SubclassOptInRequired
Annotation that marks open for subclassing classes and interfaces, and makes implementation and extension of such declarations as requiring an explicit opt-in.
When applied, any attempt to subclass the target declaration will trigger an opt-in with the corresponding level and message.
The intended uses of subclass opt-in markers include, but are not limited to the following API:
Stable to use, but unstable to implement due to its further evolution.
Stable to use, but closed for 3rd-part implementations due to internal or technical reasons.
Stable to use, but delicate or fragile to implement.
Stable to use, but with a contract that may be weakened in the future in a backwards-incompatible manner for external implementations.
Contrary to regular RequiresOptIn, there are three ways to opt-in into the subclassing requirement:
Annotate declaration with the marker annotation, making it propagating.
Annotate declaration with OptIn in order to opt in into the provided guarantees in a non-propagating manner.
Annotate declaration with SubclassOptInRequired with the same marker class, making it further propagating only for subclassing.
Uses of this annotation are limited to open and abstract classes, and non-fun
interfaces. Any other uses allowed by CLASS
annotation target yield a compilation error.
Since Kotlin
2.1See also
for a detailed description of opt-in semantics and propagation rules.