检查两个对象是否实现了 Swift 协议及其关联类型

Question

我有一个 Swift 3 项目，我在其中声明了一个具有如下关联类型的协议：

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

我想检查两个对象是否实现 ViewModelContainer 并且它的关联类型 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
}

但我不能将container 转换为ViewModelContainer：

Protocol 'ViewModelContainer' 只能用作通用约束，因为它具有 Self 或关联的类型要求

---

我目前的解决方法是直接回退到特定类及其关联类型，但这会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 UIViewControllers，但因为 Objective-C doesn't recognize generic Swift classes 而卡住了，因此无法在 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)
 }

[EDIT] 以供将来参考，这里有一个 Bool 返回以实现类型兼容性：

 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 
    }
 }

这将允许一个组合条件：

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

Answer 2

这是将associatedtype ViewModelType 更改为协议的想法。

protocol ViewModelProtocol {
}

protocol ViewModelContainer {

    var viewModel: ViewModelProtocol? { get set }
}

class MediaPlaySelectionViewModel: ViewModelProtocol {

    var title: String?
    func play() {
        print("playing")
    }
}

class SearchResultsViewModel: ViewModelProtocol {

    var results: [String]?
}

class MediaPlaySelectionViewController: UIViewController, ViewModelContainer {

    var viewModel: ViewModelProtocol?

    // So the view itself know which kind of vm it wants.
    var myViewModel: MediaPlaySelectionViewModel? {
        return viewModel as? MediaPlaySelectionViewModel
    }

    override func viewWillAppear(_ animated: Bool) {
        print(myViewModel?.title ?? "Undefined")
    }
}

class SearchResultsViewController: UIViewController, ViewModelContainer {

    var viewModel: ViewModelProtocol?

    // So the view itself know which kind of vm it wants.
    var myViewModel: SearchResultsViewModel? {
        return viewModel as? SearchResultsViewModel
    }

    override func viewWillAppear(_ animated: Bool) {
        print(myViewModel?.results?.joined(separator: ", ") ?? "No Result")
    }
}

class MenuViewController: UITableViewController {

    override func prepare(for segue: UIStoryboardSegue, sender: Any?) {

        // NOTE: Swift doesn't allow me to use 'let' here
        if var container = segue.destination as? ViewModelContainer, let cell = sender as? UITableViewCell, let vm = viewModel(for: cell) {
            container.viewModel = vm
        }
    }

    // NOTE: One difficulty here, how could you decide which ViewModel to prepare?  I guess you need a Factory.
    func viewModel(for cell: UITableViewCell) -> ViewModelProtocol! {

        if let index = tableView.indexPath(for: cell) {

            if index.item == 0 {

                let vm = MediaPlaySelectionViewModel()
                vm.title = "My Video"

                return vm
            }
            else if index.item == 1 {

                let vm = SearchResultsViewModel()
                vm.results = ["Apple", "Banana"]

                return vm
            }
        }

        return nil
    }
}