How to perform interface injection in Dagger?

To perform interface injection in Dagger, you need to provide a concrete implementation for it using @Binds or @Provides.

• @Binds: Preferred method if the implementation is a class that Dagger can directly instantiate (e.g., with an @Inject constructor).

  interface MyInterface {  
      fun doSomething()  
  }  
  
  class MyInterfaceImpl @Inject constructor() : MyInterface {  
      override fun doSomething() {  
          // ... implementation  
      } }  
  
  @Module  
  interface MyModule {  
      @Binds  
      fun bindMyInterface(impl: MyInterfaceImpl): MyInterface  
  }  
  

• @Provides: Used when the implementation requires more complex creation logic, such as using a third-party library or configuration.

  interface AnotherInterface {  
      fun getData(): String  
  }  
  
  class AnotherInterfaceImpl : AnotherInterface {  
      private val someValue: String  
  
      constructor(value: String) {  
          someValue = value  
      }  
  
      override fun getData(): String {  
          return someValue  
      } }  
  
  @Module  
  object AnotherModule {  
      @Provides  
      fun provideAnotherInterface(): AnotherInterface {  
          // Complex creation logic  
          return AnotherInterfaceImpl("some data")  
      } }  
  

Then, the Dagger component should include these modules:

@Component(modules = [MyModule::class, AnotherModule::class])  
interface AppComponent {  
    fun inject(activity: MyActivity)  
    // ...  
}  

Now, in the class where the interface needs to be injected, simply declare a field with the @Inject annotation:

class MyActivity : AppCompatActivity() {  

    @Inject  
    lateinit var myInterface: MyInterface  

    @Inject  
    lateinit var anotherInterface: AnotherInterface  

    override fun onCreate(savedInstanceState: Bundle?) {  
        super.onCreate(savedInstanceState)  
        (application as MyApplication).appComponent.inject(this)  

        myInterface.doSomething()  
        val data = anotherInterface.getData()  
        // ...  
    }  
}