ios - 无法分配给受限于类的协议(protocol)中的属性 - Swift 编译器错误

Question

我使用 Xcode 7 beta 6 并且我有这样的代码:

public protocol ViewForViewModel {
    typealias ViewModelType
    var viewModel: ViewModelType! { get set }
    func bindToViewModel()
}

func afterViewInstantiated <V : ViewForViewModel where V: UIViewController, V.ViewModelType: AnyObject>(view : V, viewModel: V.ViewModelType) -> V {
    //Cannot assign to property: 'view' is a 'let' constant
    view.viewModel = viewModel // error here

    VMTracker.append(viewModel, view: view)

    return view
}

编译器在赋值时提示 view.viewModel = viewModel。我明白 ViewForViewModel 协议(protocol)本身并不受限于类，但 V 类型受限于 UIViewController 类。是错误还是功能？

UPD:它甚至提示 UITableViewCell 变量:

func registerBinding<V: BindableCellView where V: UITableViewCell>(viewType: V.Type) {
    let typeName = nameOfType(V.ViewModelType.self)

    bindings[typeName] = { [unowned self] viewModel, indexPath in
        let view = self.tableView.dequeueReusableCellWithIdentifier(V.CellIdentifier, forIndexPath: indexPath) as! V

        //Cannot assign to 'viewModel' because 'view' is a 'let' constant
        //However view is UITableViewCell that support ViewForViewModel protocol
        view.viewModel = viewModel as! V.ViewModelType

        self.onWillBind?(view, indexPath)
        view.bindToViewModel()
        self.onDidBind?(view, indexPath)

        return view
    }
}

Answer 1

如果编译器无法推断出参数将始终是引用类型，您始终可以将 class 添加到协议(protocol)声明中:

public protocol ViewForViewModel: class {
  typealias ViewModelType
  var viewModel: ViewModelType! { get set }
  func bindToViewModel()
}

一旦协议(protocol)被标记为那样，即使对象存储在常量中，您也应该能够为属性赋值。