Bridging Existentials & Generics in Swift 2
In an earlier blog post I pointed out some incompatibilities between type information that is statically known at compile time (Generics) and type information that is dynamically available at runtime (Existentials).
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Given a heterogenous list of different instances that can be persisted we want to automatically find & call the DAO based on the type of object we encounter.
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The
.Selfmember, which would refer to the concrete type of the existential doesn’t exist in Swift 2. However, we can access the concrete type of the existential usingSelffrom within protocols & protocol extensions.Using a clever inversion of control we can use that
Selftype from within thePersistedTypeprotocol (which all persisted types implement) to dynamically specify the generic type parameter of ourGenericDAO<T>:
