an extension

Extensions consist of two main parts, metadata.json and extension.js.

The metadata.json file contains compatibility information and, well, some meta data:

{
    "shell-version": ["3.18"],
    "uuid": "planets@gergely.polonkai.eu",
    "name": "Planets",
    "description": "Display current planet positions"
}

Here, shell-version must contain all versions of GNOME Shell that is known to load and display your extension correctly. You can insert minor versions here, like I did, or exact version numbers, like 3.18.1.

In the extension.js file, which contains the actual extension code, the only thing you actually need is an init() function:

function init(extensionMeta) {
    // Do whatever it takes to initialize your extension, like
    // initializing the translations. However, never do any widget
    // magic here yet.

    // Then return the controller object
    return new ExtensionController(extensionMeta);
}

Extension controller

So far so good, but what is this extension controller thing? It is an object which is capable of managing your GNOME Shell extension. Whenever the extension is loaded, its enable() method is called; when the extension is unloaded, you guessed it, the disable() method gets called.

function ExtensionController(extensionMeta) {
    return {
        extensionMeta: extensionMeta,
        extension: null,

        enable: function() {
            this.extension = new PlanetsExtension(this.extensionMeta);

            Main.panel.addToStatusArea("planets",
                                       this.extension,
                                       0, "right");
        },

        disable: function() {
            this.extension.actor.destroy();
            this.extension.destroy();

            this.extension = null;
        }
    }
}

This controller will create a new instance of the PlanetsExtension class and add it to the panel’s right side when loaded. Upon unloading, the extension’s actor gets destroyed (which, as you will see later, gets created behind the scenes, not directly by us), together with the extension itself. Also, for safety measures, the extension is set to null.

The extension

The extension is a bit more tricky, as, for convenience reasons, it should extend an existing panel widget type.

function PlanetsExtension(extensionMeta) {
    this._init(extensionMeta);
}

PlanetsExtension.prototype = {
    __proto__ = PanelMenu.Button.prototype,

    _init: function(extensionMeta) {
        PanelMenu.Button.prototype._init.call(this, 0.0);

        this.extensionMeta = extensionMeta;

        this.panelContainer = new St.BoxLayout({style_class: 'panel-box'});
        this.actor.add_actor(this.panelContainer);
        this.actor.add_style_class_name('panel-status-button');

        this.panelLabel = new St.Label({
            text: 'Loading',
            y_align: Clutter.ActorAlign.CENTER
        });

        this.panelContainer.add(this.panelLabel);
    }
};

Here we extend the Button class of panelMenu, so we will be able to do some action upon activate.

The only parameter passed to the parent’s _init() function is menuAlignment, with the value 0.0, which is used to position the menu arrow. (Note: I cannot find any documentation on this, but it seems that with the value 0.0, a menu arrow is not added.)

The extension class in its current form is capable of creating the actual panel button displaying the text “Loading” in its center.

Loading up the extension

Now with all the necessary import lines added:

// The PanelMenu module that contains Button
const PanelMenu = imports.ui.panelMenu;
// The St class that contains lots of UI functions
const St = imports.gi.St;
// Clutter, which is used for displaying everything
const Clutter = imports.gi.Clutter;

As soon as this file is ready, you can restart your Shell (press Alt-F2 and enter the command r), and load the extension with e.g. the GNOME Tweak Tool. You will see the Planets button on the right. This little label showing the static text “Planets”, however, is pretty boring, so let’s add some action.

Adding some periodical change

Since the planets’ position continuously change, we should update our widget every minute or so. Let’s patch our _init() a bit:

this.last_update = 0;

MainLoop.timeout_add(1, Lang.bind(this, function() {
    this.last_update++;
    this.panelLabel.set_text("Update_count: " + this.last_update);
}))

This, of course, needs a new import line for MainLoop to become available:

const MainLoop = imports.mainloop;
const Lang = imports.lang;

Now if you restart your Shell, your brand new extension will increase its counter every second. This, however, presents some problems.

SWE GLib queries can sometimes be expensive, both in CPU and disk operations, so updating our widget every second may present problems. Also, planets don’t go that fast. We may update our timeout value from 1 to 60 or something, but why don’t just give our user a chance to set it?

Introducing settings

Getting settings from GSettings is barely straightforward, especially for software installed in a non-GNOME directory (which includes extensions). To make our lives easier, I copied over a convenience library from the Hamster project’s extension, originally written by Giovanni Campagna. The relevant function here is getSettings():

/**
 * getSettings:
 * @schema: (optional): the GSettings schema id
 *
 * Builds and return a GSettings schema for @schema, using schema files
 * in extensionsdir/schemas. If @schema is not provided, it is taken from
 * metadata['settings-schema'].
 */
function getSettings(schema) {
    let extension = ExtensionUtils.getCurrentExtension();

    schema = schema || extension.metadata['settings-schema'];

    const GioSSS = Gio.SettingsSchemaSource;

    // check if this extension was built with "make zip-file", and thus
    // has the schema files in a subfolder
    // otherwise assume that extension has been installed in the
    // same prefix as gnome-shell (and therefore schemas are available
    // in the standard folders)
    let schemaDir = extension.dir.get_child('schemas');
    let schemaSource;
    if (schemaDir.query_exists(null))
        schemaSource = GioSSS.new_from_directory(schemaDir.get_path(),
                                                 GioSSS.get_default(),
                                                 false);
    else
        schemaSource = GioSSS.get_default();

    let schemaObj = schemaSource.lookup(schema, true);
    if (!schemaObj)
        throw new Error('Schema ' + schema + ' could not be found for extension '
                        + extension.metadata.uuid + '. Please check your installation.');

    return new Gio.Settings({ settings_schema: schemaObj });
}

You can either incorporate this function into your extension.js file, or just use convenience.js file like I (and the Hamster applet) did and import it:

const ExtensionUtils = imports.misc.extensionUtils;
const Me = ExtensionUtils.getCurrentExtension;
const Convenience = Me.imports.convenience;

Now let’s create the settings definition. GSettings schema files are XML files. We want to add only one settings for now, the refresh interval.

<?xml version="1.0" encoding="utf-8"?>
<schemalist>
    <schema id="org.gnome.shell.extensions.planets" path="/org/gnome/shell/extensions/planets/">
        <key name="refresh-interval" type="i">
            <default>30</default>
            <summary>Refresh interval of planet data</summary>
            <description>Interval in seconds. Sets how often the planet positions are recalculated. Setting this too low (e.g. below 30) may raise performance issues.</description>
        </key>
    </schema>
</schemalist>

you need to compile these settings with

glib-compile-schemas --strict schemas/

Now let’s utilize this new setting. In the extension’s _init() function, add the following line:

this._settings = Convenience.getSettings();

And, for getSettings() to work correctly, we also need to extend our metadata.json file:

    "settings-schema": "planets"

After another restart (please, GNOME guys, add an option to reload extensions!), your brand new widget will refresh every 30 seconds.

Displaying the planet positions

The settings panel

Start an application