ETUnionViewpoint class documentationETUnionViewpoint : ETCollectionViewpoint
ETCollectionViewpoint turns a collection property belonging to a represented object into a immutable or mutable collection proxy that implements the collection protocols.
Using it as an immutable collection proxy doesn't bring a lot of benefits. However ETCollectionViewpoint can use Key-Value-Coding accessors to support mutating the collection without implementing ETCollection and ETCollectionMutation on the represented object (for the property that holds the collection).
A model object usually implements collection protocols providing access to its dominant/main collection aspect, but the represent object doesn't support these protocols to access other collections exposed as properties. ETCollectionViewpoint exposes these other collections as the main collection is usually exposed.