SwiftUI - @State 属性未更新

Question

我在 SwiftUI 中遇到了这个非常奇怪的问题/错误。在该setupSubscription方法中，我创建一个订阅subject并将其插入到cancellables集合中。然而，当我打印 的计数时cancellables，我得到零。如果我刚刚插入一个元素，集合怎么会是空的呢？这可能就是handleValue当我点击按钮时不调用该方法的原因。这是控制台的完整输出：

init
begin setupSubscription
setupSubscription subject sink: receive subscription: (CurrentValueSubject)
setupSubscription subject sink: request unlimited
setupSubscription subject sink: receive value: (initial value)
handleValue: 'initial value'
setupSubscription: cancellables.count: 0
setupSubscription subject sink: receive cancel
sent value: 'value 38'
cancellables.count: 0
sent value: 'value 73'
cancellables.count: 0
sent value: 'value 30'
cancellables.count: 0

我究竟做错了什么？为什么我的订阅被subject取消了？为什么handleValue我点击按钮后没有接到电话？

import SwiftUI
import Combine

struct Test: View {
    
    @State private var cancellables: Set<AnyCancellable> = []
    
    let subject = CurrentValueSubject<String, Never>("initial value")
    
    init() {
        print("init")
        self.setupSubscription()
    }
    
    var body: some View {
        VStack {
            Button(action: {
                let newValue = "value \(Int.random(in: 0...100))"
                self.subject.send(newValue)
                print("sent value: '\(newValue)'")
                print("cancellables.count:", cancellables.count)
            }, label: {
                Text("Tap Me")
            })
        }
    }
    
    func setupSubscription() {
        print("begin setupSubscription")
        
        let cancellable = self.subject
            .print("setupSubscription subject sink")
            .sink(receiveValue: handleValue(_:))
        
        self.cancellables.insert(cancellable)
        
        print("setupSubscription: cancellables.count:", cancellables.count) 
        // prints "setupSubscription: cancellables.count: 0"
    
    }
    
    
    func handleValue(_ value: String) {
        print("handleValue: '\(value)'")
    }
    
    
}

Answer 1

你在这里做错了一些事情。

切勿尝试将内容存储在 swiftUI 结构中。每次您的视图发生更改时，它们都会失效并重新加载。这可能就是您的订阅被取消的原因。

对于这样的事情，您应该使用具有已发布属性的 ObservableObject 或 StateObject。当 ObservableObjects 或 StateObjects 发生变化时。包含它们的视图会像 @State 或 @Binding 一样重新加载：

// ObservedObjects have an implied objectWillChange publisher that causes swiftUI views to reload any time a published property changes. In essence they act like State or Binding variables.
class ViewModel: ObservableObject {
    // Published properties ARE combine publishers
    @Published var subject: String = "initial value"
}

那么在你看来：

@ObservedObject var viewModel: ViewModel = ViewModel()

如果您确实需要使用出版商​​。或者，如果您需要在可观察对象属性发生更改时执行某些操作。您不需要使用.sink。这主要用于使用组合的 UIKit 应用程序。SwiftUI 有一个 .onReceive viewmodifier 可以做同样的事情。

以下是我的上述建议的具体实施：

struct Test: View {

    class ViewModel: ObservedObject {
        @Published var subject: String = "initial value"
    }

    @ObservedObject var viewModel: Self.ViewModel

    var body: some View {
        VStack {
            Text("\(viewModel.subject)")

            Button {
                viewModel.subject = "value \(Int.random(in: 0...100))"
            } label: {
                Text("Tap Me")
            }
        }
        .onReceive(viewModel.$subject) { [self] newValue in
            handleValue(newValue)
        }
    }

    func handleValue(_ value: String) {
        print("handleValue: '\(value)'")
    }
}

Answer 2

您只是错误地使用了状态 - 它与视图相关，并且仅在呈现视图时（即在上下文中）才可用（准备好后台存储）body。在 init 中还没有状态后台存储，所以你的可取消项就消失了。

这是可能的工作方法（但是我建议将所有与主题相关的内容移动到单独的视图模型中）

使用 Xcode 12 / iOS 14 进行测试

struct Test: View {

    private var cancellable: AnyCancellable?
    private let subject = CurrentValueSubject<String, Never>("initial value")

    init() {
        cancellable = self.subject
            .print("setupSubscription subject sink")
            .sink(receiveValue: handleValue(_:))
    }

    var body: some View {
        VStack {
            Button(action: {
                let newValue = "value \(Int.random(in: 0...100))"
                self.subject.send(newValue)
                print("sent value: '\(newValue)'")
            }, label: {
                Text("Tap Me")
            })
        }
    }

    func handleValue(_ value: String) {
        print("handleValue: '\(value)'")
    }
}