Mailspring/docs/PackageOverview.md

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---
Title: Building a Package
Section: Guides
Order: 1
---
Packages lie at the heart of N1. Each part of the core experience is a separate package that uses the Nylas Package API to add functionality to the client. Want to make a read-only mail client? Remove the core `Composer` package and you'll see reply buttons and composer functionality disappear.
Let's explore the files in a simple package that adds a Translate option to the Composer. When you tap the Translate button, we'll display a popup menu with a list of languages. When you pick a language, we'll make a web request and convert your reply into the desired language.
### Package Structure
Each package is defined by a `package.json` file that includes its name, version and dependencies. Packages may also declare dependencies which are loaded from npm - in this case, the [request](https://github.com/request/request) library. You'll need to `npm install` these dependencies locally when developing the package.
```
{
"name": "translate",
"version": "0.1.0",
"main": "./lib/main",
"description": "An example package for N1",
"license": "Proprietary",
"engines": {
"atom": "*"
},
"dependencies": {
"request": "^2.53"
}
}
```
Our package also contains source files, a spec file with complete tests for the behavior the package adds, and a stylesheet for CSS:
```
- package.json
- lib/
- main.coffee
- translate-button.cjsx
- spec/
- main-spec.coffee
- stylesheets/
- translate.less
```
`package.json` lists `lib/main` as the root file of our package. Since N1 runs NodeJS, we can `require` other source files, Node packages, etc.
N1 can read `js`, `coffee`, `jsx`, and `cjsx` files automatically.
2015-10-05 06:24:08 +08:00
Inside `main.coffee`, there are three important functions being exported:
```coffee
require './translate-button'
module.exports =
# Activate is called when the package is loaded. If your package previously
# saved state using `serialize` it is provided.
#
activate: (@state) ->
ComponentRegistry.register TranslateButton,
role: 'Composer:ActionButton'
# Serialize is called when your package is about to be unmounted.
# You can return a state object that will be passed back to your package
# when it is re-activated.
#
serialize: ->
{}
# This optional method is called when the window is shutting down,
# or when your package is being updated or disabled. If your package is
# watching any files, holding external resources, providing commands or
# subscribing to events, release them here.
#
deactivate: ->
ComponentRegistry.unregister(TranslateButton)
```
> N1 uses CJSX, a CoffeeScript version of JSX, which makes it easy to express Virtual DOM in React `render` methods! You may want to add the [Babel](https://github.com/babel/babel-sublime) plugin to Sublime Text, or the [CJSX Language](https://atom.io/packages/language-cjsx) for syntax highlighting.
### Package Stylesheets
Style sheets for your package should be placed in the _styles_ directory. Any style sheets in this directory will be loaded and attached to the DOM when your package is activated. Style sheets can be written as CSS or [Less](http://lesscss.org/), but Less is recommended.
Ideally, you won't need much in the way of styling. We've provided a standard set of components which define both the colors and UI elements for any package that fits into N1 seamlessly.
If you _do_ need special styling, try to keep only structural styles in the package stylesheets. If you _must_ specify colors and sizing, these should be taken from the active theme's [ui-variables.less][ui-variables]. For more information, see the [theme variables docs][theme-variables]. If you follow this guideline, your package will look good out of the box with any theme!
An optional `stylesheets` array in your `package.json` can list the style sheets by name to specify a loading order; otherwise, all style sheets are loaded.
### Package Assets
Many packages need other static files, like images. You can add static files anywhere in your package directory, and reference them at runtime using the `nylas://` url scheme:
```
<img src="nylas://my-package-name/assets/goofy.png">
a = new Audio()
a.src = "nylas://my-package-name/sounds/bloop.mp3"
a.play()
```
### Installing a Package
N1 ships with many packages already bundled with the application. When the application launches, it looks for additional packages in `~/.nylas/dev/packages`. Each package you create belongs in its own directory inside this folder.
In the future, it will be possible to install packages directly from within the client.