From e0088d9d1a4617295cbbaa94a36630318a3e7194 Mon Sep 17 00:00:00 2001
From: Patrik Oldsberg <poldsberg@gmail.com>
Date: Fri, 8 Sep 2023 11:17:42 +0200
Subject: [PATCH] frontend-app-api: implement api factory discovery

Co-authored-by: Camila Belo <camilaibs@gmail.com>
Signed-off-by: Patrik Oldsberg <poldsberg@gmail.com>
---
 packages/frontend-app-api/package.json        |  1 +
 packages/frontend-app-api/src/createApp.tsx   | 66 +++++++++++++++----
 .../frontend-app-api/src/extensions/Core.tsx  | 34 ++++++++++
 yarn.lock                                     |  1 +
 4 files changed, 89 insertions(+), 13 deletions(-)
 create mode 100644 packages/frontend-app-api/src/extensions/Core.tsx

diff --git a/packages/frontend-app-api/package.json b/packages/frontend-app-api/package.json
index 33b6067858..17f4b1bef3 100644
--- a/packages/frontend-app-api/package.json
+++ b/packages/frontend-app-api/package.json
@@ -34,6 +34,7 @@
   ],
   "dependencies": {
     "@backstage/config": "workspace:^",
+    "@backstage/core-app-api": "workspace:^",
     "@backstage/core-plugin-api": "workspace:^",
     "@backstage/frontend-plugin-api": "workspace:^",
     "@backstage/plugin-graphiql": "workspace:^",
diff --git a/packages/frontend-app-api/src/createApp.tsx b/packages/frontend-app-api/src/createApp.tsx
index c50b507642..77f0d1948a 100644
--- a/packages/frontend-app-api/src/createApp.tsx
+++ b/packages/frontend-app-api/src/createApp.tsx
@@ -21,6 +21,7 @@ import {
   coreExtensionData,
 } from '@backstage/frontend-plugin-api';
 import { CoreRouter } from './extensions/CoreRouter';
+import { Core } from './extensions/Core';
 import {
   createExtensionInstance,
   ExtensionInstance,
@@ -31,8 +32,13 @@ import {
   readAppExtensionParameters,
 } from './wiring/parameters';
 import { RoutingProvider } from './routing/RoutingContext';
-import { RouteRef } from '@backstage/core-plugin-api';
+import { ApiHolder, RouteRef } from '@backstage/core-plugin-api';
 import { getAvailablePlugins } from './wiring/discovery';
+import {
+  ApiFactoryRegistry,
+  ApiProvider,
+  ApiResolver,
+} from '@backstage/core-app-api';
 
 /** @public */
 export function createApp(options: { plugins: BackstagePlugin[] }): {
@@ -40,7 +46,7 @@ export function createApp(options: { plugins: BackstagePlugin[] }): {
 } {
   const appConfig = ConfigReader.fromConfigs(process.env.APP_CONFIG as any);
 
-  const builtinExtensions = [CoreRouter];
+  const builtinExtensions = [CoreRouter, Core];
   const discoveredPlugins = getAvailablePlugins();
 
   // pull in default extension instance from discovered packages
@@ -115,25 +121,59 @@ export function createApp(options: { plugins: BackstagePlugin[] }): {
 
   const routePaths = extractRouteInfoFromInstanceTree(rootInstances);
 
+  const coreInstance = rootInstances.find(({ id }) => id === 'core');
+  if (!coreInstance) {
+    throw Error('Unable to find core extension instance');
+  }
+
+  const apiHolder = createApiHolder(coreInstance);
+
   return {
     createRoot() {
-      const rootComponents = rootInstances.map(
-        e =>
-          e.data.get(
-            coreExtensionData.reactComponent.id,
-          ) as typeof coreExtensionData.reactComponent.T,
-      );
+      const rootComponents = rootInstances
+        .map(
+          e =>
+            e.data.get(
+              coreExtensionData.reactComponent.id,
+            ) as typeof coreExtensionData.reactComponent.T,
+        )
+        .filter(Boolean);
       return (
-        <RoutingProvider routePaths={routePaths}>
-          {rootComponents.map((Component, i) => (
-            <Component key={i} />
-          ))}
-        </RoutingProvider>
+        <ApiProvider apis={apiHolder}>
+          <RoutingProvider routePaths={routePaths}>
+            {rootComponents.map((Component, i) => (
+              <Component key={i} />
+            ))}
+          </RoutingProvider>
+        </ApiProvider>
       );
     },
   };
 }
 
+function createApiHolder(coreExtension: ExtensionInstance): ApiHolder {
+  const factoryRegistry = new ApiFactoryRegistry();
+
+  const apiFactories =
+    coreExtension.attachments
+      .get('apis')
+      ?.map(
+        e =>
+          e.data.get(
+            coreExtensionData.apiFactory.id,
+          ) as typeof coreExtensionData.apiFactory.T,
+      )
+      .filter(Boolean) ?? [];
+
+  for (const factory of apiFactories) {
+    factoryRegistry.register('default', factory);
+  }
+
+  ApiResolver.validateFactories(factoryRegistry, factoryRegistry.getAllApis());
+
+  return new ApiResolver(factoryRegistry);
+}
+
 /** @internal */
 export function extractRouteInfoFromInstanceTree(
   roots: ExtensionInstance[],
diff --git a/packages/frontend-app-api/src/extensions/Core.tsx b/packages/frontend-app-api/src/extensions/Core.tsx
new file mode 100644
index 0000000000..cb93b2888f
--- /dev/null
+++ b/packages/frontend-app-api/src/extensions/Core.tsx
@@ -0,0 +1,34 @@
+/*
+ * Copyright 2023 The Backstage Authors
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+import {
+  coreExtensionData,
+  createExtension,
+} from '@backstage/frontend-plugin-api';
+
+export const Core = createExtension({
+  id: 'core',
+  at: 'root',
+  inputs: {
+    apis: {
+      extensionData: {
+        api: coreExtensionData.apiFactory,
+      },
+    },
+  },
+  output: {},
+  factory() {},
+});
diff --git a/yarn.lock b/yarn.lock
index 7075f315dc..4a9b30d532 100644
--- a/yarn.lock
+++ b/yarn.lock
@@ -4317,6 +4317,7 @@ __metadata:
   dependencies:
     "@backstage/cli": "workspace:^"
     "@backstage/config": "workspace:^"
+    "@backstage/core-app-api": "workspace:^"
     "@backstage/core-plugin-api": "workspace:^"
     "@backstage/frontend-plugin-api": "workspace:^"
     "@backstage/plugin-graphiql": "workspace:^"