Generic class inheritance in Swift

Question

I have the following class:

class BaseCache<T: Equatable>: NSObject {

    var allEntities = [T]()

    // MARK: - Append

    func appendEntities(newEntities: [T]) {
        ....
    }
}

Now I want to subclass it, but I get annoying error, that my type "does not conform to protocol 'Equatable'":

It seems generics in Swift are real pain-in-the-ass.

Answer 1

Your class definition of TrackingCache is wrong. It repeats the generic parameter:

class TrackingCache<AftershipTracking>: BaseCache<AftershipTracking> { }

It should be left out:

class TrackingCache: BaseCache<AftershipTracking> { }

This triggers the underlying swift error Classes derived from generic classes must also be generic. You can work around this issue by specifying a type parameter that is required to be or inherit from AftershipTracking:

class TrackingCache<T: AftershipTracking>: BaseCache<AftershipTracking> { }

Full example:

class BaseCache<T: Equatable>: NSObject {
  var items: [T] = []

  func appendItems( items: [T]) {
    self.items += items
    didAppendItems()
  }

  func didAppendItems() {} // for overriding
}

class AftershipTracking: NSObject {
  var identifier: Int
  init( identifier: Int) {
    self.identifier = identifier
    super.init()
  }
}

extension AftershipTracking: Equatable { }

func ==( lhs: AftershipTracking, rhs: AftershipTracking) -> Bool {
  return lhs.identifier == rhs.identifier
}

class TrackingCache<T: AftershipTracking>: BaseCache<AftershipTracking> {
  override func didAppendItems() {
    // do something
  }
}

let a = TrackingCache<AftershipTracking>()
let b = TrackingCache<AftershipTracking>()

a.appendItems( [AftershipTracking( identifier: 1)])
b.appendItems( [AftershipTracking( identifier: 1)])

let result = a.items == b.items // true