Demystifying Dagger

Dagger isn't magic, it simply generates boiler-plate code, similar to how we'd write it manually if we had to. In this talk, I discuss dependency injection, and we look at some code that Dagger generates to see how it works under the hood. This helps illustrate how Dagger works to break down its complex veneer and make it more understandable.

Transcript

1. 🗡 Demystifying Dagger Ryan Harter Mobile Summit 2023
2. The Grid Graph
3. Backend Property CrashReporter 🗡
4. Backend Property CrashReporter 🗡
5. Property CrashReporter API Key HttpClient Backend 🗡
6. Property CrashReporter API Key HttpClient Backend 🗡
7. Property CrashReporter API Key HttpClient Backend The Graph 🗡
8. Dependency Injection 🗡
9. Dependency Injection 🗡 class CrashReporter Property("awesome") Backend(token = "abc123") private val userProp: Property private val backend: Backend = = { / / . .. }
10. Dependency Injection 🗡 class CrashReporter private val userProp: Property private val backend: Backend = = { / / . .. } MyDependencies.property MyDependencies.backend
11. Dependency Injection 🗡 class CrashReporter private val userProp: Property private val backend: Backend = = { / / . .. } get() get()
12. Dependency Injection 🗡 class CrashReporter private val userProp: Property private val backend: Backend / / . .. } ( , , { )
13. javax.inject 🗡
14. javax.inject 🗡 Provide
15. javax.inject 🗡 Provider<T>
16. javax.inject 🗡 interface Provider<T> { fun get(): T }
17. Injection • Constructor Injection • Member Injection • Provider Injection 🗡
18. Constructor Injection 🗡
19. class CrashReporter @Inject constructor( private val userProp: Property, private val backend: Backend, ) { // . .. } @Inject constructor userProp: Property backend: Backend Constructor Injection 🗡
20. class CrashReporter @Inject constructor(
21. class CrashReporter @Inject constructor(
22. class CrashReporter @Inject constructor( private val userProp: Property, private val backend: Backend, ) { // . .. } @Inject constructor userProp: Property backend: Backend Constructor Injection 🗡
23. class CrashReporter_Factory( private val userPropProvider: Provider<Property>, private val backendProvider: Provider<Backend>, ) : Provider<CrashReporter> { override fun get(): CrashReporter { return CrashReporter( userPropProvider.get(), backendProvider.get(), ) } } CrashReporter_Factory userPropProvider: Provider<Property> backendProvider: Provider<Backend> Provider<CrashReporter> override fun get(): CrashReporter CrashReporter( userPropProvider.get(), backendProvider.get(), ) Constructor Injection 🗡
24. class CrashReporter_Factory(
25. class CrashReporter_Factory(
26. class CrashReporter_Factory(
27. class CrashReporter_Factory(
28. class CrashReporter_Factory( userPropProvider.get(), backendProvider.get(), ) Constructor Injection 🗡
29. class CrashReporter_Factory( private val userPropProvider: Provider<Property>, private val backendProvider: Provider<Backend>, ) : Provider<CrashReporter> { override fun get(): CrashReporter { return CrashReporter( userPropProvider.get(), backendProvider.get(), ) } } CrashReporter_Factory userPropProvider: Provider<Property> backendProvider: Provider<Backend> Provider<CrashReporter> override fun get(): CrashReporter CrashReporter( userPropProvider.get(), backendProvider.get(), ) Constructor Injection 🗡
30. Member Injection 🗡
31. interface MembersInjector<T> { fun injectMembers(instance: T) } Member Injection 🗡
32. class MyFragment : Fragment() { @Inject lateinit var crashReporter: CrashReporter } Member Injection 🗡
33. class MyFragment_MembersInjector( private val crashReporterProvider: Provider<CrashReporter>, ) : MembersInjector<MyFragment> { override fun injectMembers(instance: MyFragment) { instance.crashReporter = crashReporterProvider.get() } } MyFragment_MembersInjector MembersInjector<MyFragment> private val crashReporterProvider: Provider<CrashReporter> override fun injectMembers(instance: MyFragment) instance.crashReporter = crashReporterProvider.get() Member Injection 🗡
34. class MyFragment_MembersInjector(
35. class MyFragment_MembersInjector(
36. class MyFragment_MembersInjector(
37. class MyFragment_MembersInjector(
38. class MyFragment_MembersInjector(
39. class MyFragment_MembersInjector(
40. class MyFragment_MembersInjector( private val crashReporterProvider: Provider<CrashReporter>, ) : MembersInjector<MyFragment> { override fun injectMembers(instance: MyFragment) { instance.crashReporter = crashReporterProvider.get() } } MyFragment_MembersInjector MembersInjector<MyFragment> private val crashReporterProvider: Provider<CrashReporter> override fun injectMembers(instance: MyFragment) instance.crashReporter = crashReporterProvider.get() Member Injection 🗡
41. Provider Injection 🗡
42. @Module class CrashModule { @Provides fun provideBackend(token: ApiToken): Backend { return Backend(token) } } @Module class CrashModule { @Provides } fun provideBackend(token: ApiToken) Backend return Backend(token) Provider Injection 🗡
43. @Module
44. @Module
45. @Module
46. @Module
47. @Module
48. @Module
49. @Module
50. @Module class CrashModule { @Provides } fun provideBackend(token: ApiToken) Backend return Backend(token) : { } Provider Injection 🗡
51. @Module @Provides } fun provideBackend(token: ApiToken) Backend return Backend(token) : { } class CrashModule { abstract Provider Injection 🗡
52. @Module @Provides } fun provideBackend(token: ApiToken) Backend return Backend(token) : { } class CrashModule { abstract Provider Injection 🗡
53. @Module @Provides } fun provideBackend(token: ApiToken) Backend return Backend(token) : { } class CrashModule { abstract @Binds Provider Injection 🗡
54. @Module @Provides } fun provideBackend(token: ApiToken) Backend return Backend(token) : { } class CrashModule { abstract @Binds abstract fun bindService() Provider Injection 🗡
55. @Module @Provides } fun provideBackend(token: ApiToken) Backend return Backend(token) : { } class CrashModule { abstract @Binds abstract fun bindService(backend: Backend) Provider Injection 🗡
56. @Module @Provides } fun provideBackend(token: ApiToken) Backend return Backend(token) : { } class CrashModule { abstract @Binds abstract fun bindService(backend: Backend): Service Provider Injection 🗡
57. { { CrashModule_ProvideBackendFactory ) } } class ( private val module: CrashModule, , private val tokenProvider: Provider<ApiToken> Provider<Backend?> : override fun get(): Backend return module.provideBackend( ) tokenProvider.get() Provider Injection 🗡
58. { { CrashModule_ProvideBackendFactory ) } } class ( private val module: CrashModule, , private val tokenProvider: Provider<ApiToken> Provider<Backend?> : override fun get(): Backend return module.provideBackend( ) tokenProvider.get() Provider Injection 🗡
59. { { CrashModule_ProvideBackendFactory ) } } class ( private val module: CrashModule, , private val tokenProvider: Provider<ApiToken> Provider<Backend?> : override fun get(): Backend return module.provideBackend( ) tokenProvider.get() Provider Injection 🗡
60. { { CrashModule_ProvideBackendFactory ) } } class ( private val module: CrashModule, , private val tokenProvider: Provider<ApiToken> Provider<Backend?> : override fun get(): Backend return module.provideBackend( ) tokenProvider.get() Provider Injection 🗡
61. { { CrashModule_ProvideBackendFactory ) } } class ( private val module: CrashModule, , private val tokenProvider: Provider<ApiToken> Provider<Backend?> : override fun get(): Backend return module.provideBackend( ) tokenProvider.get() Provider Injection 🗡
62. { { CrashModule_ProvideBackendFactory ) } } class ( private val module: CrashModule, , private val tokenProvider: Provider<ApiToken> Provider<Backend?> : override fun get(): Backend return module.provideBackend( ) tokenProvider.get() Provider Injection 🗡
63. { { CrashModule_ProvideBackendFactory ) } } class ( private val module: CrashModule, , private val tokenProvider: Provider<ApiToken> Provider<Backend?> : override fun get(): Backend return module.provideBackend( ) tokenProvider.get() Provider Injection 🗡
64. { { CrashModule_ProvideBackendFactory ) } } class ( private val module: CrashModule, , private val tokenProvider: Provider<ApiToken> Provider<Backend?> : override fun get(): Backend return module.provideBackend( ) tokenProvider.get() Provider Injection 🗡
65. { { CrashModule_ProvideBackendFactory ) } } class ( private val module: CrashModule, , private val tokenProvider: Provider<ApiToken> Provider<Backend?> : override fun get(): Backend return module.provideBackend( ) tokenProvider.get() Provider Injection 🗡
66. Qualifiers 🗡
67. class CrashReporter @Inject constructor( private val userProp: Property, private val backend: Backend, ) Property Backend Qualifiers 🗡
68. class CrashReporter @Inject constructor(
69. class CrashReporter @Inject constructor( @Inject lateinit var crashReporter: CrashReporter } Property Backend CrashReporter Qualifiers 🗡
70. class CrashReporter @Inject constructor(
71. class CrashModule { @Provides fun provideBackend(token: ApiToken): Backend { return Backend(token) } } class MyFragment : Fragment() {
73. @Module class CrashModule { } @Provides return Backend(token) } ): Backend { ApiToken String Qualifiers 🗡 fun provideBackend(token:
74. @Module class CrashModule { } @Provides return Backend(token) } ): Backend { ApiToken String Qualifiers 🗡 fun provideBackend(token:
75. @Module
76. @Module
77. @Module
78. @Module
79. @Module
80. @Module class CrashModule { } @Provides return Backend(token) } ): Backend { String @Provides fun provideApiToken(): String = "abcd" @ApiToken fun provideBackend( token: @ApiToken Qualifiers 🗡
81. Property CrashReporter API Key HttpClient Backend The Graph
82. Component
83. @Component(modules = [CrashModule :: class]) interface AppComponent class DaggerAppComponent {
84. class DaggerAppComponent { DaggerAppComponent companion object { fun builder(): Builder { // .. . } } class Builder { // ... fun build(): AppComponent { // .. . } } private class AppComponentImpl : AppComponent { // .
85. class DaggerAppComponent { DaggerAppComponent companion object { .
86. class DaggerAppComponent { DaggerAppComponent companion object { fun builder(): Builder { // .. . } } class Builder { // ... fun build(): AppComponent { // .. . } } private class AppComponentImpl : AppComponent { .
87. class DaggerAppComponent { DaggerAppComponent companion object { / / . ..
88. class DaggerAppComponent { DaggerAppComponent private class AppComponentImpl : AppComponent { .
89. class DaggerAppComponent { DaggerAppComponent private class AppComponentImpl : AppComponent { // Providers private val userPropProvider: Provider<Property> private val backendProvider: Provider<Backend> private val tokenProvider: Provider<ApiToken> private val crashReporterProvider: Provider<CrashReporter> // MembersInjectors private val myFragmentMembersInjector: MembersInjector<MyFragment> } } .
90. @Component(modules = [CrashModule :: class]) interface AppComponent { } fun inject(instance: MyFragment)
91. @Component(modules = [CrashModule
92. class DaggerAppComponent { DaggerAppComponent private class AppComponentImpl : AppComponent { // Providers private val userPropProvider: Provider<Property> private val backendProvider: Provider<Backend> private val tokenProvider: Provider<ApiToken> private val crashReporterProvider: Provider<CrashReporter> // MembersInjectors private val myFragmentMembersInjector: MembersInjector<MyFragment> } } .
93. class DaggerAppComponent { DaggerAppComponent private class AppComponentImpl : AppComponent { myFragmentMembersInjector.injectMembers(instance) }
94. @Component(modules = [CrashModule :: class]) interface AppComponent { } fun inject(instance: MyFragment)
95. @Component(modules = [CrashModule
96. class DaggerAppComponent { DaggerAppComponent private class AppComponentImpl : AppComponent { // Providers private val userPropProvider: Provider<Property> private val backendProvider: Provider<Backend> private val tokenProvider: Provider<ApiToken> private val crashReporterProvider: Provider<CrashReporter> // MembersInjectors private val myFragmentMembersInjector: MembersInjector<MyFragment> } } myFragmentMembersInjector.injectMembers(instance) }
97. private class AppComponentImpl : AppComponent { get() = crashReporterProvider.get()
98. @Component(modules = [CrashModule :: class]) interface AppComponent { } fun inject(instance: MyFragment) val crashReporter: CrashReporter
99. @Component(modules = [CrashModule
100. @Component(modules = [CrashModule
101. @Component(modules = [CrashModule
102. class ScopedProvider<T>( private val implProvider: Provider<T> ) : Provider<T> { private var instance: T? = null override fun get(): T { if (instance = = null) { instance = implProvider.get() } return instance } }
103. private class AppComponentImpl : AppComponent { // Providers private val userPropProvider: Provider<Property> private val backendProvider: Provider<Backend> private val tokenProvider: Provider<ApiToken> private val crashReporterProvider: Provider<CrashReporter> // MembersInjectors private val myFragmentMembersInjector: MembersInjector<MyFragment> } } myFragmentMembersInjector.injectMembers(instance) } override val crashReporter: CrashReporter get() = crashReporterProvider.get()
104. private class AppComponentImpl : AppComponent { // . .. , crashReporterProvider: Provider<CrashReporter>, ) { // . .. this.crashReporterProvider = } crashReporterProvider
105. private class AppComponentImpl : AppComponent { // . .. , crashReporterProvider: Provider<CrashReporter>, ) { // . .. this.crashReporterProvider = } ScopedProvider(crashReporterProvider) crashReporterProvider
106. AppComponent
107. AppComponent @UserId +
108. AppComponent @UserId + = UserComponent
109. AppComponent + = UserComponent @UserId
110. AppComponent + = UserComponent @UserId Subcomponent
111. AppComponent + = UserComponent @UserId Subcomponent UserSubcomponent
112. ⚒ Anvil
113. { } fun inject(instance: MyFragment) { @Component(modules = [CrashModule :: class]) interface AppComponent ⚒
114. { } fun inject(instance: MyFragment) fun inject(instance: ListActivity) fun inject(instance: ItemListFragment) fun inject(instance: ItemDetailFragment) fun inject(instance: ShareFragment) fun inject(instance: AccountActivity) fun inject(instance: FriendsFragment) fun inject(instance: SettingsActivity) @Component(modules = [CrashModule :: class]) interface AppComponent ⚒
115. { } fun inject(instance: MyFragment) fun inject(instance: ListActivity) fun inject(instance: ItemListFragment) fun inject(instance: ItemDetailFragment) fun inject(instance: ShareFragment) fun inject(instance: AccountActivity) fun inject(instance: FriendsFragment) fun inject(instance: SettingsActivity) @Component(modules = [ CrashModule :: class ]) interface AppComponent , AnalyticsModule :: class, NetworkModule : : class BillingModule : : class DatabaseModule :: class FileModule :: class UserModule :: class , , , , , ⚒
116. @Component(modules = [ CrashModule :: class ]) interface AppComponent , AnalyticsModule :: class, NetworkModule : : class BillingModule : : class DatabaseModule :: class FileModule :: class UserModule :: class , , , , , ⚒ : MyFragmentInjector, ListActivityInjector, ItemListFragmentInjector, ItemDetailFragmentInjector, ShareFragmentInjector, AccountActivityInjector, FriendsFragmentInjector, SettingsActivityInjector,
117. ⚒ abstract class AppScope
118. ⚒ abstract class AppObjectGraph
119. ⚒ abstract class AppObjectGraph interface MyFragmentInjector { fun inject(instance: MyFragment) }
120. ⚒ abstract class AppObjectGraph interface MyFragmentInjector { :: class)
121. ⚒ abstract class AppObjectGraph interface MyFragmentInjector { :
122. ⚒ abstract class AppObjectGraph interface MyFragmentInjector { :: class)
123. @Component(modules = [ CrashModule :: class ]) interface AppComponent , AnalyticsModule :: class, NetworkModule : : class BillingModule : : class DatabaseModule :: class FileModule :: class UserModule :: class , , , , , ⚒ : MyFragmentInjector, ListActivityInjector, ItemListFragmentInjector, ItemDetailFragmentInjector, ShareFragmentInjector, AccountActivityInjector, FriendsFragmentInjector, SettingsActivityInjector, @MergeComponent(scope = AppObjectGraph :
124. interface AppComponent ⚒ @MergeComponent(scope = AppObjectGraph :: class)
125. ⚒ class RealCrashReporter @Inject constructor( private val userProp: Property, private val backend: Backend, ): CrashReporter { // . .. }
126. ⚒ @Module interface CrashModule { @Binds } fun bindCrashReporter(impl: RealCrashReporter): CrashReporter @ContributesTo(AppObjectGraph :: class) class RealCrashReporter @Inject constructor( private val userProp: Property, private val backend: Backend, ): CrashReporter { // . .. }
127. ⚒ class RealCrashReporter @Inject constructor( :: class)
128. ⚒ Anvil
129. ⚒ Anvil Dagger 🗡 +
130. 🗡 Demystifying Dagger Ryan Harter Mobile Summit 2023