Swift MVVM架构下编译器报'has no member'错误的解决方案咨询

我正在编写MVVM架构的样板代码，尝试在ViewModel中抽象View接口而非绑定属性，但编译器报错：Type 'V.Command' has no member 'reload'。目前临时解决办法是强制将命令转换为V.Command，但不够优雅，寻求更优解决方案。

相关代码 #

接口定义 #

protocol Actionable {
    associatedtype Command
    func perform(command: Command)
}

extension Actionable {
    func perform(command: Command) {}
}

protocol Rendering {
    associatedtype Model
    func render(with model: Model)
}

extension Rendering {
    func render(with dependencies: Model) {}
}

protocol VMView: Rendering, Actionable {}

protocol ViewModel {
    associatedtype Command
    associatedtype Dependencies
    associatedtype Model
}

View实现 #

final class NewViewVC: UIViewController, VMView {
    typealias Model = NewViewVM<NewViewVC>.Model
    typealias Command = NewViewVM<NewViewVC>.Command
    
    func perform(command: Command) {
        print("performing \(command)")
    }
    
    func render(with model: Model) {
        print("rendering \(model)")
    }
    
    var viewModel: NewViewVM<NewViewVC>!
    
    override func viewDidLoad() {
        super.viewDidLoad()
    }
}

extension NewViewVC {
    static func create(dependencies: NewViewVM<NewViewVC>.Dependencies) -> NewViewVC {
        let vc = NewViewVC()
        let viewModel = NewViewVM.create(dependencies: dependencies, services: .init(), view: vc)
        vc.viewModel = viewModel
        return vc
    }
    
    static func createMock(dependencies: NewViewVM<NewViewVC>.Dependencies, services: NewViewVM<NewViewVC>.Dependencies.Services) -> NewViewVC {
        let vc = NewViewVC()
        let viewModel = NewViewVM.create(dependencies: dependencies, services: services, view: vc)
        vc.viewModel = viewModel
        return vc
    }
}

ViewModel实现 #

protocol NewViewVMInterface {
    func reloadData()
}

final class NewViewVM<V: VMView>: ViewModel, NewViewVMInterface {
    
    struct Model {}
    
    enum Command {
        case start, reload
    }
    
    struct Dependencies {
        struct Services {
        }
    }
    
    private var view: V!
    private var dependencies: Dependencies!
    private var services: Dependencies.Services!
    
    static func create(dependencies: Dependencies, services: Dependencies.Services, view: V) -> NewViewVM
    where V.Model == Model, V.Command == Command {
        let viewModel = NewViewVM()
        viewModel.dependencies = dependencies
        viewModel.services = services
        viewModel.view = view
        return viewModel
    }
    
    func reloadData() {
        // fetch data
        view.perform(command: .reload) // <- 报错位置
    }
}

问题根源 #

报错的核心原因是：NewViewVM类的泛型约束仅指定V: VMView，虽然create方法通过where子句限定了V.Model == Model和V.Command == Command，但这个约束只对create方法内部生效，类的其他方法（比如reloadData）无法感知到V.Command与自身定义的Command枚举是同一类型，因此编译器认为V.Command可能是任意类型，不存在reload成员。

优化解决方案 #

方案一：将类型约束提升至类级别 #

直接在NewViewVM的泛型定义中添加类型匹配约束，让整个类内部都能明确V的关联类型与ViewModel自身类型的匹配关系：

final class NewViewVM<V: VMView>: ViewModel, NewViewVMInterface where V.Model == Model, V.Command == Command {
    
    struct Model {}
    
    enum Command {
        case start, reload
    }
    
    struct Dependencies {
        struct Services {
        }
    }
    
    private var view: V!
    private var dependencies: Dependencies!
    private var services: Dependencies.Services!
    
    // 此时create方法无需额外where约束
    static func create(dependencies: Dependencies, services: Dependencies.Services, view: V) -> NewViewVM {
        let viewModel = NewViewVM()
        viewModel.dependencies = dependencies
        viewModel.services = services
        viewModel.view = view
        return viewModel
    }
    
    func reloadData() {
        view.perform(command: .reload) // 编译器可正确识别类型
    }
}

方案二：调整ViewModel协议的关联关系 #

修改ViewModel协议，让它直接关联对应的VMView类型，并默认将自身的Command和Model与View的关联类型绑定，进一步强化类型绑定的清晰度：

// 调整ViewModel协议
protocol ViewModel {
    associatedtype View: VMView
    associatedtype Command = View.Command
    associatedtype Model = View.Model
    associatedtype Dependencies
}

// 修改NewViewVM实现
final class NewViewVM<V: VMView>: ViewModel, NewViewVMInterface {
    typealias View = V
    
    struct Model {}
    
    enum Command {
        case start, reload
    }
    
    struct Dependencies {
        struct Services {
        }
    }
    
    private var view: V!
    private var dependencies: Dependencies!
    private var services: Dependencies.Services!
    
    static func create(dependencies: Dependencies, services: Dependencies.Services, view: V) -> NewViewVM {
        let viewModel = NewViewVM()
        viewModel.dependencies = dependencies
        viewModel.services = services
        viewModel.view = view
        return viewModel
    }
    
    func reloadData() {
        view.perform(command: .reload) // 类型匹配关系明确，编译通过
    }
}

两种方案都能彻底解决编译错误，避免强制类型转换的不优雅写法，同时保证类型安全，让ViewModel与View的关联关系更加清晰。

内容的提问来源于stack exchange，提问作者sam k