Hilt Android — Step by Step : Part2

On first story, we started with very basic implementation of Hilt, check here.

In this article, we will move forward one step: provide interface and how to inject them. Let’s create an interface with a basic implementation.

Providing Interfaces:

interface LoggerCallback {
    fun log()
}
//Implementation
class LoggerImpl1 @Inject constructor() : LoggerCallback {

    override fun log() {
        Log.d("abc", "LoggerImpl1")
    }
}

• There is a problem, how can we provide an interface? Here we will use @Binds that provided specifically for interfaces.

@Provides
fun provideLoggerImpl1(): LoggerCallback ?? 

• Lets add @Binds in our module:

@InstallIn(ActivityComponent::class)
@Module
abstract class LoggerModule {

    @Provides
    fun provideLogger3(@ActivityContext context: Context): Logger3 = Logger3(context)

    @Binds
    abstract fun provideLoggerImpl1(): LoggerCallback
}

error: A @Module may not contain both non-static and abstract binding methods

• We cannot use @Binds and @Provides inside same module. Create another one for @Binds. Both module and bind method should be abstract.

@InstallIn(ActivityComponent::class)
@Module
abstract class LoggerBindsModule {

    @Binds
    abstract fun bindLoggerImpl1(loggerImpl1: LoggerImpl1): LoggerCallback

}

• Inject in Activity and done!

@Inject
lateinit var loggerCallback: LoggerCallback

Different Implementation for Interfaces:

• Now, we will create second implementation for LoggerCallback and pass activity as parameter. Then @Binds inside our binding module.

class LoggerImpl2 @Inject constructor(private val activity: FragmentActivity) : LoggerCallback {

    override fun log() {
        Log.d("abc", "LoggerImpl2 : ${activity.localClassName}")
    }
}
//LoggerBindsModule
@Binds
abstract fun bindLoggerImpl2(locationChecker: LoggerImpl2): LoggerCallback

• Run and try:

error: [Dagger/DuplicateBindings] com.example.vacation.hilt.LoggerCallback is bound multiple times:
public abstract static class ApplicationC implements VacationApplication_GeneratedInjector,
^
@org.jetbrains.annotations.NotNull @Provides com.example.vacation.hilt.LoggerCallback com.example.vacation.hilt.LoggerModule.provideLoggerImpl1()
@org.jetbrains.annotations.NotNull @Provides com.example.vacation.hilt.LoggerCallback com.example.vacation.hilt.LoggerModule.provideLoggerImpl2()

• Ups, we provide same instance two times and they need to be identified. Here we will use Qualifiers:

@Qualifier
@Retention(AnnotationRetention.BINARY)
annotation class First

@Qualifier
@Retention(AnnotationRetention.BINARY)
annotation class Second

• Now we need use them for our binds

@Binds
@First
abstract fun bindLoggerImpl1(loggerImpl1: LoggerImpl1): LoggerCallback

@Binds
@Second
abstract fun bindLoggerImpl2(locationChecker: LoggerImpl2): LoggerCallback

• Run..

error: [Dagger/MissingBinding] com.example.vacation.hilt.LoggerCallback cannot be provided without an @Provides-annotated method.

• We forgot to tell which logger we want inside activity:

@Inject
@First
lateinit var loggerCallback: LoggerCallback

• Done.

Conclusion:

• We learned how to provide interfaces with @Binds annotation and how to differentiate them with @Qualifier.
• We will continue with different lifecycle scopes on next article.