Navigating Dependency Injection with Metro

Metro is yet another compile-time dependency injection framework. How is this one different though? In this talk we’ll explore how it balances blazing-fast compile times without sacrificing features, deep dive into how it works under the hood, and how to start using it seamlessly in your existing projects. From compiler plugins to graph theory to multiplatform, this talk will cover a bit of everything!

Transcript

1. Navigating Dependency Injection with Metro Zac Sweers
2. Navigating Dependency Injection with Metro 🌴 Zac Sweers
3. TODO SLIDES URL Formerly @ Slack, Uber, Flipboard Author of "Dagger Party Tricks" blogs/talks Past contributor to Anvil, author of Anvil-KSP, a few compiler plugins Led multiple Slack OSS projects like Circuit, compose-lints, etc Running, football, dogs 🐶
4. Dependency Injection (a primer)
5. What is Dependency Injection?
6. What is Dependency Injection? class WeatherApp { fun run(location: String) { val service = WeatherService() } }
7. What is Dependency Injection? class WeatherApp { fun run(location: String) { val service = WeatherService() val formatter = WeatherFormatter() } }
8. What is Dependency Injection? class WeatherApp { fun run(location: String) { val service = WeatherService() val formatter = WeatherFormatter() val forecast = service.getForecast(location) println(formatter.format(forecast)) } }
9. What is Dependency Injection? class WeatherApp { fun run(location: String) { val service = WeatherService() val formatter = WeatherFormatter() // ... } }
10. What is Dependency Injection? class WeatherApp { private val service = WeatherService() private val formatter = WeatherFormatter() fun run(location: String) { // ... } }
11. What is Dependency Injection? class WeatherApp { private val service =aWeatherService() private val formatter = WeatherFormatter() fun run(location: String) { // ... } } New York 🗽 24° Tokyo 🇯🇵 25° Curaçao 🇨🇼 31°
12. Singletons class WeatherApp { private val service = WeatherService.getInstance() }
13. Singletons class WeatherApp { private var service = WeatherService.getInstance() inner class Backdoor { fun setService(service: WeatherService) = //... } }
14. Service Locator class WeatherApp { private val service: WeatherService = getApplication().getWeatherService() }
15. Service Locator class WeatherApp { private val service: WeatherService by injector.instance() }
16. Service Locator class WeatherApp { private val service: WeatherService by inject() }
17. Service Locator class WeatherApp { private val service: WeatherService by inject() } @Test fun exampleTest() { startKoin { modules( module { single { WeatherService() } }) } // … }
18. class WeatherApp { private val service: WeatherService = //... }
19. class WeatherApp( private val service: WeatherService )
20. "Don't call us, we'll call you" —The Hollywood Principle
21. Manual Injection class WeatherApp( private val service: WeatherService )
22. Manual Injection class WeatherApp( private val service: WeatherService ) @Test fun exampleTest() { val app = WeatherApp(WeatherService()) //... }
23. Manual Injection class WeatherApp( private val service: WeatherService ) @Test fun exampleTest() { val app = WeatherApp(FakeWeatherService()) //... }
24. Manual Injection class WeatherApp( private val service: WeatherService private val formatter: WeatherFormatter ) { // ... } fun main() { val service = WeatherService() val formatter = WeatherFormatter() val app = WeatherApp(service, formatter) }
25. Manual Injection class WeatherApp( private val service: WeatherService private val formatter: WeatherFormatter ) { // ... } fun main() { val httpClient = HttpClient() val service = WeatherService(httpClient) val formatter = WeatherFormatter() val app = WeatherApp(service, app) }
26. Manual Injection class WeatherApp( private val service: WeatherService private val formatter: WeatherFormatter ) { // ... } fun main() { val cache = Cache() val httpClient = HttpClient(cache) val service = WeatherService(httpClient) val formatter = WeatherFormatter() val app = WeatherApp(service, app) }
27. class Cache(…) class HttpClient(private val cache: Cache) class WeatherService(private val httpClient: HttpClient) class WeatherFormatter(…) class WeatherApp( private val service: WeatherService private val formatter: WeatherFormatter ) fun main() { val cache = Cache() val httpClient = HttpClient(cache) val service = WeatherService(httpClient) val formatter = WeatherFormatter() val app = WeatherApp(service, app) } Manual Injection
28. class Cache(…) class HttpClient(private val cache: Cache) class WeatherService(private val httpClient: HttpClient) class WeatherFormatter(…) class WeatherApp( private val service: WeatherService private val formatter: WeatherFormatter ) fun main() { val app = TODO() } Manual Injection
29. Introducing Metro
30. Why another DI framework? Dagger-style code gen Kotlin- fi rst Anvil aggregation Multiplatform Compiler plugin IDE diagnostics* Metro
31. Why another DI framework? Dagger-style code gen Kotlin- fi rst Anvil aggregation Multiplatform Compiler plugin IDE diagnostics* Metro moshix, RCP Circuit, RCP anvil-ksp compose-lints anvil
32. Why another DI framework? Kotlin- fi rst Anvil aggregation Multiplatform Compiler plugin IDE diagnostics* Metro Dagger-style code gen moshix, RCP Circuit, RCP anvil-ksp compose-lints anvil
33. Getting Started plugins { id("dev.zacsweers.metro") version "0.6.6" }
34. Metro DI Constructor/assisted/member/function injection Providers Multibindings Scopes Runtime intrinsics
35. Metro DI class Cache(…) class HttpClient(private val cache: Cache) class WeatherService(private val httpClient: HttpClient) class WeatherFormatter(…) class WeatherApp( private val service: WeatherService private val formatter: WeatherFormatter )
36. Metro DI @Inject class Cache(…) @Inject class HttpClient(private val cache: Cache) @Inject class WeatherService(private val httpClient: HttpClient) @Inject class WeatherFormatter(…) @Inject class WeatherApp( private val service: WeatherService private val formatter: WeatherFormatter )
37. Metro DI fun main() { val cache = Cache() val httpClient = HttpClient() val service = WeatherService(httpClient) val formatter = WeatherFormatter() val app = WeatherApp(service, formatter) }
38. Metro DI @DependencyGraph interface AppGraph { val app: WeatherApp } fun main() { val app = createGraph<AppGraph>().app }
39. Metro DI @DependencyGraph interface AppGraph { val app: WeatherApp }
40. Metro DI @DependencyGraph interface AppGraph { val app: WeatherApp }
41. Metro DI @DependencyGraph interface AppGraph { val app: WeatherApp } class HttpClient class WeatherFormatter class Cache class WeatherService
42. Metro DI @DependencyGraph interface AppGraph { val app: WeatherApp } @Component interface AppGraph { val app: WeatherApp }
43. Metro DI @DependencyGraph interface AppGraph { val app: WeatherApp } fun main() { val app = createGraph<AppGraph>().app }
44. Metro DI @DependencyGraph interface AppGraph { val app: WeatherApp } fun main() { val app = createGraph<AppGraph>().app }
45. Metro DI @DependencyGraph interface AppGraph { val app: WeatherApp } fun main() { val app = AppGraph.$$MetroGraph().app }
46. Metro DI @DependencyGraph interface AppGraph { val app: WeatherApp } fun main() { val app = createGraph<AppGraph>().app }
47. Metro DI @DependencyGraph interface AppGraph { val app: WeatherApp @DependencyGraph.Factory interface Factory { fun create(@Provides useMetric: Boolean) : AppGraph } } fun main() { val app = createGraph<AppGraph>().app }
48. Metro DI @DependencyGraph interface AppGraph { val app: WeatherApp @DependencyGraph.Factory interface Factory { fun create(@Provides useMetric: Boolean) : AppGraph } } fun main() { val app = createGraph<AppGraph>().app }
49. Metro DI @DependencyGraph interface AppGraph { val app: WeatherApp @DependencyGraph.Factory interface Factory { fun create(@Provides useMetric: Boolean) : AppGraph } } fun main() { val app = createGraphFactory<AppGraph.Factory>() .create(useMetric = true) .app }
50. Metro DI @DependencyGraph interface AppGraph { val app: WeatherApp }
51. Metro DI @DependencyGraph interface AppGraph { val app: WeatherApp @Provides fun provideCache() : Cache = Cache() }
52. Metro DI @DependencyGraph interface AppGraph { val app: WeatherApp @Provides fun provideCache(fs: FileSystem) : Cache = Cache(fs) }
53. Metro DI @DependencyGraph interface AppGraph : CacheProviders { val app: WeatherApp } interface CacheProviders { @Provides fun provideCache(fs: FileSystem) : Cache = Cache(fs) }
54. Metro DI @DependencyGraph interface AppGraph { val app: WeatherApp @Provides fun provideCache() : Cache = Cache() }
55. Metro DI @DependencyGraph interface AppGraph { val app: WeatherApp @Provides fun provideCache() : Cache = Cache() @Provides fun provideFormatterCache() : Cache = Cache() }
56. Metro DI @Qualif i er
57. Metro DI @Qualif i er annotation class Named(val value: String)
58. Metro DI @DependencyGraph interface AppGraph { val app: WeatherApp @Provides fun provideCache() : Cache = Cache() @Named("FormatterCache") @Provides fun provideFormatterCache() : Cache = Cache() }
59. Metro DI @Named("FormatterCache") Cache
60. Metro DI @Named("FormatterCache") Cache Type
61. Metro DI @Named("FormatterCache") Cache Type Quali fi er
62. Metro DI @Named("FormatterCache") Cache TypeKey
63. What's a Binding?
64. What's a Binding? @Provides fun provideCache() : Cache = Cache()
65. Injection class HttpClient(private val cache: Cache)
66. Injection @Inject class HttpClient(private val cache: Cache)
67. Injection class HttpClient( private val cache: Cache, private val timeout: Duration, ) { @Inject constructor(cache: Cache) : this(cache, 30.seconds) }
68. Metro DI @Inject class Cache @Inject class HttpClient( private val cache: Cache ) @Inject class WeatherService( private val httpClient: HttpClient ) @Inject class WeatherFormatter @Inject class WeatherApp( private val service: WeatherService private val formatter: WeatherFormatter ) @DependencyGraph interface AppGraph { val app: WeatherApp } fun main() { val app = createGraph<AppGraph>().app }
69. Scoping @Scope
70. Scoping @Scope annotation class SingleIn(val scope: KClass <*> )
71. Scoping @SingleIn(AppScope :: class)
72. Scoping @DependencyGraph interface AppGraph { val app: WeatherApp } @SingleIn(AppScope :: class)
73. Scoping @SingleIn(AppScope :: class) @DependencyGraph interface AppGraph { val app: WeatherApp }
74. Scoping @SingleIn(AppScope :: class) @Singleton
75. Scoping @SingleIn(AppScope :: class) @DependencyGraph interface AppGraph { val app: WeatherApp }
76. Scoping @Inject class WeatherService( private val httpClient: HttpClient )
77. Scoping @Inject @SingleIn(AppScope :: class) class WeatherService( private val httpClient: HttpClient )
78. Scoping @SingleIn(AppScope :: class) @Inject class WeatherService( private val httpClient: HttpClient ) @Inject class WeatherApp( private val service: WeatherService private val formatter: WeatherFormatter ) @DependencyGraph interface AppGraph { val app: WeatherApp } Always same instance
79. Scoping All bindings and graphs are unscoped by default Scoped bindings are lazily initialized with thread-safe, doubly-checked locking Scope is declared at the binding, rather than the consumer of it needing to know about it Certain binding types can’t be scoped
80. More Bindings
81. @Binds
82. @Binds interface Bindings { @Binds fun bind(int: Int) : Number @Binds val String.bind: CharSequence }
83. @Binds @Binds fun bind(int: Int) : Number @Inject class HttpClient(timeoutMillis: Number)
84. @Binds @Binds fun bind(int: Int) : Number @Inject class HttpClient(timeoutMillis: Number)
85. @Binds @Binds fun bind(int: Int) : Number @Inject class HttpClient(timeoutMillis: Number)
86. @Binds Binding impl types as interfaces they implement @Binds val Int.bind: Number
87. @Binds Binding impl types as interfaces they implement Binding types into multibindings @Binds val Int.bind: Number @Binds @IntoSet val Int.bind: Number
88. @Binds Binding impl types as interfaces they implement Binding types into multibindings Binding types as nullable variants @Binds val Int.bind: Number @Binds @IntoSet val Int.bind: Number @Binds val Int.bind: Int?
89. @Binds Binding impl types as interfaces they implement Binding types into multibindings Binding types as nullable variants Binding types with different quali fi ers @Binds val Int.bind: Number @Binds @IntoSet val Int.bind: Number @Binds val Int.bind: Int? @Binds @Named("timeout") val Int.bind: Int
90. Assisted Injection
91. Assisted Injection @Inject class UserRepository( val id: String, val httpClient: HttpClient, val db: UserDb )
92. Assisted Injection @Inject class UserRepository( @Assisted val id: String, val httpClient: HttpClient, val db: UserDb )
93. Assisted Injection @Inject class UserRepository( @Assisted val id: String, val httpClient: HttpClient, val db: UserDb ) { @AssistedFactory interface Factory { fun create(id: String) : UserRepository } }
94. Assisted Injection @Inject class UserRepository( @Assisted val id: String, val httpClient: HttpClient, val db: UserDb ) { @AssistedFactory interface Factory { fun create(id: String) : UserRepository } }
95. Assisted Injection @Inject class UserRepository( @Assisted val id: String, val httpClient: HttpClient, val db: UserDb ) { @AssistedFactory interface Factory { fun create(id: String) : UserRepository } }
96. Assisted Injection @Inject class WeatherApp( val factory: UserRepository.Factory ) { fun run() { val userRepo = factory.create("d3938c4891ae") } } @Inject class UserRepository( @Assisted val id: String, val httpClient: HttpClient, val db: UserDb ) { @AssistedFactory interface Factory { fun create(id: String) : UserRepository } }
97. Multibindings @DependencyGraph interface AppGraph { }
98. Multibindings @DependencyGraph interface AppGraph { val interceptors: Set<Interceptor> val eventHandlers: Map<String, EventHandler> }
99. Multibindings @DependencyGraph interface AppGraph { val interceptors: Set<Interceptor> val eventHandlers: Map<String, EventHandler> @Provide @IntoSet fun provideLoggingInterceptor() : Interceptor = LoggingInterceptor() }
100. Multibindings @DependencyGraph interface AppGraph { val interceptors: Set<Interceptor> val eventHandlers: Map<String, EventHandler> @Provide @IntoSet fun provideLoggingInterceptor() : Interceptor = LoggingInterceptor() } @Inject @IntoMap @StringKey("ping") class PingHandler : EventHandler
101. Multibindings @DependencyGraph interface AppGraph { @Multibinds val interceptors: Set<Interceptor> @Multibinds val eventHandlers: Map<String, EventHandler> }
102. Multibindings @DependencyGraph interface AppGraph { @Multibinds(allowEmpty = true) val interceptors: Set<Interceptor> @Multibinds(allowEmpty = true) val eventHandlers: Map<String, EventHandler> }
103. @MapKey Multibindings
104. @MapKey annotation class ClassKey(val value: KClass <*> ) Multibindings
105. @MapKey annotation class StringKey(val value: String) Multibindings
106. @Inject @IntoMap @StringKey("ping") class PingHandler : EventHandler Multibindings
107. @Provides @ElementsIntoSet fun provideInterceptors() : Set<Interceptor> = setOf(…) Multibindings
108. Aggregation
109. Aggregation @DependencyGraph interface AppGraph
110. Aggregation @DependencyGraph(scope = AppScope :: class) interface AppGraph
111. Aggregation @DependencyGraph(scope = AppScope :: class) interface AppGraph @SingleIn(scope = AppScope :: class)
112. Aggregation @DependencyGraph(AppScope :: class) // @SingleIn(AppScope :: class) // Implicit! interface AppGraph
113. What's a Contribution? @DependencyGraph(AppScope :: class) interface AppGraph { val cache: Cache }
114. What's a Contribution? @DependencyGraph(AppScope :: class) interface AppGraph { val cache: Cache } @Inject class CacheImpl(…) : Cache
115. What's a Contribution? @DependencyGraph(AppScope :: class) interface AppGraph { val cache: Cache } @Inject class CacheImpl(…) : Cache
116. What's a Contribution? @DependencyGraph(AppScope :: class) interface AppGraph { val cache: Cache @Binds val CacheImpl.bind: Cache } @Inject class CacheImpl(…) : Cache
117. What's a Contribution? @DependencyGraph(AppScope :: class) interface AppGraph { val cache: Cache } @ContributesBinding(AppScope :: class) @Inject class CacheImpl(…) : Cache
118. What's a Contribution? @DependencyGraph(AppScope :: class) interface AppGraph : CacheImpl.AppScope$$Contribution { val cache: Cache } @ContributesBinding(AppScope :: class) @Inject class CacheImpl(…) : Cache { @MetroContribution interface AppScope$$Contribution { @Binds val CacheImpl.bind: Cache } }
119. What's a Contribution? @DependencyGraph(AppScope :: class) interface AppGraph { val cache: Cache } @ContributesBinding(AppScope :: class) @Inject class CacheImpl(…) : Cache
120. What's a Contribution? @DependencyGraph(AppScope :: class) interface AppGraph { val cache: Cache } @ContributesBinding(AppScope :: class) @Inject class CacheImpl(…) : Cache, Closeable
121. What's a Contribution? @DependencyGraph(AppScope :: class) interface AppGraph { val cache: Cache } @ContributesBinding(AppScope :: class, binding = binding<Cache>()) @Inject class CacheImpl(…) : Cache, Closeable
122. What's a Contribution? @DependencyGraph(AppScope :: class) interface AppGraph { val cache: Cache<Forecast> } @ContributesBinding(AppScope :: class, binding = binding<Cache<Forecast >> ()) @Inject class CacheImpl(…) : Cache<Forecast>, Closeable
123. What's a Contribution? @DependencyGraph(AppScope :: class) interface AppGraph { val cache: Cache } @ContributesBinding(AppScope :: class) @Inject class CacheImpl(…) : Cache
124. What's a Contribution? @DependencyGraph(AppScope :: class) interface AppGraph { val interceptors: Set<Interceptor> } @ContributesIntoSet(AppScope :: class) @Inject class LogcatInterceptor(…) : Interceptor
125. What's a Contribution? @DependencyGraph(AppScope :: class) interface AppGraph { val eventHandlers: Map<String, EventHandler> } @Inject @ContributesIntoMap(AppScope :: class) @StringKey("ping") class PingHandler : EventHandler
126. What's a Contribution? @DependencyGraph(AppScope :: class) interface AppGraph @ContributesTo(AppScope :: class) interface Accessors { val eventHandlers: Map<String, EventHandler> }
127. What's a Contribution? @DependencyGraph(AppScope :: class) interface AppGraph : Accessors @ContributesTo(AppScope :: class) interface Accessors { val eventHandlers: Map<String, EventHandler> }
128. @DependencyGraph(AppScope :: class) interface AppGraph { val cache: Cache val interceptors: Set<Interceptor> val eventHandlers: Map<String, EventHandler> } @ContributesIntoSet(AppScope :: class) @Inject class LogcatInterceptor(…) : Interceptor @Inject @ContributesIntoMap(AppScope :: class) @StringKey("ping") class PingHandler : EventHandler @ContributesBinding(AppScope :: class) @Inject class CacheImpl(…) : Cache :lib - network :app :lib - events :lib - cache
129. Intrinsics
130. Intrinsics Provider Lazy @Inject class HttpClient(val cache: Cache) { init { val cacheInstance = cache } }
131. Intrinsics Provider Lazy @Inject class HttpClient(val cache: Provider<Cache>) { init { val cacheInstance = cache() } }
132. Intrinsics Provider Lazy @Inject class HttpClient(val cache: Lazy<Cache>) { init { val cacheInstance = cache.value } }
133. Graph Extensions
134. Graph Extensions @GraphExtension interface LoggedInGraph { val user: User }
135. Graph Extensions @GraphExtension interface LoggedInGraph { val user: User } @DependencyGraph interface AppGraph { val loggedInGraph: LoggedInGraph }
136. Graph Extensions Factories that you can declare instead @GraphExtension interface LoggedInGraph { val user: User } @DependencyGraph interface AppGraph { val loggedInGraph: LoggedInGraph }
137. Graph Extensions Factories that you can declare instead @GraphExtension interface LoggedInGraph { val user: User @GraphExtension.Factory interface Factory { fun create(@Provides id: String) : LoggedInGraph } } @DependencyGraph interface AppGraph { val loggedInGraphFactory: LoggedInGraph.Factory }
138. Graph Extensions Factories that you can declare instead @GraphExtension interface LoggedInGraph { val user: User @GraphExtension.Factory interface Factory { fun createLoggedInGraph() : LoggedInGraph } } @DependencyGraph interface AppGraph : LoggedInGraph.Factory / / usage val loggedInGraph = appGraph.createLoggedInGraph()
139. Graph Extensions Factories that you can declare instead Scope annotations @GraphExtension @SingleIn(LoggedInScope :: class) interface LoggedInGraph { val user: User }
140. Graph Extensions Factories that you can declare instead Scope annotations Aggregation scopes @GraphExtension(LoggedInScope :: class) interface LoggedInGraph { val user: User }
141. Graph Extensions Factories that you can declare instead Scope annotations Aggregation scopes Provider/Binds callables @GraphExtension(LoggedInScope :: class) interface LoggedInGraph { val user: User @Provides fun provideUser() : User = … }
142. Graph Extensions Factories that you can declare instead Scope annotations Aggregation scopes Provider/Binds callables Their own extensions! @GraphExtension interface LoggedInActivityGraph { val user: User val activity: Activity } @GraphExtension interface LoggedInGraph { val user: User val loggedInActivityGraph: LoggedInActivityGraph } @DependencyGraph interface AppGraph { val loggedInGraph: LoggedInGraph }
143. Build Pipeline @DependencyGraph interface AppGraph { val app: WeatherApp }
144. Simple Build Pipeline @DependencyGraph interface AppGraph { val app: WeatherApp } kotlinc (Frontend and Backend) compileKotlin .class f i les
145. Multiplatform Build Pipeline @DependencyGraph interface AppGraph { val app: WeatherApp } kotlinc (Frontend and Backend) compileKotlin .class .js .wasm ...
146. KSP Build Pipeline @DependencyGraph interface AppGraph { val app: WeatherApp } <compiled binaries> kotlinc (Frontend only) KSP source f i les kotlinc (Frontend and Backend) compileKotlin
147. kotlinc (Frontend only) Stub gen .java f i les Kapt (javac) Kapt source f i les kotlinc (Frontend and Backend) compileKotlin .class f i les KAPT Build Pipeline
148. kotlinc (Frontend only) Stub gen .java f i les Kapt (javac) Kapt source f i les kotlinc (Frontend and Backend) compileKotlin .class f i les javac compileJava More .class f i les KAPT Build Pipeline
149. kotlinc (Frontend only) Stub gen .java f i les Kapt (javac) Kapt source f i les kotlinc (Frontend and Backend) compileKotlin .class f i les javac compileJava More .class f i les HILT Build Pipeline javac :hiltJavaCompileDebug Even more .class f i les
150. kotlinc (Frontend only) Stub gen .java f i les Kapt (javac) Kapt source f i les kotlinc (Frontend and Backend) compileKotlin .class f i les javac compileJava More .class f i les HILT Build Pipeline javac :hiltJavaCompileDebug Even more .class f i les
152. Metro Build Pipeline @DependencyGraph interface AppGraph { val app: WeatherApp } kotlinc (Frontend and Backend) compileKotlin binary f i les
153. Metro Build Pipeline
154. https://zacsweers.github.io/metro/latest/ performance.html Metro KIA Anvil-KSP Anvil-KAPT
155. Metro Build Pipeline @DependencyGraph interface AppGraph { val app: WeatherApp } kotlinc (Frontend and Backend) compileKotlin binary f i les
156. Metro Build Pipeline binary f i les Frontend Backend Source f i les
157. Metro Build Pipeline binary f i les Frontend Backend FIR IR Lowering Parse/PSI Source f i les
158. Metro Build Pipeline binary f i les Frontend Backend FIR IR Lowering Parse/PSI fi r2ir Source f i les
159. Frontend Parsing sources Reports diagnostics Build a resolved frontend IR (FIR) tree
160. Frontend Source f i les
161. Frontend Parse/PSI Source f i les
162. Frontend FIR Parse/PSI Source f i les
163. Frontend FIR Parse/PSI Source f i les object FirCastOperatorsChecker
164. Frontend FIR
165. Frontend FIR AnalysisHandlerExtension
166. Frontend FIR
167. Frontend FIR Declaration generation Supertype generation Status transformation
168. Declaration Generation Constructor-injected classes/providers @Inject class Cache { // Generated by Metro class $$MetroFactory : Factory<Cache> { constructor() companion object { fun create() : $$MetroFactory fun newInstance() : Cache } } }
169. Declaration Generation Constructor-injected classes/providers Assisted factory impls @AssistedFactory interface Factory { fun create(id: String) : UserRepository // Generated by Metro class $$Impl : Factory }
170. Declaration Generation Constructor-injected classes/providers Assisted factory impls Contribution interfaces @ContributesBinding(AppScope :: class) @Inject class CacheImpl(…) : Cache { // Generated by Metro @MetroContribution interface AppScope$$Contribution }
171. Declaration Generation Constructor-injected classes/providers Assisted factory impls Contribution interfaces Etc
172. Declaration Generation @DependencyGraph.Factory interface Factory { fun create() : AppGraph // Generated in FIR class $$Impl : Factory { constructor() } }
173. Declaration Generation @DependencyGraph.Factory interface Factory { fun create() : AppGraph // Generated in FIR class $$Impl : Factory { constructor() } } @DependencyGraph.Factory interface Factory { fun create() : AppGraph class $$Impl : Factory { constructor() { } // Implemented in IR override fun create() : AppGraph = AppGraph.$$MetroGraph() } }
174. Supertype Generation
175. Supertype Generation @DependencyGraph(AppScope :: class) interface AppGraph
176. Supertype Generation @DependencyGraph(AppScope :: class) interface AppGraph @ContributesBinding(AppScope :: class) @Inject class CacheImpl(…) : Cache { // Generated by Metro @MetroContribution interface AppScope$$Contribution } @ContributesTo(AppScope :: class) interface NetworkProviders { // ... }
177. Status Transformers
178. Status Transformers
179. Status Transformers FirProvidesStatusTransformer FirAccessorOverrideStatusTransformer (Metro 0.6.6+)
180. Frontend FIR
181. Frontend FIR Checkers
182. Checkers Has a single abstract function That function returns a dependency graph That function has valid parameters Nearly 100 more! object DependencyGraphCreatorChecker
183. Checkers
184. Checkers
185. 🚧 IDE Support 🚧 https://zacsweers.github.io/metro/latest/installation.html#ide-support
186. Metro Build Pipeline binary f i les Frontend Backend FIR IR Lowering Parse/PSI fi r2ir Source f i les
187. IR Transformation
188. IR Transformation 1. Generate binds functions for contributions / aggregate contributions 2. Everything else 🙃
189. Metro IR Pipeline 1. Transform non-graph classes as needed 2. Transform dependency graphs A. Collect graph data B. Process extensions (go to step 2) C. Build a BindingGraph D. Seal E. (If seal successful) implement the IR graph • Populate bindings • Hybrid Tarjan validation + Kahn sorting • Return result of top sorted, reachable, and deferred keys
190. @Inject @SingleIn(AppScope : : class) class WeatherService( httpClient: HttpClient ) @Inject class WeatherFormatter(…) @Inject class WeatherApp( service: WeatherService formatter: WeatherFormatter ) @DependencyGraph @SingleIn(AppScope : : class) interface AppGraph { val app: WeatherApp @Provides fun provideCache() : Cache = Cache() @Provides fun provideHttpClient(cache: Cache) : HttpClient = HttpClient(cache) } Metro IR Pipeline
191. Metro IR Pipeline @DependencyGraph @SingleIn(AppScope :: class) interface AppGraph { val app: WeatherApp @Provides fun provideCache() : Cache = Cache() @Provides fun provideHttpClient(cache: Cache) : HttpClient = HttpClient(cache) }
192. Metro IR Pipeline Scope — @SingleIn(AppScope :: class) @DependencyGraph interface AppGraph { val app: WeatherApp @Provides fun provideCache() : Cache = Cache() @Provides fun provideHttpClient( cache: Cache ) : HttpClient = HttpClient(cache) }
193. Metro IR Pipeline Scope — @SingleIn(AppScope :: class) IrBinding.Provided — Cache @DependencyGraph interface AppGraph { val app: WeatherApp @Provides fun provideHttpClient( cache: Cache ) : HttpClient = HttpClient(cache) }
194. Metro IR Pipeline Scope — @SingleIn(AppScope :: class) IrBinding.Provided — Cache IrBinding.Provided — HttpClient @DependencyGraph interface AppGraph { val app: WeatherApp }
195. Metro IR Pipeline Scope — @SingleIn(AppScope :: class) IrBinding.Provided — Cache IrBinding.Provided — HttpClient Accessor — app: WeatherApp @DependencyGraph interface AppGraph
196. Metro IR Pipeline Scope — @SingleIn(AppScope :: class) IrBinding.Provided — Cache IrBinding.Provided — HttpClient Accessor — app: WeatherApp AppGraph
197. Metro IR Pipeline Scope — @SingleIn(AppScope :: class) Accessor — app: WeatherApp AppGraph BindingLookup Cache: IrBinding.Provided HttpClient: IrBinding.Provided
198. Metro IR Pipeline WeatherApp
199. Metro IR Pipeline WeatherApp class WeatherFormatter class WeatherService
200. Metro IR Pipeline WeatherApp class HttpClient class WeatherFormatter class Cache class WeatherService BindingLookup Cache: IrBinding.Provided HttpClient: IrBinding.Provided
201. Metro IR Pipeline WeatherApp class HttpClient class WeatherFormatter class Cache class WeatherService
202. Metro IR Pipeline WeatherApp (IrBinding.ConstructorInjected) WeatherFormatter (IrBinding.ConstructorInjected) WeatherService (IrBinding.ConstructorInjected) Cache (IrBinding.Provided) HttpClient (IrBinding.Provided)
203. Metro IR Pipeline Tarjan's Algorithm WeatherApp (IrBinding.ConstructorInjected) WeatherFormatter (IrBinding.ConstructorInjected) WeatherService (IrBinding.ConstructorInjected) Cache (IrBinding.Provided) HttpClient (IrBinding.Provided)
204. Metro IR Pipeline Tarjan's Algorithm njected) tructorInjected) ctorInjected)
205. Metro IR Pipeline Kahn's Topo Sort WeatherApp (IrBinding.ConstructorInjected) WeatherFormatter (IrBinding.ConstructorInjected) WeatherService (IrBinding.ConstructorInjected) Cache (IrBinding.Provided) HttpClient (IrBinding.Provided)
206. Metro IR Pipeline Kahn's Topo Sort Cache (IrBinding.Provided) HttpClient (IrBinding.Provided) WeatherFormatter (IrBinding.ConstructorInjected) WeatherService (IrBinding.ConstructorInjected) WeatherApp (IrBinding.ConstructorInjected)
207. Metro IR Pipeline Cache (IrBinding.Provided) HttpClient (IrBinding.Provided) WeatherFormatter (IrBinding.ConstructorInjected) WeatherService (IrBinding.ConstructorInjected) WeatherApp (IrBinding.ConstructorInjected)
208. Metro IR Pipeline @DependencyGraph @SingleIn(AppScope :: class) interface AppGraph { val app: WeatherApp } Cache (IrBinding.Provided) HttpClient (IrBinding.Provided) WeatherFormatter (IrBinding.ConstructorInjected) WeatherService (IrBinding.ConstructorInjected) WeatherApp (IrBinding.ConstructorInjected)
209. Metro IR Pipeline class $$MetroGraph : AppGraph { }
210. Metro IR Pipeline class $$MetroGraph : AppGraph { } Cache (IrBinding.Provided) HttpClient (IrBinding.Provided) WeatherFormatter (IrBinding.ConstructorInjected) WeatherService (IrBinding.ConstructorInjected) WeatherApp (IrBinding.ConstructorInjected)
211. Metro IR Pipeline class $$MetroGraph : AppGraph { } ProviderFieldCollector Cache (IrBinding.Provided) HttpClient (IrBinding.Provided) WeatherFormatter (IrBinding.ConstructorInjected) WeatherService (IrBinding.ConstructorInjected) WeatherApp (IrBinding.ConstructorInjected)
212. Metro IR Pipeline class $$MetroGraph : AppGraph { } WeatherService ProviderFieldCollector
213. Metro IR Pipeline Cache (IrBinding.Provided) HttpClient (IrBinding.Provided) WeatherFormatter (IrBinding.ConstructorInjected) WeatherApp (IrBinding.ConstructorInjected) class $$MetroGraph : AppGraph { private val weatherServiceProvider: Provider<WeatherService> }
214. Metro IR Pipeline WeatherFormatter (IrBinding.ConstructorInjected) WeatherApp (IrBinding.ConstructorInjected) class $$MetroGraph : AppGraph { private val weatherServiceProvider: Provider<WeatherService> = DoubleCheck.provider(…) }
215. Metro IR Pipeline WeatherFormatter (IrBinding.ConstructorInjected) WeatherApp (IrBinding.ConstructorInjected) class $$MetroGraph : AppGraph { private val weatherServiceProvider: Provider<WeatherService> = DoubleCheck.provider( WeatherService.$$MetroFactory.create( $$HttpClientProvider.create( $$CacheProvider() ) ) ) }
216. Metro IR Pipeline class $$MetroGraph : AppGraph { private val weatherServiceProvider: Provider<WeatherService> = DoubleCheck.provider( WeatherService.$$MetroFactory.create( $$HttpClientProvider.create( $$CacheProvider() ) ) ) override val app: WeatherApp get() = WeatherApp$$MetroFactory.create( this.weatherServiceProvider, WeatherFormatter$$MetroFactory.create() ).invoke() }
217. Metro IR Pipeline class $$MetroGraph : AppGraph { private val weatherServiceProvider: Provider<WeatherService> = DoubleCheck.provider( WeatherService.$$MetroFactory.create( $$HttpClientProvider.create( $$CacheProvider() ) ) ) override val app: WeatherApp get() = WeatherApp$$MetroFactory.create( this.weatherServiceProvider, WeatherFormatter$$MetroFactory.create() ).invoke() }
218. Other Stuff • Incremental compilation • Transform createGraph() and createGraphFactory() calls val graph = createGraph<AppGraph>() val graph = AppGraph.$$MetroGraph()
219. What if validation fails?
220. What if validation fails?
221. What if validation fails?
222. What if validation fails?
223. Advanced Usage Binding containers @Includes Member injection Valid cycles Optional Top-level function injection Contribution replacements/excludes Compose integration Reports Circuit
224. Interop & Adoption https://zacsweers.github.io/metro/latest/adoption.html
225. Component Interop @Component interface StartupComponent { val httpClient: HttpClient //... } @DependencyGraph interface AppGraph { @DependencyGraph.Factory interface Factory { fun create( @Includes startupComponent: StartupComponent ) : AppGraph } } https://zacsweers.github.io/metro/latest/adoption.html
226. Annotations Interop @Component interface StartupComponent { val httpClient: HttpClient } class HttpClient @Inject constructor( val cache: Provider<Cache> ) metro { interop { enableDagger() enableAnvil() } } https://zacsweers.github.io/metro/latest/adoption.html
227. Annotations Interop @Component interface StartupComponent { val httpClient: HttpClient } class HttpClient @Inject constructor( val cache: Provider<Cache> )
228. Annotations Interop @dagger.Component interface StartupComponent { val httpClient: HttpClient } class HttpClient @jakarta.inject.Inject constructor( val cache: jakarta.inject.Provider<Cache> )
229. Custom Code Generators https://zacsweers.github.io/metro/latest/adoption.html
230. Migration https://zacsweers.github.io/metro/latest/adoption.html
231. FAQ & Future
232. FAQ Are compiler plugins safe to use? Solo maintainer? SPI / diagnostics? Debugging? Fast init IDE support/IDE plugin Dynamic/adhoc graphs Kotlin 2.3.0 support/known caveats Future
233. Metro Docs: https://zacsweers.github.io/metro/ API: https://zacsweers.github.io/metro/api/0.x/ Repo: https://github.com/zacsweers/metro Me: https://zacsweers.dev