feat: Add dependency resolution logic for plugins

README.md

@@ -198,6 +198,31 @@ await app.ready()
 
 For more advanced use cases, check the official [awilix documentation](https://github.com/jeffijoe/awilix).
 
+## Defining awilix dependencies in plugins
+
+Relying on dependency injection in plugins is problematic, because plugins cannot know upfront how they can resolve dependencies from a fastify application.
+
+For this purpose there is a helper class available in `@fastify/awilix`:
+
+```ts
+import { FastifyDependencyProvider } from '@fastify/awilix'
+
+app = fastify({ logger: true })
+await app.register(fastifyAwilixPlugin, {
+  container: diContainer,
+})
+app.diContainer.register({
+  maxUserName: asValue('username'),
+})
+
+// ...
+
+function plugin (fastify, opts, next) {
+  const maxUserName = resolveDependencyFromApp<string>(fastify, 'maxUserName')
+  // 'username'
+}
+```
+
 ## License
 
 Licensed under [MIT](./LICENSE).

lib/fastifyAwilixPlugin.js

@@ -100,8 +100,13 @@ const fastifyAwilixPlugin = fp(plugin, {
   name: '@fastify/awilix'
 })
 
+function resolveDependencyFromApp (app, dependencyKey) {
+  return app.diContainer.resolve(dependencyKey)
+}
+
 module.exports = {
   diContainer: diContainerProxy,
+  resolveDependencyFromApp,
   diContainerProxy,
   diContainerClassic,
   fastifyAwilixPlugin

lib/index.d.ts

@@ -1,5 +1,5 @@
 import { AwilixContainer } from 'awilix'
-import { FastifyPluginCallback } from 'fastify'
+import { FastifyInstance, FastifyPluginCallback } from 'fastify'
 import { AwilixManager } from 'awilix-manager'
 
 export interface Cradle {}
@@ -17,6 +17,11 @@ declare module 'fastify' {
   }
 }
 
+/**
+ * Tries to resolve the dependency and throws an error if it fails
+ */
+export function resolveDependencyFromApp<T> (app: FastifyInstance<any, any, any, any, any>, dependencyKey: string): T
+
 export type FastifyAwilixOptions = {
   disposeOnResponse?: boolean
   disposeOnClose?: boolean

test/fastifyAwilixPlugin.test.js

@@ -6,6 +6,7 @@ const { describe, it, afterEach } = require('node:test')
 const assert = require('node:assert')
 
 const { diContainer, diContainerClassic, fastifyAwilixPlugin } = require('../lib')
+const { resolveDependencyFromApp } = require('../lib/fastifyAwilixPlugin')
 
 class UserServiceClassic {
   constructor (userRepository, maxUserName, maxEmail) {
@@ -169,4 +170,21 @@ describe('fastifyAwilixPlugin', () => {
       await assert.rejects(app.ready(), /failed to init/)
     })
   })
+
+  describe('FastifyDependencyProvider', () => {
+    it('resolves dependencies', async () => {
+      app = fastify({ logger: true })
+      await app.register(fastifyAwilixPlugin, {
+        container: diContainer,
+      })
+      app.diContainer.register({
+        maxUserName: asValue('username'),
+      })
+
+      const maxUserName = resolveDependencyFromApp(app, 'maxUserName')
+      assert.equal(maxUserName, 'username')
+
+      await app.close()
+    })
+  })
 })