Refactor and expose resolveFeatures() from frontend-defaults

Signed-off-by: Eric Peterson <ericpeterson@spotify.com>

.changeset/mira-color-like.md

@@ -0,0 +1,5 @@
+---
+'@backstage/frontend-defaults': minor
+---
+
+Feature resolution logic used in `createApp` is now exposed via the `resolveFeatures` function.

packages/frontend-defaults/report.api.md

@@ -3,6 +3,7 @@
 > Do not edit this file. It is a report generated by [API Extractor](https://api-extractor.com/).
 
 ```ts
+import { Config } from '@backstage/config';
 import { ConfigApi } from '@backstage/frontend-plugin-api';
 import { CreateAppRouteBinder } from '@backstage/frontend-app-api';
 import { ExtensionFactoryMiddleware } from '@backstage/frontend-app-api';
@@ -43,4 +44,10 @@ export interface CreateAppOptions {
 export function createPublicSignInApp(options?: CreateAppOptions): {
   createRoot(): React_2.JSX.Element;
 };
+
+// @public (undocumented)
+export function resolveFeatures(options: {
+  config: Config;
+  providedFeatures?: (FrontendFeature | CreateAppFeatureLoader)[];
+}): Promise<FrontendFeature[]>;
 ```

packages/frontend-defaults/src/createApp.tsx

@@ -16,14 +16,12 @@
 
 import React, { JSX, ReactNode } from 'react';
 import { ConfigApi, coreExtensionData } from '@backstage/frontend-plugin-api';
-import { stringifyError } from '@backstage/errors';
 // eslint-disable-next-line @backstage/no-relative-monorepo-imports
 import { defaultConfigLoaderSync } from '../../core-app-api/src/app/defaultConfigLoader';
 // eslint-disable-next-line @backstage/no-relative-monorepo-imports
 import { overrideBaseUrlConfigs } from '../../core-app-api/src/app/overrideBaseUrlConfigs';
-import { getAvailableFeatures } from './discovery';
+import { resolveFeatures } from './discovery';
 import { ConfigReader } from '@backstage/config';
-import appPlugin from '@backstage/plugin-app';
 import {
   CreateAppRouteBinder,
   ExtensionFactoryMiddleware,
@@ -89,29 +87,12 @@ export function createApp(options?: CreateAppOptions): {
         overrideBaseUrlConfigs(defaultConfigLoaderSync()),
       );
 
-    const discoveredFeatures = getAvailableFeatures(config);
-
-    const providedFeatures: FrontendFeature[] = [];
-    for (const entry of options?.features ?? []) {
-      if ('load' in entry) {
-        try {
-          const result = await entry.load({ config });
-          providedFeatures.push(...result.features);
-        } catch (e) {
-          throw new Error(
-            `Failed to read frontend features from loader '${entry.getLoaderName()}', ${stringifyError(
-              e,
-            )}`,
-          );
-        }
-      } else {
-        providedFeatures.push(entry);
-      }
-    }
-
     const app = createSpecializedApp({
       config,
-      features: [appPlugin, ...discoveredFeatures, ...providedFeatures],
+      features: await resolveFeatures({
+        config,
+        providedFeatures: options?.features,
+      }),
       bindRoutes: options?.bindRoutes,
       extensionFactoryMiddleware: options?.extensionFactoryMiddleware,
     });

packages/frontend-defaults/src/discovery.ts

@@ -15,7 +15,10 @@
  */
 
 import { Config, ConfigReader } from '@backstage/config';
+import { stringifyError } from '@backstage/errors';
 import { FrontendFeature } from '@backstage/frontend-app-api';
+import { CreateAppFeatureLoader } from './createApp';
+import appPlugin from '@backstage/plugin-app';
 
 interface DiscoveryGlobal {
   modules: Array<{ name: string; export?: string; default: unknown }>;
@@ -94,3 +97,29 @@ function isBackstageFeature(obj: unknown): obj is FrontendFeature {
   }
   return false;
 }
+
+/** @public */
+export async function resolveFeatures(options: {
+  config: Config;
+  providedFeatures?: (FrontendFeature | CreateAppFeatureLoader)[];
+}): Promise<FrontendFeature[]> {
+  const discoveredFeatures = getAvailableFeatures(options.config);
+  const features = [appPlugin, ...discoveredFeatures];
+  for (const entry of options.providedFeatures ?? []) {
+    if ('load' in entry) {
+      try {
+        const result = await entry.load({ config: options.config });
+        features.push(...result.features);
+      } catch (e) {
+        throw new Error(
+          `Failed to read frontend features from loader '${entry.getLoaderName()}', ${stringifyError(
+            e,
+          )}`,
+        );
+      }
+    } else {
+      features.push(entry);
+    }
+  }
+  return features;
+}

packages/frontend-defaults/src/index.ts

@@ -26,3 +26,4 @@ export {
   type CreateAppFeatureLoader,
 } from './createApp';
 export { createPublicSignInApp } from './createPublicSignInApp';
+export { resolveFeatures } from './discovery';