From b6557c098811208cfb4f311422bb8110c5a26ffa Mon Sep 17 00:00:00 2001
From: Patrik Oldsberg <poldsberg@gmail.com>
Date: Fri, 4 Dec 2020 13:50:37 +0100
Subject: [PATCH] core-api: update ApiFactory type to correctly infer API type
 and disallow mismatched implementations

---
 .changeset/calm-icons-jump.md                 | 23 +++++++++++++++++++
 .../src/apis/system/ApiFactoryRegistry.ts     | 10 ++++----
 packages/core-api/src/apis/system/helpers.ts  | 15 ++++++------
 packages/core-api/src/apis/system/types.ts    | 16 +++++++++----
 packages/dev-utils/src/devApp/render.tsx      |  8 ++++---
 5 files changed, 54 insertions(+), 18 deletions(-)
 create mode 100644 .changeset/calm-icons-jump.md

diff --git a/.changeset/calm-icons-jump.md b/.changeset/calm-icons-jump.md
new file mode 100644
index 0000000000..16054a80e6
--- /dev/null
+++ b/.changeset/calm-icons-jump.md
@@ -0,0 +1,23 @@
+---
+'@backstage/core-api': patch
+'@backstage/dev-utils': patch
+---
+
+Update ApiFactory type to correctly infer API type and disallow mismatched implementations.
+
+This fixes for example the following code:
+
+```ts
+interface MyApi {
+  myMethod(): void
+}
+
+const myApiRef = createApiRef<MyApi>({...});
+
+createApiFactory({
+  api: myApiRef,
+  deps: {},
+  // This should've caused an error, since the empty object does not fully implement MyApi
+  factory: () => ({}),
+})
+```
diff --git a/packages/core-api/src/apis/system/ApiFactoryRegistry.ts b/packages/core-api/src/apis/system/ApiFactoryRegistry.ts
index 556f63589c..9d9b058eac 100644
--- a/packages/core-api/src/apis/system/ApiFactoryRegistry.ts
+++ b/packages/core-api/src/apis/system/ApiFactoryRegistry.ts
@@ -55,9 +55,9 @@ export class ApiFactoryRegistry implements ApiFactoryHolder {
    * A factory will not be added to the registry if there is already
    * an existing factory with the same or higher priority.
    */
-  register<Api, Deps extends { [name in string]: unknown }>(
+  register<Api, Impl extends Api, Deps extends { [name in string]: unknown }>(
     scope: ApiFactoryScope,
-    factory: ApiFactory<Api, Deps>,
+    factory: ApiFactory<Api, Impl, Deps>,
   ) {
     const priority = ScopePriority[scope];
     const existing = this.factories.get(factory.api);
@@ -69,12 +69,14 @@ export class ApiFactoryRegistry implements ApiFactoryHolder {
     return true;
   }
 
-  get<T>(api: ApiRef<T>): ApiFactory<T, { [x: string]: unknown }> | undefined {
+  get<T>(
+    api: ApiRef<T>,
+  ): ApiFactory<T, T, { [x: string]: unknown }> | undefined {
     const tuple = this.factories.get(api);
     if (!tuple) {
       return undefined;
     }
-    return tuple.factory as ApiFactory<T, { [x: string]: unknown }>;
+    return tuple.factory as ApiFactory<T, T, { [x: string]: unknown }>;
   }
 
   getAllApis(): Set<AnyApiRef> {
diff --git a/packages/core-api/src/apis/system/helpers.ts b/packages/core-api/src/apis/system/helpers.ts
index 0ccd0cdb80..cabff73060 100644
--- a/packages/core-api/src/apis/system/helpers.ts
+++ b/packages/core-api/src/apis/system/helpers.ts
@@ -25,18 +25,19 @@ export function createApiFactory<
   Api,
   Impl extends Api,
   Deps extends { [name in string]: unknown }
->(factory: ApiFactory<Api, Deps>): ApiFactory<Api, Deps>;
-export function createApiFactory<Api>(
+>(factory: ApiFactory<Api, Impl, Deps>): ApiFactory<Api, Impl, Deps>;
+export function createApiFactory<Api, Impl extends Api>(
   api: ApiRef<Api>,
-  instance: Api,
-): ApiFactory<Api, {}>;
+  instance: Impl,
+): ApiFactory<Api, Impl, {}>;
 export function createApiFactory<
   Api,
+  Impl extends Api,
   Deps extends { [name in string]: unknown }
 >(
-  factory: ApiFactory<Api, Deps> | ApiRef<Api>,
-  instance?: Api,
-): ApiFactory<Api, Deps> {
+  factory: ApiFactory<Api, Impl, Deps> | ApiRef<Api>,
+  instance?: Impl,
+): ApiFactory<Api, Impl, Deps> {
   if ('id' in factory) {
     return {
       api: factory,
diff --git a/packages/core-api/src/apis/system/types.ts b/packages/core-api/src/apis/system/types.ts
index b0c19e551c..6234a55c71 100644
--- a/packages/core-api/src/apis/system/types.ts
+++ b/packages/core-api/src/apis/system/types.ts
@@ -34,16 +34,24 @@ export type ApiHolder = {
   get<T>(api: ApiRef<T>): T | undefined;
 };
 
-export type ApiFactory<Api, Deps extends { [name in string]: unknown }> = {
+export type ApiFactory<
+  Api,
+  Impl extends Api,
+  Deps extends { [name in string]: unknown }
+> = {
   api: ApiRef<Api>;
   deps: TypesToApiRefs<Deps>;
-  factory(deps: Deps): Api;
+  factory(deps: Deps): Impl;
 };
 
-export type AnyApiFactory = ApiFactory<unknown, { [key in string]: unknown }>;
+export type AnyApiFactory = ApiFactory<
+  unknown,
+  unknown,
+  { [key in string]: unknown }
+>;
 
 export type ApiFactoryHolder = {
   get<T>(
     api: ApiRef<T>,
-  ): ApiFactory<T, { [key in string]: unknown }> | undefined;
+  ): ApiFactory<T, T, { [key in string]: unknown }> | undefined;
 };
diff --git a/packages/dev-utils/src/devApp/render.tsx b/packages/dev-utils/src/devApp/render.tsx
index cca3470f3f..869de01024 100644
--- a/packages/dev-utils/src/devApp/render.tsx
+++ b/packages/dev-utils/src/devApp/render.tsx
@@ -56,9 +56,11 @@ class DevAppBuilder {
   /**
    * Register an API factory to add to the app
    */
-  registerApi<Api, Deps extends { [name in string]: unknown }>(
-    factory: ApiFactory<Api, Deps>,
-  ): DevAppBuilder {
+  registerApi<
+    Api,
+    Impl extends Api,
+    Deps extends { [name in string]: unknown }
+  >(factory: ApiFactory<Api, Impl, Deps>): DevAppBuilder {
     this.apis.push(factory);
     return this;
   }