Dagger

What is Dagger?

Dagger is a fully static and compile-time dependency injection framework. Compile-time means that issues in the dependency graph (such as cycles or missing providers) are caught during build-time.

Components & Subcomponents

Dagger creates the dependency graph using components and subcomponents

• Components are top-level containers of providers that are pulled from modules that component is configured to include
• Subcomponents are also containers, and may contain other subcomponents
• Subcomponents automatically inherit all the dependencies from their parent components
• Components/subcomponents can automatically collect dependencies for which they are scoped

Scopes

Scopes are compile-time annotations associated both with a component/subcomponent and either injectable objects or providers of objects

Injectables & Providers

Dagger supports two types of injections: field and constructors

• All objects that are injected need to have an @Inject-declared constructor
• Any parameters passed into an @Inject-declared constructor will be retrieved from the dependency graph
• Fields can be marked as @Inject-able, but a separate inject() method in a component needs to be added for that class to initialize those fields, and the class must call this method

Note: Classes can have their providers inferred just by being qualified and having an @Inject-able constructor--no need for a Dagger module

Modules

Dagger modules are defined in separate classes annotated with the @Module tag

• Modules can provide an implementation with @Provides
• Modules can bind one type to another type using @Binds

Its Usage in Oppia Android

• Dagger object lifetimes need to be compatible with Android object lifecycles
• Prefer constructor injection over field injection to encourage encapsulation
• Result are activity/fragment/view presenter classes that are field-injected into their corresponding Android objects, but themselves support constructor injection

There's an Android-specific dependency hierarchy: Root application component for @Singleton that's initialized in a custom Application class Per-activity and per-fragment subcomponents that are initialized in base activity and fragment classes, respectively Inheritance such that all application-level classes can be injected in activities, and all activity-level classes can be injected into fragments (but not vice versa for either) Activity & fragment controllers for each activity and fragment that are @Inject-constructed, and can inject all needed dependencies and perform necessary UI logic Activity and fragment classes become boilerplate classes that extend base classes that enable DI for them, and delegate UI callbacks to their controller classes for processing View models are also injectable for use in controllers per a ProAndroidDev article Similar to Dagger Android, but with better encapsulation

You can understand it with this example :

This is a Singleton-scoped object with dependency. Note that because Factory is @Singleton scoped, it can inject everything in the Singleton component including blocking dispatcher.

@Singleton
class Factory @Inject constructor(@BlockingDispatcher private val blockingDispatcher: CoroutineDispatcher) {
  fun <T: Any> create(): InMemoryBlockingCache<T> {
    return InMemoryBlockingCache(blockingDispatcher)
  }
}

These are Singleton-scoped providers with custom qualifiers. Note also that to distinguish between two of the same types, we can use custom qualifier annotations like @BackgroundDispatcher and @BlockingDispatcher.

@Module
class DispatcherModule {
 @Provides
 @BackgroundDispatcher
 @Singleton
 fun provideBackgroundDispatcher(): CoroutineDispatcher {
   return Executors.newFixedThreadPool(4).asCoroutineDispatcher()
 }

 @Provides
 @BlockingDispatcher
 @Singleton
 fun provideBlockingDispatcher(): CoroutineDispatcher {
   return Executors.newSingleThreadExecutor().asCoroutineDispatcher()
 }
}

How to write Tests with Dagger

• Dependencies can be replaced at test time
• This is especially useful for API endpoints! We can replace Retrofit instances with mocks that let us carefully control request/response pairs
• This is also useful for threading! We can synchronize coroutines and ensure they complete before continuing test operations
• Tests can declare their own scoped modules in-file
• Tests themselves create a test application component and inject dependencies directly into @Inject-able fields
• Bazel (#59) will make this even easier since test modules could then be shareable across tests

Here is an example of testing with Oppia Dagger. This shows setting up a test component and using it to inject dependencies for testing purposes. It also shows how to create a test-specific dependency that can be injected into a test for manipulation.

class InMemoryBlockingCacheTest {
 @ExperimentalCoroutinesApi @Inject @field:TestDispatcher lateinit var testDispatcher: TestCoroutineDispatcher
 @ExperimentalCoroutinesApi private val backgroundTestCoroutineScope by lazy { CoroutineScope(backgroundTestCoroutineDispatcher) }
 @ExperimentalCoroutinesApi private val backgroundTestCoroutineDispatcher by lazy { TestCoroutineDispatcher() }

 @Before @ExperimentalCoroutinesApi fun setUp() { setUpTestApplicationComponent() }

 @Test @ExperimentalCoroutinesApi fun `test with testDispatcher since it's connected to the blocking dispatcher`() = runBlockingTest(testDispatcher) { /* ... */ }

 private fun setUpTestApplicationComponent() {
   DaggerInMemoryBlockingCacheTest_TestApplicationComponent.builder().setApplication(ApplicationProvider.getApplicationContext()).build().inject(this)
 }

 @Qualifier annotation class TestDispatcher
 @Module
 class TestModule {
   @ExperimentalCoroutinesApi @Singleton @Provides @TestDispatcher fun provideTestDispatcher(): TestCoroutineDispatcher { return TestCoroutineDispatcher() }
   @ExperimentalCoroutinesApi @Singleton @Provides @BlockingDispatcher
   fun provideBlockingDispatcher(@TestDispatcher testDispatcher: TestCoroutineDispatcher): CoroutineDispatcher { return testDispatcher }
 }

 @Singleton
 @Component(modules = [TestModule::class])
 interface TestApplicationComponent {
   @Component.Builder interface Builder { @BindsInstance fun setApplication(application: Application): Builder fun build(): TestApplicationComponent }
   fun inject(inMemoryBlockingCacheTest: InMemoryBlockingCacheTest)
 }
}

Points to Note

Dagger compile-time errors can be hard to understand

• When you encounter one: scan the error for the dependency name (it's likely a dependency you just imported into the file failing to compile)
• Search for the Dagger module you want to use to provide that dependency
• Make sure your Gradle module or Bazel build file depends on the library that contains the module you need
• Note that Gradle modules cannot depend on the app module, which means any Dagger modules in the app Gradle module are inaccessible outside of the app module