docs: update plugin create and reference docs for top-level routable extensions

docs/architecture.drawio

@@ -0,0 +1 @@
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docs/plugins/plugin-development.md

@@ -23,35 +23,60 @@ browser APIs or by depending on external modules to do the work.
 
 ### Routing
 
-Each plugin is responsible for registering its components to corresponding
-routes in the app.
+Each plugin can export routable extensions, which are then imported into the app
+and mounted at a path.
 
-The app will call the `createPlugin` method on each plugin, passing in a
-`router` object with a set of methods on it.
+First you will need a `RouteRef` instance to serve as the mount point of your
+extensions. This can be used within your own plugin to create a link to the
+extension page using `useRouteRef`, as well as for other plugins to link to your
+extension.
 
-```jsx
+It is best to place these in a separate top-level `src/routes.ts` file, in order
+to avoid import cycles, for example like this:
+
+```tsx
+/* src/routes.ts */
+import { createRouteRef } from '@backstage/core';
+
+// Note: This route ref is for internal use only, don't export it from the plugin
+export const rootRouteRef = createRouteRef({
+  title: 'Example Page',
+});
+```
+
+Now that we have a `RouteRef`, we import it into `src/plugin.ts`, create our
+plugin instance with `createPlugin`, as well as create and wrap our routable
+extension using `createRoutableExtension` from `@backstage/core`:
+
+```tsx
+/* src/plugin.ts */
 import { createPlugin, createRouteRef } from '@backstage/core';
 import ExampleComponent from './components/ExampleComponent';
 
-export const rootRouteRef = createRouteRef({
-  path: '/new-plugin',
-  title: 'New plugin',
-});
-
-export const plugin = createPlugin({
-  id: 'new-plugin',
-  register({ router }) {
-    router.addRoute(rootRouteRef, ExampleComponent);
+// Create a plugin instance and export this from your plugin package
+export const examplePlugin = createPlugin({
+  id: 'example',
+  routes: {
+    root: rootRouteRef, // This is where the route ref should be exported for usage in the app
   },
 });
+
+// This creates a routable extension, which are typically full pages of content.
+// Each extension should also be exported from your plugin package.
+export const ExamplePage = examplePlugin.provide(
+  createRoutableExtension({
+    // The component needs to be lazy-loaded. It's what will actually be rendered in the end.
+    component: () =>
+      import('./components/ExampleComponent').then(m => m.ExampleComponent),
+    // This binds the extension to this route ref, which allows for routing within and across plugin extensions
+    mountPoint: rootRouteRef,
+  }),
+);
 ```
 
-#### `router` API
+This extension can then be imported and used in the app as follow, typically
+placed within the top-level `<FlatRoutes>`:
 
-```typescript
-addRoute(
-  target: RouteRef,
-  Component: ComponentType<any>,
-  options?: RouteOptions,
-): void;
+```tsx
+<Route route="/any-path" element={<ExamplePage />} />
 ```

docs/plugins/structure-of-a-plugin.md

@@ -28,6 +28,7 @@ new-plugin/
         index.ts
         plugin.test.ts
         plugin.ts
+        routes.ts
     jest.config.js
     jest.setup.ts
     package.json
@@ -56,26 +57,30 @@ package.json to declare the plugin dependencies, metadata and scripts.
 In the `src` folder we get to the interesting bits. Check out the `plugin.ts`:
 
 ```jsx
-import { createPlugin, createRouteRef } from '@backstage/core';
-import ExampleComponent from './components/ExampleComponent';
+import { createPlugin, createRoutableExtension } from '@backstage/core';
 
-export const rootRouteRef = createRouteRef({
-  path: '/new-plugin',
-  title: 'New plugin',
-});
+import { rootRouteRef } from './routes';
 
-export const plugin = createPlugin({
-  id: 'new-plugin',
-  register({ router }) {
-    router.addRoute(rootRouteRef, ExampleComponent);
+export const examplePlugin = createPlugin({
+  id: 'example',
+  routes: {
+    root: rootRouteRef,
   },
 });
+
+export const ExamplePage = examplePlugin.provide(
+  createRoutableExtension({
+    component: () =>
+      import('./components/ExampleComponent').then(m => m.ExampleComponent),
+    mountPoint: rootRouteRef,
+  }),
+);
 ```
 
-This is where the plugin is created and where it hooks into the app by declaring
-what component should be shown on what URL. See reference docs for
-[createPlugin](../reference/createPlugin.md) or
-[router](../reference/createPlugin-router.md).
+This is where the plugin is created and where it creates and exports extensions
+that can be imported and used the app. See reference docs for
+[createPlugin](../reference/createPlugin.md) or introduction to the new
+[Composability System](./composability.md).
 
 ## Components
 
@@ -91,12 +96,15 @@ You may tweak these components, rename them and/or replace them completely.
 
 ## Connecting the plugin to the Backstage app
 
-There are two things needed for a Backstage app to start making use of a plugin.
+There are three things needed for a Backstage app to start making use of a
+plugin.
 
 1. Add plugin as dependency in `app/package.json`
 2. `import` plugin in `app/src/plugins.ts`
+3. Import and use one or more plugin extensions, for example in
+   `app/src/App.tsx`.
 
-Luckily these two steps happen automatically when you create a plugin with the
+Luckily these three steps happen automatically when you create a plugin with the
 Backstage CLI.
 
 ## Talking to the outside world

docs/reference/createPlugin-router.md

@@ -1,31 +0,0 @@
----
-id: createPlugin-router
-title: createPlugin - router
-description: Documentation on createPlugin - router
----
-
-The router that is passed to the `register` function makes it possible for
-plugins to hook into routing of the Backstage app and provide the end users with
-new views to navigate to. This is done by utilising the following methods on the
-`router`:
-
-```typescript
-addRoute(
-  target: RouteRef,
-  Component: ComponentType<any>,
-  options?: RouteOptions,
-): void;
-```
-
-## RouteRef
-
-`addRoute` method is using mutable RouteRefs, which can be created as following:
-
-```ts
-import { createRouteRef } from '@backstage/core';
-
-const myPluginRouteRef = createRouteRef({
-  path: '/my-plugin',
-  title: 'My Plugin',
-});
-```

docs/reference/createPlugin.md

@@ -17,32 +17,24 @@ type PluginConfig = {
 };
 
 type PluginHooks = {
-  router: RouterHooks;
+  featureFlags: FeatureFlagsHooks;
 };
 ```
 
-- [Read more about the router here](createPlugin-router.md)
 - [Read more about feature flags here](createPlugin-feature-flags.md)
 
 ## Example Uses
 
 ### Creating a basic plugin
 
-Showcasing adding a route and a feature flag.
+Showcasing adding a feature flag.
 
 ```jsx
-import { createPlugin, createRouteRef } from '@backstage/core';
-import ExampleComponent from './components/ExampleComponent';
-
-export const rootRouteRef = createRouteRef({
-  path: '/new-plugin',
-  title: 'New Plugin',
-});
+import { createPlugin } from '@backstage/core';
 
 export default createPlugin({
   id: 'new-plugin',
   register({ router, featureFlags }) {
-    router.addRoute(rootRouteRef, ExampleComponent);
     featureFlags.register('enable-example-component');
   },
 });