generics.swift

/*
 Here's our goal:

 let localDataStore = UserDataStore()
 let remoteDataStore = UserApi()
 let dataStore = CacheBackedDataStore(localDataStore, remoteDataStore)

 dataStore.fetch(userID) { result in
   // handle result
 }
 */

protocol LocalStore {
  associatedtype StoredType

  func fetch(_ identifier: String) -> StoredType?
  func persist(_ item: StoredType)
}

protocol RemoteStore {
  associatedtype StoredType

  func fetch(_ identifier: String, _ completion: (StoredType?) -> Void)
}

struct CacheBackedDataStore<Local: LocalStore, Remote: RemoteStore> where Local.StoredType == Remote.StoredType {
  let localStore: Local
  let remoteStore: Remote

  func fetch(_ identifier: String, completion: @escaping (Remote.StoredType?) -> Void)  {
    if let object = localStore.fetch(identifier) {
      completion(object)
    } else {
      remoteStore.fetch(identifier, { object in
        if let unwrappedObject = object {
          localStore.persist(unwrappedObject)
        }
        completion(object)
      })
    }
  }
}

struct User {}
struct Video {}

class UserDataStore: LocalStore {
  var user: User?

  func fetch(_ identifier: String) -> User? {
    print("FETCH")
    return nil
  }

  func persist(_ item: User) {
    print("PERSIST")
    user = item
  }
}

class RemoteUsers: RemoteStore {
  func fetch(_ identifier: String, _ completion: (User?) -> Void) {
    print("REMOTE FETCH")
    completion(User())
  }
}

let local = UserDataStore()
let remote = RemoteUsers()

class VideosDataStore: LocalStore {
  func fetch(_ identifier: String) -> Video? {
    return nil
  }

  func persist(_ item: Video) {
    //
  }
}

class RemoteVideos: RemoteStore {
  func fetch(_ identifier: String, _ completion: (Video?) -> Void) {
    completion(nil)
  }
}

let local2 = VideosDataStore()
let remote2 = RemoteVideos()

let cacheBacked = CacheBackedDataStore(localStore: local, remoteStore: remote)
let cacheBacked2 = CacheBackedDataStore(localStore: local2, remoteStore: remote2)

cacheBacked.fetch("1") { user in
  print("user fetched: \(user)")
}