Initial Starlight Plugins support (#942)

Co-authored-by: Chris Swithinbank <357379+delucis@users.noreply.github.com>

.changeset/lovely-keys-wash.md

@@ -0,0 +1,7 @@
+---
+'@astrojs/starlight': minor
+---
+
+Adds plugin API
+
+See the [plugins reference](https://starlight.astro.build/reference/plugins/) to learn more about creating plugins for Starlight using this new API.

docs/src/content/docs/reference/configuration.mdx

@@ -529,3 +529,18 @@ starlight({
 ```
 
 See the [Overrides Reference](/reference/overrides/) for details of all the components that you can override.
+
+### `plugins`
+
+**type:** [`StarlightPlugin[]`](/reference/plugins/#quick-api-reference)
+
+Extend Starlight with custom plugins.
+Plugins apply changes to your project to modify or add to Starlight's features.
+
+```js
+starlight({
+	plugins: [starlightPlugin()],
+});
+```
+
+See the [Plugins Reference](/reference/plugins/) for details about creating your own plugins.

docs/src/content/docs/reference/plugins.md

@@ -0,0 +1,163 @@
+---
+title: Plugins Reference
+description: An overview of the Starlight plugin API.
+tableOfContents:
+  maxHeadingLevel: 4
+---
+
+Starlight plugins can customize Starlight configuration, UI, and behavior, while also being easy to share and reuse.
+This reference page documents the API that plugins have access to.
+
+Lean more about using a Starlight plugin in the [Configuration Reference](/reference/configuration/#plugins).
+
+## Quick API Reference
+
+A Starlight plugin has the following shape.
+See below for details of the different properties and hook parameters.
+
+```ts
+interface StarlightPlugin {
+  name: string;
+  hooks: {
+    setup: (options: {
+      config: StarlightUserConfig;
+      updateConfig: (newConfig: StarlightUserConfig) => void;
+      addIntegration: (integration: AstroIntegration) => void;
+      astroConfig: AstroConfig;
+      command: 'dev' | 'build' | 'preview';
+      isRestart: boolean;
+      logger: AstroIntegrationLogger;
+    }) => void | Promise<void>;
+  };
+}
+```
+
+## `name`
+
+**type:** `string`
+
+A plugin must provide a unique name that describes it. The name is used when [logging messages](#logger) related to this plugin and may be used by other plugins to detect the presence of this plugin.
+
+## `hooks`
+
+Hooks are functions which Starlight calls to run plugin code at specific times. Currently, Starlight supports a single `setup` hook.
+
+### `hooks.setup`
+
+Plugin setup function called when Starlight is initialized (during the [`astro:config:setup`](https://docs.astro.build/en/reference/integrations-reference/#astroconfigsetup) integration hook).
+The `setup` hook can be used to update the Starlight configuration or add Astro integrations.
+
+This hook is called with the following options:
+
+#### `config`
+
+**type:** `StarlightUserConfig`
+
+A read-only copy of the user-supplied [Starlight configuration](/reference/configuration).
+This configuration may have been updated by other plugins configured before the current one.
+
+#### `updateConfig`
+
+**type:** `(newConfig: StarlightUserConfig) => void`
+
+A callback function to update the user-supplied [Starlight configuration](/reference/configuration).
+Provide the root-level configuration keys you want to override.
+To update nested configuration values, you must provide the entire nested object.
+
+To extend an existing config option without overriding it, spread the existing value into your new value.
+In the following example, a new [`social`](/reference/configuration/#social) media account is added to the existing configuration by spreading `config.social` into the new `social` object:
+
+```ts {6-11}
+// plugin.ts
+export default {
+  name: 'add-twitter-plugin',
+  hooks: {
+    setup({ config, updateConfig }) {
+      updateConfig({
+        social: {
+          ...config.social,
+          twitter: 'https://twitter.com/astrodotbuild',
+        },
+      });
+    },
+  },
+};
+```
+
+#### `addIntegration`
+
+**type:** `(integration: AstroIntegration) => void`
+
+A callback function to add an [Astro integration](https://docs.astro.build/en/reference/integrations-reference/) required by the plugin.
+
+In the following example, the plugin first checks if [Astro’s React integration](https://docs.astro.build/en/guides/integrations-guide/react/) is configured and, if it isn’t, uses `addIntegration()` to add it:
+
+```ts {14} "addIntegration,"
+// plugin.ts
+import react from '@astrojs/react';
+
+export default {
+  name: 'plugin-using-react',
+  hooks: {
+    plugin({ addIntegration, astroConfig }) {
+      const isReactLoaded = astroConfig.integrations.find(
+        ({ name }) => name === '@astrojs/react'
+      );
+
+      // Only add the React integration if it's not already loaded.
+      if (!isReactLoaded) {
+        addIntegration(react());
+      }
+    },
+  },
+};
+```
+
+#### `astroConfig`
+
+**type:** `AstroConfig`
+
+A read-only copy of the user-supplied [Astro configuration](https://docs.astro.build/en/reference/configuration-reference/).
+
+#### `command`
+
+**type:** `'dev' | 'build' | 'preview'`
+
+The command used to run Starlight:
+
+- `dev` - Project is executed with `astro dev`
+- `build` - Project is executed with `astro build`
+- `preview` - Project is executed with `astro preview`
+
+#### `isRestart`
+
+**type:** `boolean`
+
+`false` when the dev server starts, `true` when a reload is triggered.
+Common reasons for a restart include a user editing their `astro.config.mjs` while the dev server is running.
+
+#### `logger`
+
+**type:** `AstroIntegrationLogger`
+
+An instance of the [Astro integration logger](https://docs.astro.build/en/reference/integrations-reference/#astrointegrationlogger) that you can use to write logs.
+All logged messages will be prefixed with the plugin name.
+
+```ts {6}
+// plugin.ts
+export default {
+  name: 'long-process-plugin',
+  hooks: {
+    plugin({ logger }) {
+      logger.info('Starting long process…');
+      // Some long process…
+    },
+  },
+};
+```
+
+The example above will log a message that includes the provided info message:
+
+```shell
+[long-process-plugin] Starting long process…
+```

packages/starlight/__tests__/plugins/config.test.ts

@@ -0,0 +1,126 @@
+import { describe, expect, test } from 'vitest';
+import config from 'virtual:starlight/user-config';
+import { getSidebar } from '../../utils/navigation';
+import { runPlugins } from '../../utils/plugins';
+import { createTestPluginContext } from '../test-plugin-utils';
+
+test('reads and updates a configuration option', () => {
+	expect(config.title).toBe('Plugins - Custom');
+});
+
+test('overwrites a configuration option', () => {
+	expect(getSidebar('/', undefined)).toMatchObject([{ href: '/showcase', label: 'Showcase' }]);
+});
+
+test('runs plugins in the order that they are configured and always passes down the latest user config', () => {
+	expect(config.description).toBe('plugin 1 - plugin 2 - plugin 3');
+});
+
+test('receives the user provided configuration without any Zod `transform`s applied', () => {
+	/**
+	 * If the `transform` associated to the favicon schema was applied, the favicon `href` would be
+	 * `invalid.svg`.
+	 * @see {@link file://./vitest.config.ts} for more details in the `test-plugin-1` plugin.
+	 */
+	expect(config.favicon.href).toBe('valid.svg');
+});
+
+test('receives the user provided configuration including the plugins list', async () => {
+	expect.assertions(1);
+
+	await runPlugins(
+		{ title: 'Test Docs' },
+		[
+			{ name: 'test-plugin-1', hooks: { setup: () => {} } },
+			{ name: 'test-plugin-2', hooks: { setup: () => {} } },
+			{
+				name: 'test-plugin-3',
+				hooks: {
+					setup: ({ config }) => {
+						expect(config.plugins?.map(({ name }) => name)).toMatchObject([
+							'test-plugin-1',
+							'test-plugin-2',
+							'test-plugin-3',
+						]);
+					},
+				},
+			},
+		],
+		createTestPluginContext()
+	);
+});
+
+describe('validation', () => {
+	test('validates starlight configuration before running plugins', async () => {
+		expect(
+			async () =>
+				await runPlugins(
+					// @ts-expect-error - invalid sidebar config.
+					{ title: 'Test Docs', sidebar: true },
+					[],
+					createTestPluginContext()
+				)
+		).rejects.toThrowError(/Invalid config passed to starlight integration/);
+	});
+
+	test('validates plugins configuration before running them', async () => {
+		expect(
+			async () =>
+				await runPlugins(
+					{ title: 'Test Docs' },
+					// @ts-expect-error - invalid plugin with no `hooks` defined.
+					[{ name: 'invalid-plugin' }],
+					createTestPluginContext()
+				)
+		).rejects.toThrowError(/Invalid plugins config passed to starlight integration/);
+	});
+
+	test('validates configuration updates from plugins do not update the `plugins` config key', async () => {
+		expect(
+			async () =>
+				await runPlugins(
+					{ title: 'Test Docs' },
+					[
+						{
+							name: 'test-plugin',
+							hooks: {
+								setup: ({ updateConfig }) => {
+									// @ts-expect-error - plugins cannot update the `plugins` config key.
+									updateConfig({ plugins: [{ name: 'invalid-plugin' }] });
+								},
+							},
+						},
+					],
+					createTestPluginContext()
+				)
+		).rejects.toThrowError(
+			/The 'test-plugin' plugin tried to update the 'plugins' config key which is not supported./
+		);
+	});
+
+	test('validates configuration updates from plugins', async () => {
+		expect(
+			async () =>
+				await runPlugins(
+					{ title: 'Test Docs' },
+					[
+						{
+							name: 'test-plugin',
+							hooks: {
+								setup: ({ updateConfig }) => {
+									// @ts-expect-error - invalid sidebar config update.
+									updateConfig({ description: true });
+								},
+							},
+						},
+					],
+					createTestPluginContext()
+				)
+		).rejects.toThrowError(/Invalid config update provided by the 'test-plugin' plugin/);
+	});
+});
+
+test('does not expose plugins to the config virtual module', () => {
+	// @ts-expect-error - plugins are not serializable and thus not in the config virtual module.
+	expect(config.plugins).not.toBeDefined();
+});

packages/starlight/__tests__/plugins/integration.test.ts

@@ -0,0 +1,77 @@
+import type { AstroIntegration } from 'astro';
+import { expect, test } from 'vitest';
+import { runPlugins } from '../../utils/plugins';
+import { createTestPluginContext } from '../test-plugin-utils';
+
+test('returns all integrations added by plugins without deduping them', async () => {
+	const integration1: AstroIntegration = {
+		name: 'test-integration-1',
+		hooks: {},
+	};
+
+	const integration2: AstroIntegration = {
+		name: 'test-integration-2',
+		hooks: {},
+	};
+
+	const { integrations } = await runPlugins(
+		{ title: 'Test Docs' },
+		[
+			{
+				name: 'test-plugin-1',
+				hooks: {
+					setup({ addIntegration, updateConfig }) {
+						updateConfig({ description: 'test' });
+						addIntegration(integration1);
+					},
+				},
+			},
+			{
+				name: 'test-plugin-2',
+				hooks: {
+					setup({ addIntegration }) {
+						addIntegration(integration1);
+						addIntegration(integration2);
+					},
+				},
+			},
+		],
+		createTestPluginContext()
+	);
+
+	expect(integrations).toMatchObject([
+		{ name: 'test-integration-1' },
+		{ name: 'test-integration-1' },
+		{ name: 'test-integration-2' },
+	]);
+});
+
+test('receives the Astro config with a list of integrations including the ones added by previous plugins', async () => {
+	expect.assertions(1);
+
+	await runPlugins(
+		{ title: 'Test Docs' },
+		[
+			{
+				name: 'test-plugin-1',
+				hooks: {
+					setup({ addIntegration }) {
+						addIntegration({
+							name: 'test-integration',
+							hooks: {},
+						});
+					},
+				},
+			},
+			{
+				name: 'test-plugin-2',
+				hooks: {
+					setup({ astroConfig }) {
+						expect(astroConfig.integrations).toMatchObject([{ name: 'test-integration' }]);
+					},
+				},
+			},
+		],
+		createTestPluginContext()
+	);
+});