SwiftUI MVVM AnyViewModel不传播状态更改

问题描述

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应用中实现MVVM。在我的研究中，我遇到了这篇文章，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），但是问题是state中ConcreteViewModel属性的更新未反映在MyView中。

我怀疑问题出在AnyViewModel和wrappedObjectWillChange闭包中，但是调试起来很困难。我需要与objectWillChange发布者明确地做些什么，还是@Published应该自动处理它？

非常感谢您的帮助。

解决方法

我相信SwiftUI类型检查器无法正确解析var objectWillChange: some Publisher。将其设置为匹配类型var objectWillChange: AnyPublisher<Void,Never>可以解决该错误。

请参阅：https://gist.github.com/LizzieStudeneer/c3469eb465e2f88bcb8225df29fbbb77