SwiftUI MVVM AnyViewModel 不传播状态更改

Question

我正在尝试以一种将视图与视图模型本身分离的方式在我的 SwiftUI 应用程序中实现 MVVM。在我的研究中，我看到这篇文章概述了一种策略：https://quickbirdstudios.com/blog/swiftui-architecture-redux-mvvm/

以下是其工作原理的摘要：

// ViewModel.swift

protocol ViewModel: ObservableObject where ObjectWillChangePublisher.Output == Void {
    associatedtype State
    associatedtype Event

    var state: State { get }
    func trigger(_ event: Event)
}

// AnyViewModel.swift

final class AnyViewModel<State, Event>: ObservableObject {
    private let wrappedObjectWillChange: () -> AnyPublisher<Void, Never>
    private let wrappedState: () -> State
    private let wrappedTrigger: (Event) -> Void

    var objectWillChange: some Publisher {
        wrappedObjectWillChange()
    }

    var state: State {
        wrappedState()
    }

    func trigger(_ input: Event) {
        wrappedTrigger(input)
    }

    init<V: ViewModel>(_ viewModel: V) where V.State == State, V.Event == Event {
        self.wrappedObjectWillChange = { viewModel.objectWillChange.eraseToAnyPublisher() }
        self.wrappedState = { viewModel.state }
        self.wrappedTrigger = viewModel.trigger
    }
}

// MyView.swift

extension MyView {
    enum Event {
        case onAppear
    }

    enum ViewState {
        case loading
        case details(Details)
    }

    struct Details {
        let title: String
        let detail: String
    }
}

struct MyView: View {
    @ObservedObject var viewModel: AnyViewModel<ViewState, Event>

    var body: some View { ... }
}

// ConcreteViewModel.swift

class ConcreteViewModel: ViewModel {
    @Published var state: MyView.ViewState = .loading

    func trigger(_ event: MyView.Event) {
        ...
        state = .details(...) // This gets called by my app and the state is updated.
        ...
    }
}

// Constructing MyView

let view = MyView(viewModel: AnyViewModel(ConcreteViewModel))

这成功地将视图与视图模型分离（使用 AnyViewModel 作为包装器），但问题是 ConcreteViewModel 中 state 属性的更新未反映在MyView.

我怀疑问题出在 AnyViewModel 和 wrappedObjectWillChange 闭包上，但我很难调试它。我需要明确地对 objectWillChange 发布者做些什么，还是应该 @Published 自动处理它？

非常感谢任何帮助。

Answer 1

我认为 var objectWillChange: some Publisher 没有被 SwiftUI 类型检查器正确解析。将其设置为匹配类型 var objectWillChange: AnyPublisher<Void, Never> 应该可以修复错误。

参见：https://gist.github.com/LizzieStudeneer/c3469eb465e2f88bcb8225df29fbbb77

SwiftUI MVVM AnyViewModel 不传播状态更改

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