feat: Add watching functionality to plugin detection

Signed-off-by: Jack Palmer <jackpalmer@spotify.com>

packages/cli/src/lib/bundler/discover.ts

@@ -18,13 +18,30 @@ import { BackstagePackageJson } from '@backstage/cli-node';
 import { Config } from '@backstage/config';
 import fs from 'fs-extra';
 import path from 'path';
+import chokidar from 'chokidar';
 
 import { paths as cliPaths } from '../../lib/paths';
 import { BundlingPathsOptions, resolveBundlingPaths } from './paths';
 
-type Options = { config: Config } & BundlingPathsOptions;
+type Options = { config: Config; watch: () => void } & BundlingPathsOptions;
 
-export async function writeDetectedPluginsModule(options: Options) {
+export async function buildDetectedPlugins(options: Options) {
+  const { entry, targetDir } = options;
+  const { targetPackageJson } = resolveBundlingPaths({ entry, targetDir });
+
+  if (!!options.watch) {
+    const watcher = chokidar.watch(targetPackageJson);
+
+    watcher.on('change', async () => {
+      await writeDetectedPluginsModule(options);
+      options.watch();
+    });
+  }
+
+  await writeDetectedPluginsModule(options);
+}
+
+async function writeDetectedPluginsModule(options: Options) {
   const requirePackageScript = (await detectPlugins(options))
     ?.map(pkg => `{name: '${pkg}', module: require('${pkg}')}`)
     .join(',');

packages/cli/src/lib/bundler/server.ts

@@ -31,7 +31,7 @@ import { paths as libPaths } from '../../lib/paths';
 import { loadCliConfig } from '../config';
 import { Lockfile } from '../versioning';
 import { createConfig, resolveBaseUrl } from './config';
-import { writeDetectedPluginsModule } from './discover';
+import { buildDetectedPlugins } from './discover';
 import { resolveBundlingPaths } from './paths';
 import { ServeOptions } from './types';
 
@@ -131,10 +131,13 @@ export async function serveBundle(options: ServeOptions) {
     },
   });
 
-  await writeDetectedPluginsModule({
+  await buildDetectedPlugins({
     config: fullConfig,
     entry: options.entry,
     targetDir: options.targetDir,
+    watch() {
+      server?.invalidate();
+    },
   });
 
   const compiler = webpack(config);