swift - 检查两个对象是否实现了 Swift 协议(protocol)及其关联类型

Question

我有一个 Swift 3 项目，我在其中声明一个具有关联类型的协议(protocol)，如下所示:

protocol ViewModelContainer {
    associatedtype ViewModelType
    var viewModel: ViewModelType! { get set }
}

我想检查两个对象是否实现了 ViewModelContainer 及其关联类型 ViewModelType 以“通用”方式进行分配。

理想情况下我想做这样的事情:

if let container = container as? ViewModelContainer, let model = model as? container.ViewModelType {
    container.viewModel = model
}

但我无法将 container 转换为 ViewModelContainer:

Protocol 'ViewModelContainer' can only be used as a generic constraint because it has Self or associated type requirements

---

我目前的解决方法是直接回退到特定的类及其关联类型，但这使我的代码非常冗长且容易出错:

override func prepare(for segue: UIStoryboardSegue, sender: Any?) {
    if let vc = segue.destination as? MediaPlaySelectionViewController, let vm = sender as? MediaPlaySelectionViewModel {
        vc.viewModel = vm
    }
    if let vc = segue.destination as? SearchResultsViewController, let vm = sender as? SearchResultsViewModel {
        vc.viewModel = vm
    }
    if let vc = segue.destination as? ReviewDetailsViewController, let vm = sender as? ReviewDetailsViewModel {
        vc.viewModel = vm
    }
    if let vc = segue.destination as? ReviewComposerViewController, let vm = sender as? ReviewComposerViewModel {
        vc.viewModel = vm
    }
}

我尝试使用通用的 UIViewController，但由于 Objective-C doesn't recognize generic Swift classes 而卡住了因此不能在 Storyboard 中使用。

Answer 1

这比我预期的要棘手(所以我删除了我之前的帖子以避免混淆)但我相信这对你有用:

 protocol ViewModelContainerVC 
 {
   mutating func setModel(_ :Any)
 }

 protocol ViewModelContainer:class,ViewModelContainerVC 
 {
     associatedtype ViewModelType
     var viewModel: ViewModelType! { get set }
 }

 extension ViewModelContainer
 {
    mutating func setModel(_ model:Any)
    { if model is ViewModelType { viewModel = model as! ViewModelType } }
 }

然后您可以使用 ViewModelContainerVC 进行类型转换和赋值:

 if let container = container as? ViewModelContainerVC 
 { 
   container.setModel(model)
 }

[编辑] 以供将来引用，这里有同样的事情，返回 Bool 以实现类型兼容性:

 protocol ViewModelContainerVC 
 {
   @discardableResult mutating func setModel(_ :Any) -> Bool
 }

 extension ViewModelContainer
 {
    @discardableResult mutating func setModel(_ model:Any) -> Bool
    { 
       if let validModel = model as? ViewModelType 
       { viewModel = validModel; return true }
       return false 
    }
 }

这将允许组合条件:

 if var container = container as? ViewModelContainerVC,
    container.setModel(model) 
 { ... }