SwiftUI MVVM AnyViewModel不传播状态更改

编程问答 2022-08-16

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

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