What is multi-binding in Dagger?

Multibinding in Dagger allows declaring a set of objects (for example, Set or Map) from different modules, which will be assembled into a collection during injection. This is useful when you have multiple implementations of the same interface or type that need to be handled uniformly.

There are two main types of multibinding:

1. Set Multibinding: Creates a Set from the provided objects.

   • Annotation @IntoSet.

   @Module
   abstract class MyModule {
       @Binds @IntoSet
       abstract MyInterface provideImpl1(Impl1 impl);
   
       @Binds @IntoSet
       abstract MyInterface provideImpl2(Impl2 impl);
   }
   
   // At injection point
   @Inject
   Set<MyInterface> myImplementations;
   

2. Map Multibinding: Creates a Map with keys and values.

   • Annotation @IntoMap.
   • You need to provide an annotation for the key (for example, @StringKey, @IntKey, @EnumKey).

   @Module
   class MyModule {
       @Provides @IntoMap
       @StringKey("key1")
       MyValueType provideValue1() {
           return new MyValueType("value1");
       }
   
       @Provides @IntoMap
       @StringKey("key2")
       MyValueType provideValue2() {
           return new MyValueType("value2");
       }
   }
   
   // At injection point
   @Inject
   Map<String, MyValueType> myMap;
   

Multibindings simplify adding new elements to a collection without changing the code where this collection is used.