docs: ui slot external config

Author: johnvente

docs/how_tos/create_an_external_plugin_ui_slot.rst

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+########################################
+How to create an external plugin UI slot
+########################################
+
+To create a new plugin that will live outside the MFE host, it will be in npm. The plugin needs to be compiled by `Babel <https://babeljs.io/>`_, that way webpack will be available to read the plugin. Here's an example for a small plugin:
+
+
+**1. Plugin Structure:**
+
+The plugin will have the following structure:
+
+    - root
+       - package.json
+       - .babelrc
+       - Makefile
+       - src
+          - index.jsx
+          - index.css
+
+
+**Example files:**
+
+**1.1 package.json:**
+
+This file contains the necessary dependencies for the plugin and also a Babel script to transpile the files inside the ``src`` folder of the plugin.
+
+.. code-block:: json
+
+   {
+     "name": "@openedx-plugins/example-plugin",
+     "version": "1.0.0",
+     "scripts": {
+         "babel:compile": "babel src --out-dir dist --source-maps --ignore '**/*.test.jsx,**/*.test.js' --copy-files"
+     },
+     "peerDependencies": {
+       "@edx/browserslist-config": "^1.2.0",
+       "@edx/frontend-app-communications": "https://github.com/eduNEXT/frontend-app-communications.git#jv/pluggable-component-slot",
+       "@edx/frontend-build": "13.0.1",
+       "@edx/frontend-platform": "5.5.2",
+       "@edx/paragon": "^20.44.0",
+       "eslint-import-resolver-webpack": "^0.13.8",
+       "react": "17.0.2",
+       "prop-types": "^15.8.1",
+       "react-dom": "17.0.2"
+     },
+     "peerDependenciesMeta": {
+       "@edx/frontend-app-communications": {
+         "optional": true
+       }
+     },
+     "devDependencies": {
+       "@babel/cli": "^7.23.4",
+       "@babel/core": "^7.23.7",
+       "@babel/eslint-parser": "^7.23.3",
+       "@babel/plugin-proposal-class-properties": "^7.18.6",
+       "@babel/plugin-proposal-object-rest-spread": "^7.20.7",
+       "@babel/preset-env": "^7.23.8",
+       "@babel/preset-react": "^7.23.3",
+       "@edx/browserslist-config": "^1.2.0",
+       "@edx/frontend-build": "13.0.1"
+     }
+   }
+
+**1.2. .babelrc:**
+
+The essential configuration for Babel. This configuration works for any plugin. You could set it up if you need it.
+
+.. code-block:: json
+
+   {
+     "presets": [
+       [
+         "@babel/preset-env",
+         {
+           "modules": false
+         }
+       ],
+       "@babel/preset-react"
+     ],
+     "plugins": [
+       "@babel/plugin-proposal-object-rest-spread",
+       "@babel/plugin-proposal-class-properties"
+     ],
+     "env": {
+       "test": {
+         "plugins": ["@babel/plugin-proposal-class-properties"],
+         "presets": [["@babel/preset-env"], "@babel/preset-react"]
+       }
+     },
+     "ignore": ["**/*.test.jsx", "**/*.test.js"]
+   }
+
+
+**1.3. Makefile:**
+
+This will create a folder called ``package`` that will have the compiled Babel files ready to upload to, for example, npm.
+
+.. code-block:: makefile
+
+   .ONESHELL: clean build
+
+   clean:
+       # Remove the package folder if it exists
+       rm -rf package
+       rm -rf dist
+
+   build: clean
+       # Run npm run babel
+       npm run babel:compile
+
+       # Create the package folder
+       mkdir -p package
+
+       # Copy package.json to the package folder
+       cp package.json package/
+
+       # Move files from dist folder to package folder
+       find dist -type f ! -name "*.map" ! -name "*.test.js" ! -name "*.test.jsx" -exec cp {} package/ \;
+
+       # Rename index.js to index.jsx in the package folder
+       rm -rf  package/index.js
+       cp dist/index.js package/index.jsx
+
+
+**1.4. index.jsx:**
+
+This will be the content of the plugin. In this case, it's a simple div.
+
+.. code-block:: jsx
+
+   import React from 'react';
+   
+   import './index.css';
+   
+   const MyPlugin = () => {
+     return (
+       <div className="openedx-plugin">
+         <h1>Hello, World!</h1>
+         {/* Add your plugin UI components here */}
+       </div>
+     );
+   }
+   
+   export default MyPlugin;
+
+**1.5. index.css:**
+
+Styles for the plugin.
+
+.. code-block:: css
+
+   .openedx-plugin { 
+       background-color: red;
+       color: white;
+   }
+
+
+**2. Install Dependencies:**
+
+After having the plugin structure, you can install the dependencies:
+
+.. code-block:: bash
+
+   npm install
+
+
+**3. Compile the Plugin:**
+
+Now you can compile the plugin by running the Makefile. This will create a folder called ``package`` with the necessary compiled files. Run the following command:
+
+.. code-block:: bash
+
+   make build
+
+Notice that inside the ``package`` folder, there will always be a file called ``index.jsx`` and not ``index.js``. This ensures that when loading components, lazy loading won't have any problems.
+
+
+**4. Upload the Plugin:**
+
+Now that you have compiled the plugin, you can upload it to npm for example. Inside the ``package`` folder you can publish the plugin from there