jupyterlab/docs/extensions_dev.md

Extensions Developer Guide

JupyterLab can be extended in three ways via:

• application plugins (top level): Application plugins extend the functionality of JupyterLab itself.
• mime renderer extension (top level): Mime Renderer extensions are a convenience for creating an extension that can render mime data and potentially render files of a given type.
• document widget extensions (lower level): Document widget extensions extend the functionality of document widgets added to the application, and we cover them in the "Documents" tutorial.

A JupyterLab application is comprised of:

• A core Application object
• Plugins

Plugins

A plugin adds a core functionality to the application:

• A plugin can require other plugins for operation.
• A plugin is activated when it is needed by other plugins, or when explicitly activated.
• Plugins require and provide Token objects, which are used to provide a typed value to the plugin's activate() method.
• The module providing plugin(s) must meet the JupyterLab.IPluginModule interface, by exporting a plugin object or array of plugin objects as the default export.

The default plugins in the JupyterLab application include:

• Terminal - Adds the ability to create command prompt terminals.
• Shortcuts - Sets the default set of shortcuts for the application.
• Images - Adds a widget factory for displaying image files.
• Help - Adds a side bar widget for displaying external documentation.
• File Browser - Creates the file browser and the document manager and the file browser to the side bar.
• Editor - Add a widget factory for displaying editable source files.
• Console - Adds the ability to launch Jupyter Console instances for interactive kernel console sessions.

A dependency graph for the core JupyterLab plugins (along with links to their source) is shown here:

Application Object

The JupyterLab Application object is given to each plugin in its activate() function. The Application object has a:

• commands - used to add and execute commands in the application.
• keymap - used to add keyboard shortcuts to the application.
• shell - a JupyterLab shell instance.

JupyterLab Shell

The JupyterLab shell is used to add and interact with content in the application. The application consists of:

• A top area for things like top level menus and toolbars
• Left and right side bar areas for collapsible content
• A main area for user activity.
• A bottom area for things like status bars

Phosphor

The Phosphor library is used as the underlying architecture of JupyterLab and provides many of the low level primitives and widget structure used in the application. Phosphor provides a rich set of widgets for developing desktop-like applications in the browser, as well as patterns and objects for writing clean, well-abstracted code. The widgets in the application are primarily Phosphor widgets, and Phosphor concepts, like message passing and signals, are used throughout. Phosphor messages are a many-to-one interaction that allows information like resize events to flow through the widget hierarchy in the application. Phosphor signals are a one-to-many interaction that allow listeners to react to changes in an observed object.

Extension Authoring

An Extension is a valid npm package that meets the following criteria:

• Exports one or more JupyterLab plugins as the default export in its main file.
• Has a jupyterlab key in its package.json which has "extension" metadata. The value can be true to use the main module of the package, or a string path to a specific module (e.g. "lib/foo").

While authoring the extension, you can use the command:

jupyter labextension link <path>

This causes the builder to re-install the source folder before building the application files. You can re-build at any time using jupyter lab build and it will reinstall these packages. You can also link other npm packages that you are working on simultaneously; they will be re-installed but not considered as extensions if they lack the metadata. Linked extensions and packages are included in jupyter labextension list.

You can also use jupyter labextension install <path>, but that will only copy the current contents of the source folder.

When using linked packages, you can run the command

jupyter lab --watch

This will cause the application to incrementally rebuild when one of the linked packages changes. You can open another terminal and run

npm run watch

to automatically recompile the TypeScript files while editing. Note that only the compiled JavaScript files (and the CSS files) are watched by the WebPack process.

Note that the application is built against released versions of the core JupyterLab extensions. If your extension depends on JupyterLab packages, it should be compatible with the dependencies in the jupyterlab/package.app.json file. If you must install a extension into a development branch of JupyterLab, you have to graft it into the source tree of JupyterLab itself. This may be done using the command

npm run add:sibling <path-or-url> && npm install

in the JupyterLab root directory, where <path-or-url> refers either to an extension npm package on the local filesystem, or a URL to a git repository for an extension npm package. This operation may be subsequently reversed by running

npm run remove:package <extension-dir-name>

This will remove the package metadata from the source tree, but wil not remove any files added by the addsibling script, which should be removed manually.

The package should export EMCAScript 5 compatible JavaScript. It can import CSS using the syntax require('foo.css'). The CSS files can also import CSS from other packages using the syntax @import url('~foo/index.css'), where foo is the name of the package.

The following file types are also supported (both in JavaScript and CSS): json, html, jpg, png, gif, svg, js.map, woff2, ttf, eot.

If your package uses any other file type it must be converted to one of the above types. If your JavaScript is written in any other dialect than EMCAScript 5 it must be converted using an appropriate tool.

If you publish your extension on npm.org, users will be able to install it as simply jupyter labextension install <foo>, where <foo> is the name of the published npm package. You can alternatively provide a script that runs jupyter labextension install against a local folder path on the user's machine or a provided tarball. Any valid npm install specifier can be used in jupyter labextension install (e.g. foo@latest, bar@3.0.0.0, path/to/folder, and path/to/tar.gz).

Mime Renderer Extensions

Mime Renderer extensions are a convenience for creating an extension that can render mime data and potentially render files of a given type.

Mime renderer extensions are more declarative than standard extensions. The extension is treated the same from the command line perspective (install and link), but it does not directly create JupyterLab plugins. Instead it exports an interface given in the rendermime-interfaces package.

The JupyterLab repo has an example mime renderer extension for vega2. It provides a mime renderer for vega data and registers itself as a document renderer for vega file types.

The rendermime-interfaces package is intended to be the only JupyterLab package needed to create a mime renderer extension (using the interfaces in TypeScript or as a form of documentation if using plain JavaScript).

The only other difference from a standard extension is that has a jupyterlab key in its package.json with "mimeExtension" metadata. The value can be true to use the main module of the package, or a string path to a specific module (e.g. "lib/foo").

Themes

A theme is a JupyterLab extension that uses a ThemeManager and can be loaded and unloaded dynamically. The package must include all static assets that are referenced by url() in its CSS files. The url() paths in a CSS file served by the Jupyter server must start with the path './lab/api/themes/<foo>/', where foo is the name of the package. Note that '@import' paths are still given as relative paths, e.g. ('@import './foo.css';). The path to the theme assets is specified package.json under the "jupyterlab" key as "themeDir". See the JupyterLab Light Theme for an example. Ensure that the theme files are included in the "files" metadata in package.json. A theme can optionally specify an embed.css file that can be consumed outside of a JupyterLab application. See the JupyterLab Light Theme for an example.

Standard (General-Purpose) Extensions

See the example, How to Extend the Notebook Plugin. Notice that the mime renderer and themes extensions above use a limited, simplified interface to JupyterLab's extension system. Modifying the notebook plugin requires the full, general-purpose interface to the extension system.

Extension Settings

An extension can specify user settings using a JSON Schema. The schema definition should be in a file that resides in the schemaDir directory that is specified in the package.json file of the extension. The actual file name should use is the part that follows the package name of extension. So for example, the JupyterLab apputils-extension package hosts several plugins:

• '@jupyterlab/apputils-extension:menu'
• '@jupyterlab/apputils-extension:palette'
• '@jupyterlab/apputils-extension:settings'
• '@jupyterlab/apputils-extension:themes'

And in the package.json for @jupyterlab/apputils-extension, the schemaDir field is a directory called schema. Since the themes plugin requires a JSON schema, its schema file location is: schema/themes.json. The plugin's name is used to automatically associate it with its settings file, so this naming convention is important. Ensure that the schema files are included in the "files" metadata in package.json.

See the (fileeditor-extension)[https://github.com/jupyterlab/jupyterlab/tree/master/packages/fileeditor-extension] for another example of an extension that uses settings.

Storing Extension Data

In addition to the file system that is accessed by using the @jupyterlab/services package, JupyterLab offers two ways for extensions to store data: a client-side state database that is built on top of localStorage and a plugin settings system that allows for default setting values and user overrides.

State Database

The state database can be accessed by importing IStateDB from @jupyterlab/coreutils and adding it to the list of requires for a plugin:

const id = 'foo-extension:IFoo';

const IFoo = new Token<IFoo>(id);

interface IFoo {}

class Foo implements IFoo {}

const plugin: JupyterLabPlugin<IFoo> = {
  id,
  requires: [IStateDB],
  provides: IFoo,
  activate: (app: JupyterLab, state: IStateDB): IFoo => {
    const foo = new Foo();
    const key = `${id}:some-attribute`;

    // Load the saved plugin state and apply it once the app
    // has finished restoring its former layout.
    Promise.all([state.fetch(key), app.restored])
      .then(([saved]) => { /* Update `foo` with `saved`. */ });

    // Fulfill the plugin contract by returning an `IFoo`.
    return foo;
  },
  autoStart: true
};