gnome-shell/HACKING.md

Coding guide

Our goal is to have all JavaScript code in GNOME follow a consistent style. In a dynamic language like JavaScript, it is essential to be rigorous about style (and unit tests), or you rapidly end up with a spaghetti-code mess.

A quick note

Life isn't fun if you can't break the rules. If a rule seems unnecessarily restrictive while you're coding, ignore it, and let the patch reviewer decide what to do.

Indentation, braces and whitespace

• Use four-space indents.
• Braces are on the same line as their associated statements.
• You should only omit braces if both sides of the statement are on one line.
• One space after the function keyword.
• No space between the function name in a declaration or a call.
• One space before the parens in the if statements, or while, or for loops.

    function foo(a, b) {
        let bar;

        if (a > b)
            bar = do_thing(a);
        else
            bar = do_thing(b);

        if (var == 5) {
            for (let i = 0; i < 10; i++) {
                print(i);
            }
        } else {
            print(20);
        }
    }

Semicolons

JavaScript allows omitting semicolons at the end of lines, but don't. Always end statements with a semicolon.

js2-mode

If using Emacs, do not use js2-mode. It is outdated and hasn't worked for a while. emacs now has a built-in JavaScript mode, js-mode, based on espresso-mode. It is the de facto emacs mode for JavaScript.

File naming and creation

For JavaScript files, use lowerCamelCase-style names, with a .js extension.

We only use C where gjs/gobject-introspection is not available for the task, or where C would be cleaner. To work around limitations in gjs/gobject-introspection itself, add a new method in shell-util.[ch].

Like many other GNOME projects, we prefix our C source filenames with the library name followed by a dash, e.g. shell-app-system.c. Create a -private.h header when you want to share code internally in the library. These headers are not installed, distributed or introspected.

Imports

Use UpperCamelCase when importing modules to distinguish them from ordinary variables, e.g.

    const GLib = imports.gi.GLib;

Imports should be categorized into one of two places. The top-most import block should contain only "environment imports". These are either modules from gobject-introspection or modules added by gjs itself.

The second block of imports should contain only "application imports". These are the JS code that is in the gnome-shell codebase, e.g. imports.ui.popupMenu.

Each import block should be sorted alphabetically. Don't import modules you don't use.

    const GLib = imports.gi.GLib;
    const Gio = imports.gi.Gio;
    const St = imports.gi.St;

    const Main = imports.ui.main;
    const Params = imports.misc.params;
    const Tweener = imports.ui.tweener;
    const Util = imports.misc.util;

The alphabetical ordering should be done independently of the location of the location. Never reference imports in actual code.

Constants

We use CONSTANTS_CASE to define constants. All constants should be directly under the imports:

    const MY_DBUS_INTERFACE = 'org.my.Interface';

Variable declaration

Always use either const or let when defining a variable.

    // Iterating over an array
    for (let i = 0; i < arr.length; ++i) {
        let item = arr[i];
    }

    // Iterating over an object's properties
    for (let prop in someobj) {
        ...
    }

If you use "var" then the variable is added to function scope, not block scope. See What's new in JavaScript 1.7

Classes

There are many approaches to classes in JavaScript. We use standard ES6 classes whenever possible, that is when not inheriting from GObjects.

    var IconLabelMenuItem = class extends PopupMenu.PopupMenuBaseItem {
        constructor(icon, label) {
            super({ reactive: false });
            this.actor.add_child(icon);
            this.actor.add_child(label);
        }

        open() {
            log("menu opened!");
        }
    };

For GObject inheritence, we use the GObject.registerClass() function provided by gjs.

    var MyActor = GObject.registerClass(
    class MyActor extends Clutter.Actor {
        _init(params) {
            super._init(params);

            this.name = 'MyCustomActor';
        }
    });

GObject Introspection

GObject Introspection is a powerful feature that allows us to have native bindings for almost any library built around GObject. If a library requires you to inherit from a type to use it, you can do so:

    var MyClutterActor = GObject.registerClass(
    class MyClutterActor extends Clutter.Actor {

        vfunc_get_preferred_width(forHeight) {
             return [100, 100];
        }

        vfunc_get_preferred_height(forWidth) {
             return [100, 100];
        }

        vfunc_paint() {
             let alloc = this.get_allocation_box();
             Cogl.set_source_color4ub(255, 0, 0, 255);
             Cogl.rectangle(alloc.x1, alloc.y1,
                            alloc.x2, alloc.y2);
        }
    });

Translatable strings, environment.js

We use gettext to translate the GNOME Shell into all the languages that GNOME supports. The gettext function is aliased globally as _, you do not need to explicitly import it. This is done through some magic in the environment.js file. If you can't find a method that's used, it's probably either in gjs itself or installed on the global object from the Environment.

Use 'single quotes' for programming strings that should not be translated and "double quotes" for strings that the user may see. This allows us to quickly find untranslated or mistranslated strings by grepping through the sources for double quotes without a gettext call around them.

actor and _delegate

gjs allows us to set so-called "expando properties" on introspected objects, allowing us to treat them like any other. Because the Shell was built before you could inherit from GTypes natively in JS, we usually have a wrapper class that has a property called actor. We call this wrapper class the "delegate".

We sometimes use expando properties to set a property called _delegate on the actor itself:

    var MyClass = class {
        constructor() {
            this.actor = new St.Button({ text: "This is a button" });
            this.actor._delegate = this;

            this.actor.connect('clicked', this._onClicked.bind(this));
        }

        _onClicked(actor) {
            actor.set_label("You clicked the button!");
        }
    };

The 'delegate' property is important for anything which trying to get the delegate object from an associated actor. For instance, the drag and drop system calls the handleDragOver function on the delegate of a "drop target" when the user drags an item over it. If you do not set the _delegate property, your actor will not be able to be dropped onto.

Functional style

JavaScript Array objects offer a lot of common functional programming capabilities such as forEach, map, filter and so on. You can use these when they make sense, but please don't have a spaghetti mess of function programming messed in a procedural style. Use your best judgment.

Closures

this will not be captured in a closure, it is relative to how the closure is invoked, not to the value of this where the closure is created, because "this" is a keyword with a value passed in at function invocation time, it is not a variable that can be captured in closures.

All closures should be wrapped with Function.prototype.bind or use arrow notation.

    let closure1 = () => { this._fnorbate(); };
    let closure2 = this._fnorbate.bind(this);

A more realistic example would be connecting to a signal on a method of a prototype:

    const FnorbLib = imports.fborbLib;

    var MyClass = class {
        _init() {
            let fnorb = new FnorbLib.Fnorb();
            fnorb.connect('frobate', this._onFnorbFrobate.bind(this));
        }

        _onFnorbFrobate(fnorb) {
            this._updateFnorb();
        }
    };

Object literal syntax

In JavaScript, these are equivalent:

    foo = { 'bar': 42 };
    foo = { bar: 42 };

and so are these:

    var b = foo['bar'];
    var b = foo.bar;

If your usage of an object is like an object, then you're defining "member variables." For member variables, use the no-quotes no-brackets syntax: { bar: 42 } foo.bar.

If your usage of an object is like a hash table (and thus conceptually the keys can have special chars in them), don't use quotes, but use brackets: { bar: 42 }, foo['bar'].

Getters, setters, and Tweener

Getters and setters should be used when you are dealing with an API that is designed around setting properties, like Tweener. If you want to animate an arbitrary property, create a getter and setter, and use Tweener to animate the property.

    var ANIMATION_TIME = 2000;

    var MyClass = class {
        constructor() {
            this.actor = new St.BoxLayout();
            this._position = 0;
        }

        get position() {
            return this._position;
        }

        set position(value) {
            this._position = value;
            this.actor.set_position(value, value);
        }
    };

    let myThing = new MyClass();
    Tweener.addTween(myThing,
                     { position: 100,
                       time: ANIMATION_TIME,
                       transition: 'easeOutQuad' });