gistfile1.txt

class DataBinding<T> {
    
    typealias Handler = (T) -> Void
    private var handler: Handler?
    var value: T {
        didSet {
            self.handler?(value)
        }
    }
    
    init(value: T) {
        self.value = value
    }
    
    func bind(hdl: @escaping Handler) {
        self.handler = hdl
        handler?(value)
    }
    
}

gistfile2.txt

class ItemsViewModel {
    var items = DataBinding<[ItemModel]>(value: [])
    var error = DataBinding<Error?>(value: nil)
    func numberItems() -> Int {
      return items.value.count
    }
    func fetchData() {
        ServiceManager.fetchItems { [weak self] (items, error) in
            guard let self = self else {return}
            if error != nil {
               self.error = error
            } else {
               self.items.value = items                           
            }
        }
    }
}