Can you explain what the AssistedInject mechanism in Dagger is and what it is used for?

AssistedInject in Dagger is a mechanism that allows combining automatic dependency injection with manually passing some parameters during object creation.

Typically, Dagger creates objects by fully managing their dependencies. But sometimes, some parameters need to be passed dynamically, for example, data known only at runtime.

AssistedInject solves this problem by:

• Marking the constructor with @AssistedInject.
• Annotating parameters that will be passed manually with @Assisted.
• Creating a factory that accepts these parameters and returns an object with dependencies injected.

Example:

class UserViewModel @AssistedInject constructor(
    private val repository: UserRepository, // injected by Dagger
    @Assisted private val userId: String  // passed manually
) {
    // ...

    @AssistedFactory
    interface Factory {
        fun create(userId: String): UserViewModel
    }
}

Then, the factory can be used to create instances with different userId, while other dependencies are automatically injected by Dagger.