Composable Homepage (#6486)

feat: implement homepage plugin

plugins/home/README.md

@@ -0,0 +1,95 @@
+# Home
+
+Development is ongoing. You can follow the progress and contribute at the Backstage [Home Project Board](https://github.com/backstage/backstage/projects/7) or reach out to us in the [`#support` Discord channel](https://discord.com/channels/687207715902193673/687235481154617364).
+
+## Overview
+
+The Home plugin introduces a system for composing a Home Page for Backstage in order to surface relevant info and provide convenient shortcuts for common tasks. It's designed with composability in mind with an open ecosystem that allows anyone to contribute with any component, to be included in any Home Page.
+
+For App Integrators, the system is designed to be composable to give total freedom in designing a Home Page that suits the needs of the organization. From the perspective of a Component Developer who wishes to contribute with building blocks to be included in Home Pages, there's a convenient interface for bundling the different parts and exporting them with both error boundary and lazy loading handled under the surface.
+
+## Getting started
+
+If you have a standalone app (you didn't clone this repo), then do
+
+```bash
+# From the Backstage repository root
+cd packages/app
+yarn add @backstage/plugin-home
+```
+
+### Setting up the Home Page
+
+1. Create a Home Page Component that will be used for composition.
+
+`packages/app/src/components/home/HomePage.tsx`
+
+```tsx
+import React from 'react';
+
+export const HomePage = () => {
+  return {
+    /* TODO: Compose a Home Page here */
+  };
+};
+```
+
+2. Add a route where the homepage will live, presumably `/`.
+
+`packages/app/src/App.tsx`
+
+```tsx
+import { HomepageCompositionRoot } from '@backstage/plugin-home';
+import { HomePage } from './components/home/HomePage';
+
+// ...
+<Route path="/" element={<HomepageCompositionRoot />}>
+  <HomePage />
+</Route>;
+// ...
+```
+
+### Creating Components
+
+The Home Page can be composed with regular React components, so there's no magic in creating components to be used for composition 🪄 🎩 . However, in order to assure that your component fits into a diverse set of Home Pages, there's an extension creator for this purpose, that creates a Card-based layout, for consistency between components (read more about extensions [here](https://backstage.io/docs/plugins/composability#extensions)). The extension creator requires two fields: `title` and `components`. The `components` field is expected to be an asynchronous import that should at least contain a `Content` field. Additionally, you can optionally provide `settings`, `actions` and `contextProvider` as well. These parts will be combined to create a card, where the `content`, `actions` and `settings` will be wrapped within the `contextProvider` in order to be able to access to context and effectively communicate with one another.
+
+Finally, the `createCardExtension` also accepts a generic, such that Component Developers can indicate to App Integrators what custom props their component will accept, such as the example below where the default category of the random jokes can be set.
+
+```tsx
+export const RandomJokeHomePageComponent = homePlugin.provide(
+  createCardExtension<{ defaultCategory?: 'programming' | 'any' }>({
+    title: 'Random Joke',
+    components: () => import('./homePageComponents/RandomJoke'),
+  }),
+);
+```
+
+In summary: it is not necessary to use the `createCardExtension` extension creator to register a home page component, although it is convenient since it provides error boundary and lazy loading, and it also may hook into other functionality in the future.
+
+### Composing a Home Page
+
+Composing a Home Page is no different from creating a regular React Component, i.e. the App Integrator is free to include whatever content they like. However, there are components developed with the Home Page in mind, as described in the previous section. If created by the `createCardExtension` extension creator, they are rendered like so
+
+```tsx
+import React from 'react'
+import Grid from '@material-ui/core/Grid'
+import { RandomJokeHomePageComponent } from '@backstage/plugin-home';
+
+export const HomePage = () => {
+  return (
+    <Grid container spacing={3}>
+      <Grid item xs={12} md={4}>
+        <RandomJokeHomePageComponent>
+      </Grid>
+    </Grid>
+  )
+}
+```
+
+Additionally, the App Integrator is provided an escape hatch in case the way the card is rendered does not fit their requirements. They may optionally pass the `Renderer`-prop, which will receive the `title`, `content` and optionally `actions`, `settings` and `contextProvider`, if they exist for the component. This allows the App Integrator to render the content in any way they want.
+
+## Contributing
+
+We believe that people have great ideas for what makes a useful Home Page, and we want to make it easy for every to benefit from the effort you put in to create something cool for the Home Page. Therefore, a great way of contributing is by simply creating more Home Page Components, than can then be used by everyone when composing their own Home Page. If they are tightly coupled to an existing plugin, it is recommended to allow them to live within that plugin, for convenience and to limit complex dependencies. On the other hand, if there's no clear plugin that the component is based on, it's also fine to contribute them into the [home plugin](/plugins/home/src/homePageComponents)
+
+Additionally, the API is at a very early state, so contributing with additional use cases may expose weaknesses in the current solution that we may iterate on, to provide more flexibility and ease of use for those who wish to develop components for the Home Page.