Power of Default Associated Type

PowerOfDefaultAssociatedType.swift

// Simplify code of http://martiancraft.com/blog/2015/07/objective-c-swift-core-data/ by using default associated type to eliminate the need of defining associated type in every subclass.

class ManagedObject {}

class Person: ManagedObject {}

class Thing: ManagedObject {}

let db: [ManagedObject] = [Person(), Thing()]



protocol Fetchable {
    typealias FetchableType: ManagedObject = Self
    
    static func objectInDB() -> FetchableType?
}

extension Fetchable where Self: ManagedObject {
    static func objectInDB() -> FetchableType?
    {
        return db.first as? FetchableType
    }
}

extension Person: Fetchable {}

extension Thing: Fetchable {}



let person: Person? = Person.objectInDB()
print(person)
let thing: Thing? = Thing.objectInDB()
print(thing)