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diff --git a/HACKING.md b/HACKING.md new file mode 100644 index 0000000..bdcb9ba --- /dev/null +++ b/HACKING.md @@ -0,0 +1,341 @@ +# 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. + +```javascript + 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 It is outdated and hasn't worked for a +while.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. +```javascript + 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. +```javascript + const { GLib, Gio, St } = imports.gi; + + const Main = imports.ui.main; + const Params = imports.misc.params; + 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: +```javascript + const MY_DBUS_INTERFACE = 'org.my.Interface'; +``` + +## Variable declaration + +Always use either `const` or `let` when defining a variable. +```javascript + // 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](https://developer.mozilla.org/en/JavaScript/New_in_JavaScript/1.7#Block_scope_with_let_%28Merge_into_let_Statement%29) + +## Classes + +There are many approaches to classes in JavaScript. We use standard ES6 classes +whenever possible, that is when not inheriting from GObjects. +```javascript + 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 inheritance, we use the GObject.registerClass() function provided +by gjs. +```javascript + 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: +```javascript + 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(paintContext) { + let framebuffer = paintContext.get_framebuffer(); + let coglContext = framebuffer.get_context(); + let alloc = this.get_allocation_box(); + + let pipeline = new Cogl.Pipeline(coglContext); + pipeline.set_color4ub(255, 0, 0, 255); + + framebuffer.draw_rectangle(pipeline, + 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](http://git.gnome.org/browse/gnome-shell/tree/js/ui/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` (deprecated) 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, in some cases we have a wrapper +class that has a property called `actor` (now deprecated). We call this +wrapper class the "delegate". + +We sometimes use expando properties to set a property called `_delegate` on +the actor itself: +```javascript + var MyActor = GObject.registerClass( + class MyActor extends Clutter.Actor { + _init(params) { + super._init(params); + this._delegate = this; + } + }); +``` + +Or using the deprecated `actor`: +```javascript + 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. +In case the class is an actor itself, the `_delegate` can be just set to `this`. + +## 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. +```javascript + 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: +```javascript + 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: +```javascript + foo = { 'bar': 42 }; + foo = { bar: 42 }; +``` + +and so are these: +```javascript + 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']`. + +## Animations + +Most objects that are animated are actors, and most properties used in animations +are animatable, which means they can use implicit animations: + +```javascript + moveActor(actor, x, y) { + actor.ease({ + x, + y, + duration: 500, // ms + mode: Clutter.AnimationMode.EASE_OUT_QUAD + }); + } +``` + +The above is a convenience wrapper around the actual Clutter API, and should generally +be preferred over the more verbose: + +```javascript + moveActor(actor, x, y) { + actor.save_easing_state(); + + actor.set_easing_duration(500); + actor.set_easing_mode(Clutter.AnimationMode.EASE_OUT_QUAD); + actor.set({ + x, + y + }); + + actor.restore_easing_state(); + } +``` + +There is a similar convenience API around Clutter.PropertyTransition to animate +actor (or actor meta) properties that cannot use implicit animations: + +```javascript + desaturateActor(actor, desaturate) { + let factor = desaturate ? 1.0 : 0.0; + actor.ease_property('@effects.desaturate.factor', factor, { + duration: 500, // ms + mode: Clutter.AnimationMode.EASE_OUT_QUAD + }); + } +``` |