Extension Anatomy

In the previous section you learned how to create your first extension. In this section you will learn how this extension works under the hood.

The Hello World sample extension does three things:

• Implements onActivate() and outputs a message to the console.
• Implements onDeactivate() and outputs a message to the console.
• Registers ClusterPage so that the page is visible in the left-side menu of the cluster dashboard.

Let's take a closer look at our Hello World sample's source code and see how these three things are achieved.

Extension File Structure

.
├── .gitignore          // Ignore build output and node_modules
├── Makefile            // Config for build tasks that compiles the extension
├── README.md           // Readable description of your extension's functionality
├── src
│   └── page.tsx         // Extension's additional source code
├── main.ts              // Source code for extension's main entrypoint
├── package.json         // Extension manifest and dependencies
├── renderer.tsx         // Source code for extension's renderer entrypoint
├── tsconfig.json        // TypeScript configuration
├── webpack.config.js    // Webpack configuration

The extension directory contains the extension's entry files and a few configuration files. Three files: package.json, main.ts and renderer.tsx are essential to understanding the Hello World sample extension. We'll look at those first.

Extension Manifest

Each Lens extension must have a package.json file. It contains a mix of Node.js fields, including scripts and dependencies, and Lens-specific fields such as publisher and contributes. Some of the most-important fields include:

• name and publisher: Lens uses @<publisher>/<name> as a unique ID for the extension. For example, the Hello World sample has the ID @lensapp-samples/helloworld-sample. Lens uses this ID to uniquely identify your extension.
• main: the extension's entry point run in main process.
• renderer: the extension's entry point run in renderer process.
• engines.lens: the minimum version of Lens API that the extension depends upon. We only support the ^ range, which is also optional to specify, and only major and minor version numbers. Meaning that ^5.4 and 5.4 both mean the same thing, and the patch version in 5.4.2 is ignored.

{
  "name": "helloworld-sample",
  "publisher": "lens-samples",
  "version": "0.0.1",
  "description": "Lens helloworld-sample",
  "license": "MIT",
  "homepage": "https://github.com/lensapp/lens-extension-samples",
  "engines": {
    "node": "^14.18.12",
    "lens": "5.4"
  },
  "main": "dist/main.js",
  "renderer": "dist/renderer.js",
  "scripts": {
    "build": "webpack --config webpack.config.js",
    "dev": "npm run build --watch"
  },
  "dependencies": {
    "react-open-doodles": "^1.0.5"
  },
  "devDependencies": {
    "@k8slens/extensions": "^5.4.6",
    "ts-loader": "^8.0.4",
    "typescript": "^4.5.5",
    "@types/react": "^17.0.44",
    "@types/node": "^14.18.12",
    "webpack": "^4.44.2",
    "webpack-cli": "^3.3.11"
  }
}

Webpack configuration

The following webpack externals are provided by Lens and must be used (when available) to make sure that the versions used are in sync.

Package	webpack external syntax	Lens versions	Available in Main	Available in Renderer
mobx	var global.Mobx	>5.0.0	✅	✅
mobx-react	var global.MobxReact	>5.0.0	❌	✅
react	var global.React	>5.0.0	❌	✅
react-router	var global.ReactRouter	>5.0.0	❌	✅
react-router-dom	var global.ReactRouterDom	>5.0.0	❌	✅
react-dom	var global.ReactDOM	>5.5.0	❌	✅

What is exported is the whole of the packages as a * import (within typescript).

For example, the following is how you would specify these within your webpack configuration files.

{
  ...
  "externals": [
    ...
    {
      "mobx": "var global.Mobx"
      "mobx-react": "var global.MobxReact"
      "react": "var global.React"
      "react-router": "var global.ReactRouter"
      "react-router-dom": "var global.ReactRouterDom"
      "react-dom": "var global.ReactDOM"
    }
  ]
}

Extension Entry Files

Lens extensions can have two separate entry files. One file is used in the main process of the Lens application and the other is used in the renderer process. The main entry file exports the class that extends LensMainExtension, and the renderer entry file exports the class that extends LensRendererExtension.

Both extension classes have onActivate and onDeactivate methods. The onActivate method is executed when your extension is activated. If you need to initialize something in your extension, this is where such an operation should occur. The onDeactivate method gives you a chance to clean up before your extension becomes deactivated. For extensions where explicit cleanup is not required, you don't need to override this method. However, if an extension needs to perform an operation when Lens is shutting down (or if the extension is disabled or uninstalled), this is the method where such an operation should occur.

The Hello World sample extension does not do anything on the main process, so we'll focus on the renderer process, instead. On the renderer entry point, the Hello World sample extension defines the Cluster Page object. The Cluster Page object registers the /extension-example path, and this path renders the ExamplePage React component. It also registers the MenuItem component that displays the ExampleIcon React component and the "Hello World" text in the left-side menu of the cluster dashboard. These React components are defined in the additional ./src/page.tsx file.

import { Renderer } from "@k8slens/extensions";
import { ExampleIcon, ExamplePage } from "./page";
import React from "react";

export default class ExampleExtension extends Renderer.LensExtension {
  clusterPages = [
    {
      id: "extension-example",
      components: {
        Page: () => <ExamplePage extension={this} />,
      },
    },
  ];
}

The Hello World sample extension uses the Cluster Page capability, which is just one of the Lens extension API's capabilities. The Common Capabilities page will help you home in on the right capabilities to use with your own extensions.