AssistedFactory

Annotates an abstract class or interface used to create an instance of a type via an assisted Inject constructor.

An AssistedFactory-annotated type must obey the following constraints:

• The type must be an abstract class or interface,

• The type must contain exactly one abstract, non-default method whose return type must exactly match the type of an assisted injection type, and parameters must match the exact list of Assisted parameters in the assisted injection type's constructor (and in the same order).

Automatic Assisted Factory Generation

Metro supports automatic generation of assisted factories via opt-in compiler option. If enabled, Metro will automatically generate a default factory as a nested class within the injected type.

@Inject
class HttpClient(
  @Assisted timeoutDuration: Duration,
  cache: Cache,
) {
  // Generated by Metro
  @AssistedFactory
  fun interface Factory {
    fun create(timeoutDuration: Duration): HttpClient
  }
}

If a nested class called Factory is already present, Metro will do nothing.

Why opt-in?

The main reason this is behind an opt-in option at the moment is because compiler plugin IDE support is rudimentary at best and currently requires enabling a custom registry flag.

Because of this, it's likely better for now to just hand-write the equivalent class that Metro generates. If you still wish to proceed with using this, it can be enabled via the Gradle DSL.

metro {
  generateAssistedFactories.set(true)
}

See also