Example to instantiate models only once and update them

UniquelyInstanciatedModels.swift

class Weak<T: AnyObject> {
  weak var value : T?
  init (value: T) {
    self.value = value
  }
}

public class Model {
  let id: String
  private init(from payload: JSON) {
    ...
  }

  private static var instanciated = [String: Weak<Model>]()
  deinit {
    if Model.instanciated[id]?.value === self {
      Model.instanciated.removeValue(forKey: id)
    }
  }
  private static func getCacheOrCache(model: Model) -> Model {
    if let existingInstance = instanciated[model.id]?.value {
      update(cached: existingInstance, with: model)
      return existingInstance
    }
    instanciated[model.id] = Weak<Model>(value: model)
    return model
  }

  private static func update(cached instance: Model, with update: Model) {
    instance.someField = update.someField
  }

  static func create(from payload: JSON) Model {
    let model = Model(from: payload)
    return getCacheOrCache(model: model)
  }
}

// then instead of doing `let m = Model(…)`, you do `let m = Model.create(…)`