Fundamentals of Dependency Injection - Droidcon NYC

Dependency Injection (DI) plays a key role in building scalable software projects. The Android community has spent the last decade experimenting and iterating on DI approaches primarily focused on the benefits of facilitating testing and building more modular apps. If you weren't in the industry during that experimentation and iteration it can be easy to miss how we got to where we are today.

In this talk, we're going back to basics! We're going to look at Dependency Injection, to help you understand the concepts behind the pattern, irrespective of which library, if any, you use to implement it. We'll look at the basic approaches to dependency substitution, see what problems DI can help solve, and learn how to make our own DI implementation.

Transcript

1. Ryan Harter Dependency Injection ryan@harter.us
2. Ryan Harter Dependency Injection ryan@harter.us The “Jake Wharton” of Dropbox
3. Dagger is complicated
4. Dependency injection is complicated
5. Repository MyPresenter
6. Repository Analytics Other Repository MyPresenter
7. Analytics Other Repository API DB Repository MyPresenter
8. Analytics Other Repository API DB Repository MyPresenter
14. Dependency injection is complicated
15. Object Graph
17. Other Repository MyPresenter Repository Analytics
18. class MyPresenter { Analytics() Repository() OtherRepository() private val analytics: Analytics = = = private val otherRepository: OtherRepository private val repository: Repository }
19. class MyPresenter { Analytics() Repository() OtherRepository() private val analytics: Analytics = = = private val otherRepository: OtherRepository private val repository: Repository }
20. class MyPresenter { Analytics() Repository() OtherRepository() private val analytics: Analytics = = = private val otherRepository: OtherRepository private val repository: Repository }
21. class MyPresenter private val analytics: private val otherRepository: private val repository: ) ( , , , Analytics Repository OtherRepository
22. class MyPresenter private val analytics: private val otherRepository: private val repository: ) ( , , , Analytics Repository OtherRepository
23. class MyPresenter private val analytics: private val otherRepository: private val repository: ) ( , , , Analytics Repository OtherRepository Analytics Repository OtherRepository
24. class Analytics class Repository class OtherRepository class MyPresenter private val analytics: private val otherRepository: private val repository: ) ( , , , Analytics Repository OtherRepository
25. class class class class MyPresenter private val analytics: private val otherRepository: private val repository: ) ( , , , Analytics Repository OtherRepository Analytics Repository OtherRepository
26. Other Repository MyPresenter Repository Analytics
27. class class class private val analytics: private val otherRepository: private val repository: ) ( , , , Analytics Repository OtherRepository Analytics Repository OtherRepository MyPresenter class
28. private val analytics: private val otherRepository: private val repository: ) ( Analytics Repository OtherRepository , , , MyPresenter class
29. private val analytics: private val otherRepository: private val repository: ) ( Analytics Repository OtherRepository Analytics() Repository() OtherRepository() = = = , , , MyPresenter class
30. MyPresenter fun getMyPresenter():
31. MyPresenter fun getMyPresenter(): class ObjectGraph { }
32. MyPresenter fun getMyPresenter(): class ObjectGraph { } private fun getAnalytics(): Analytics private fun getRepository(): Repository private fun getOtherRepository(): OtherRepository ...
33. MyPresenter fun getMyPresenter(): class ObjectGraph { } = MyPresenter( ) getAnalytics(), getRepository(), getOtherRepository(), private fun getAnalytics(): Analytics private fun getRepository(): Repository private fun getOtherRepository(): OtherRepository
36. MyPresenter fun getMyPresenter(): } = MyPresenter( ) getAnalytics(), getRepository(), getOtherRepository(), class ObjectGraph { private fun getAnalytics(): Analytics private fun getRepository(): Repository private fun getOtherRepository(): OtherRepository
37. MyPresenter fun getMyPresenter(): class ObjectGraph } = MyPresenter( ) getAnalytics(), getRepository(), getOtherRepository(), { private val application: Application, ) ( private fun getAnalytics(): Analytics private fun getRepository(): Repository private fun getOtherRepository(): OtherRepository
38. MyPresenter fun getMyPresenter(): ObjectGraph } = MyPresenter( ) getAnalytics(), getRepository(), getOtherRepository(), { private val application: Application, ) ( private fun getAnalytics(): Analytics private fun getRepository(): Repository private fun getOtherRepository(): OtherRepository class
39. MyPresenter fun getMyPresenter(): } = MyPresenter( ) getAnalytics(), getRepository(), getOtherRepository(), { private val application: Application, ) ( private fun getAnalytics(): Analytics private fun getRepository(): Repository private fun getOtherRepository(): OtherRepository Component class
40. Object Graph
41. Factory
42. class private val analytics: private val otherRepository: private val repository: ( Analytics Repository OtherRepository , , , MyPresenter )
43. private val analytics: private val otherRepository: private val repository: ) Analytics Repository OtherRepository , , , class class MyPresenterFactory ( MyPresenter
44. private val analytics: private val otherRepository: private val repository: ) Analytics Repository OtherRepository , , , class class MyPresenterFactory ( MyPresenter
45. private val analytics: private val otherRepository: private val repository: ) Analytics Repository OtherRepository , , , class class MyPresenterFactory ( MyPresenter fun get(): MyPresenter } {
46. private val analytics: private val otherRepository: private val repository: ) Analytics Repository OtherRepository , , , class class MyPresenterFactory ( MyPresenter fun get(): MyPresenter } {
47. private val analytics: private val otherRepository: private val repository: ) Analytics Repository OtherRepository , , , class class MyPresenterFactory ) fun get(): MyPresenter } ( MyPresenter { ( private val analyticsFactory: private val repositoryFactory: private val otherRepositoryFactory: , , , Analytics Repository OtherRepository Factory Factory Factory
48. private val analytics: private val otherRepository: private val repository: ) Analytics Repository OtherRepository , , , class class MyPresenterFactory ) fun get(): MyPresenter = MyPresenter( ) } analyticsFactory.get(), repositoryFactory.get(), otherRepositoryFactory.get(), ( MyPresenter { ( private val analyticsFactory: private val repositoryFactory: private val otherRepositoryFactory: , , , Analytics Repository OtherRepository Factory Factory Factory
49. private val analytics: private val otherRepository: private val repository: ) Analytics Repository OtherRepository , , , class class MyPresenterFactory ) fun get(): MyPresenter = MyPresenter( ) analyticsFactory.get(), repositoryFactory.get(), otherRepositoryFactory.get(), ( MyPresenter { ( interface Provider<T> { fun get(): T } private val analyticsFactory: private val repositoryFactory: private val otherRepositoryFactory: , , , Analytics Repository OtherRepository Factory Factory Factory
50. private val otherRepository: ) OtherRepository, class MyPresenterFactory fun get(): MyPresenter = MyPresenter( ) } analyticsFactory.get(), repositoryFactory.get(), otherRepositoryFactory.get(), ( interface Provider<T> { fun get(): T } ) { Provider<T> : override private val analyticsFactory: private val repositoryFactory: private val otherRepositoryFactory: , , , Analytics Repository OtherRepository Factory Factory Factory
51. private val otherRepository: ) OtherRepository, class MyPresenterFactory private val analyticsFactory: private val repositoryFactory: private val otherRepositoryFactory: fun get(): MyPresenter = MyPresenter( ) } analyticsFactory.get(), repositoryFactory.get(), otherRepositoryFactory.get(), ( Analytics , interface Provider<T> { fun get(): T } ) { Provider<T> : override Provider< > Repository , Provider< > OtherRepository , Provider< >
52. Factory
53. Object Lifetime
54. private val analytics: private val otherRepository: private val repository: ) Analytics Repository OtherRepository , , , class class MyPresenterFactory private val analyticsFactory: private val repositoryFactory: RepositoryFactory, private val otherRepositoryFactory: OtherRepositoryFactory, ) fun get(): MyPresenter = MyPresenter( ) } analyticsFactory.get(), repositoryFactory.get(), otherRepositoryFactory.get(), ( MyPresenter { ( AnalyticsFactory,
55. private val analytics: private val otherRepository: private val repository: ) Analytics Repository OtherRepository , , , class class MyPresenterFactory private val analyticsFactory: private val repositoryFactory: RepositoryFactory, private val otherRepositoryFactory: OtherRepositoryFactory, ) fun get(): MyPresenter = MyPresenter( ) } analyticsFactory.get(), repositoryFactory.get(), otherRepositoryFactory.get(), ( MyPresenter { ( AnalyticsFactory,
56. class fun get(): Analytics = Analytics() } { AnalyticsFactory
57. class fun get(): Analytics = Analytics() } { AnalyticsFactory class Analytics { ... } val sessionId: String = randomString()
58. class fun get(): Analytics = Analytics() } { AnalyticsFactory class Analytics { ... } val sessionId: String = randomString()
59. class fun get(): Analytics = Analytics() } { AnalyticsFactory class Analytics { val sessionId: String ... } = randomString()
60. class fun get(): Analytics = Analytics() } { AnalyticsFactory class Analytics { ... } val sessionId: String = randomString()
61. class fun get(): Analytics = Analytics() } { AnalyticsFactory class Analytics { ... } val sessionId: String = randomString() val f = AnalyticsFactory()
62. class fun get(): Analytics = Analytics() } { AnalyticsFactory class Analytics { ... } val sessionId: String = randomString() val f = AnalyticsFactory() f.get().sessionId
63. class fun get(): Analytics = Analytics() } { AnalyticsFactory class Analytics { ... } val sessionId: String = randomString() val f = AnalyticsFactory() f.get().sessionId = 49bf5857-d468-4efd-a3cd-9e16044b6aaa
64. class fun get(): Analytics = Analytics() } { AnalyticsFactory class Analytics { ... } val sessionId: String = randomString() val f = AnalyticsFactory() f.get().sessionId f.get().sessionId = 49bf5857-d468-4efd-a3cd-9e16044b6aaa
65. class fun get(): Analytics = Analytics() } { AnalyticsFactory class Analytics { ... } val sessionId: String = randomString() val f = AnalyticsFactory() f.get().sessionId f.get().sessionId = 1f49aee6-2bf4-499e-bc5f-7a83d03453b4 = 49bf5857-d468-4efd-a3cd-9e16044b6aaa
66. class fun get(): Analytics = Analytics() } { AnalyticsFactory class Analytics { ... } val sessionId: String = randomString()
67. class fun get(): Analytics } { AnalyticsFactory class Analytics { ... } val sessionId: String = randomString() { }
68. class fun get(): Analytics } { AnalyticsFactory class Analytics { ... } val sessionId: String = randomString() { } private var _instance
69. class fun get(): Analytics } { AnalyticsFactory class Analytics { ... } val sessionId: String = randomString() { } private var _instance: Any
70. class fun get(): Analytics } { AnalyticsFactory class Analytics { ... } val sessionId: String = randomString() { } private var _instance: Any private val none = Any()
71. class fun get(): Analytics } { AnalyticsFactory class Analytics { ... } val sessionId: String = randomString() { } private var _instance: Any private val none = Any() = none
72. class fun get(): Analytics } { AnalyticsFactory class Analytics { ... } val sessionId: String = randomString() { } private var _instance: Any private val none = Any() = none if (_instance == none) { _instance = Analytics() }
73. class fun get(): Analytics } { AnalyticsFactory class Analytics { ... } val sessionId: String = randomString() { } private var _instance: Any private val none = Any() = none if (_instance == none) { _instance = Analytics() } return _instance as Analytics val f = AnalyticsFactory()
74. class fun get(): Analytics } { AnalyticsFactory { } private var _instance: Any private val none = Any() = none if (_instance == none) { _instance = Analytics() } return _instance as Analytics val f = AnalyticsFactory() f.get().sessionId f.get().sessionId = 49bf5857-d468-4efd-a3cd-9e16044b6aaa = 49bf5857-d468-4efd-a3cd-9e16044b6aaa
75. Object Lifetime
76. class fun get(): Analytics { AnalyticsFactory class Analytics { ... } val sessionId: String = randomString() { private var _instance: Any private val none = Any() = none if (_instance == none) { _instance = Analytics()
77. class fun get(): Analytics { AnalyticsFactory class Analytics { ... } val sessionId: String = randomString() { private var _instance: Any private val none = Any() = none if (_instance == none) { _instance = Analytics()
78. class Analytics { ... } val sessionId: String @Inject constructor( , )
79. Quali fi ers
80. class Analytics { ... } @Inject constructor( , ) val sessionId: String
81. class Analytics { ... } val sessionId: @Inject constructor( , ) String
82. Analytics Repository Other Repository API DB MyPresenter
83. Analytics Repository Other Repository API DB MyPresenter String
84. Analytics Repository Other Repository API DB MyPresenter String String
85. Analytics Repository Other Repository API DB MyPresenter String String
86. Analytics Repository Other Repository API DB MyPresenter String String Tag1 + Tag2 +
87. Analytics Repository Other Repository API DB MyPresenter String String Tag1 + Tag2 +
88. String String Tag1 + Tag2 + Analytics + Repository + Other Repository + ∅ ∅ ∅ API + ∅ DB + ∅ MyPresenter + ∅
89. class Analytics { ... } @Inject constructor( , ) val sessionId: String
90. class Analytics { ... } @Inject constructor( ) @SessionId , val sessionId: String
91. class Analytics { ... } @Inject constructor( ) @SessionId , val sessionId: String
92. data class DependencyId( val qualifier: Qualifier?, val klass: KClass<*>, ) class Analytics { ... } @Inject constructor( ) @SessionId , val sessionId: String
93. data class DependencyId( val qualifier: Qualifier?, val klass: KClass<*>, ) val dependencyGraph: Map<DependencyId, Provider<*>> class Analytics { ... } @Inject constructor( ) @SessionId , val sessionId: String
94. data class DependencyId( val qualifier: Qualifier?, val klass: KClass<*>, ) val dependencyGraph: Map<DependencyId, Provider<*>> mapOf( = DependencyId(SessionId::class, String::class) to SessionIdFactory, ) class Analytics { ... } @Inject constructor( ) @SessionId , val sessionId: String
95. class Analytics { ... } @Inject constructor( ) @SessionId , val sessionId: String mapOf( DependencyId(SessionId::class, String::class) to SessionIdFactory, ) DependencyId(null, Analytics::class) to AnalyticsFactory, val dependencyGraph: Map<DependencyId, Provider<*>> = data class DependencyId( val qualifier: Qualifier?, val klass: KClass<*>, )
96. Child Graphs
105. class ObjectGraph } { private val application: Application, ) ( private fun getAnalytics(): Analytics private fun getNetworkClient(): NetworkClient
106. class ObjectGraph } { private val application: Application, ) ( class UserApi @Inject constructor( @UserId userId: String, client: NetworkClient, analytics: Analytics, ) { // ... } private fun getAnalytics(): Analytics private fun getNetworkClient(): NetworkClient
107. class ObjectGraph } { private val application: Application, ) ( class UserApi @Inject constructor( @UserId userId: String, client: NetworkClient, analytics: Analytics, ) { // ... } private fun getAnalytics(): Analytics private fun getNetworkClient(): NetworkClient
108. class ObjectGraph } { private val application: Application, ) ( class UserApi @Inject constructor( @UserId userId: String, client: NetworkClient, analytics: Analytics, ) { // ... } private fun getAnalytics(): Analytics private fun getNetworkClient(): NetworkClient
109. class ObjectGraph } { private val application: Application, ) ( class UserApi @Inject constructor( @UserId userId: String, client: NetworkClient, analytics: Analytics, ) { // ... } private fun getAnalytics(): Analytics private fun getNetworkClient(): NetworkClient
110. class ObjectGraph } { private val application: Application, ) ( class UserApi @Inject constructor( @UserId userId: String, client: NetworkClient, analytics: Analytics, ) { // ... } private fun getAnalytics(): Analytics private fun getNetworkClient(): NetworkClient fun getUserApi(@UserId userId: String): UserApi = UserApi(userId, getNetworkClient(), getAnalytics())
111. class ObjectGraph } { private val application: Application, ) ( class UserApi @Inject constructor( @UserId userId: String, client: NetworkClient, analytics: Analytics, ) { // ... } private fun getAnalytics(): Analytics private fun getNetworkClient(): NetworkClient
114. class ObjectGraph } { private val application: Application, ) ( class UserApi @Inject constructor( @UserId userId: String, client: NetworkClient, analytics: Analytics, ) { // ... } private fun getAnalytics(): Analytics private fun getNetworkClient(): NetworkClient
115. class ObjectGraph } { private val application: Application, ) ( class UserApi @Inject constructor( @UserId userId: String, client: NetworkClient, analytics: Analytics, ) { // ... } private fun getAnalytics(): Analytics private fun getNetworkClient(): NetworkClient inner class ChildGraph
116. class ObjectGraph } { private val application: Application, ) ( class UserApi @Inject constructor( @UserId userId: String, client: NetworkClient, analytics: Analytics, ) { // ... } private fun getAnalytics(): Analytics private fun getNetworkClient(): NetworkClient inner class ChildGraph( @UserId private val userId: String )
117. class ObjectGraph } { private val application: Application, ) ( class UserApi @Inject constructor( @UserId userId: String, client: NetworkClient, analytics: Analytics, ) { private fun getAnalytics(): Analytics private fun getNetworkClient(): NetworkClient inner class ChildGraph( @UserId private val userId: String ) { } fun getUserApi(): UserApi = UserApi( ) getAnalytics() getNetworkClient(), , userId,
118. class ObjectGraph } { private val application: Application, ) ( class UserApi @Inject constructor( @UserId userId: String, client: NetworkClient, analytics: Analytics, ) { private fun getAnalytics(): Analytics private fun getNetworkClient(): NetworkClient inner class ChildGraph( @UserId private val userId: String ) { } fun getUserApi(): UserApi = UserApi( ) getAnalytics() getNetworkClient(), , userId,
119. class ObjectGraph } { private val application: Application, ) ( class UserApi @Inject constructor( @UserId userId: String, client: NetworkClient, analytics: Analytics, ) { private fun getAnalytics(): Analytics private fun getNetworkClient(): NetworkClient inner class ChildGraph( @UserId private val userId: String ) { } fun getUserApi(): UserApi = UserApi( ) getAnalytics() getNetworkClient(), , userId,
120. class ObjectGraph } { private val application: Application, ) ( class UserApi @Inject constructor( @UserId userId: String, private fun getAnalytics(): Analytics private fun getNetworkClient(): NetworkClient inner class ChildGraph( @UserId private val userId: String ) { } fun getUserApi(): UserApi = UserApi( ) getAnalytics() getNetworkClient(), , userId, @UserId userId: String fun createChildGraph( ): ChildGraph = ChildGraph(userId)
121. client: NetworkClient, analytics: Analytics, ) { // ... } val childGraph = objectGraph.createChildGraph(userId) childGraph.getUserApi()
122. client: NetworkClient, analytics: Analytics, ) { // ... } val childGraph = objectGraph.createChildGraph(userId) childGraph.getUserApi()
123. Child Graphs
124. Recap • Dependency injection is complicated • Key concepts • Object Graph • Factories • Object Lifetime • Quali fi ers • Child Graphs
125. Ryan Harter Dependency Injection ryan@harter.us