ios - @Published和.assign对值更新没有反应

SwiftUI和Combine noob在这里，我在操场上隔离了我遇到的问题。这是操场。

final class ReactiveContainer<T: Equatable> {
    @Published var containedValue: T?
}

class AppContainer {
    static let shared = AppContainer()

    let text = ReactiveContainer<String>()
}

struct TestSwiftUIView: View {

    @State private var viewModel = "test"

    var body: some View {
        Text("\(viewModel)")
    }

    init(textContainer: ReactiveContainer<String>) {

        textContainer.$containedValue.compactMap {
            print("compact map \($0)")
            return $0
        }.assign(to: \.viewModel, on: self)
    }
}

AppContainer.shared.text.containedValue = "init"


var testView = TestSwiftUIView(textContainer: AppContainer.shared.text)
print(testView)

print("Executing network request")
DispatchQueue.main.asyncAfter(deadline: .now() + 3) {
    AppContainer.shared.text.containedValue = "Hello world"
    print(testView)
}

我在操场上跑的时候是这样的:

compact map Optional("init")
TestSwiftUIView(_viewModel: SwiftUI.State<Swift.String>(_value: "test", _location: nil))
Executing network request
TestSwiftUIView(_viewModel: SwiftUI.State<Swift.String>(_value: "test", _location: nil))

如您所见，那里有两个问题:

精简 map 关闭仅在订阅时调用一次，但在运行分派时不调用

从不将分配运算符称为

在过去的几个小时中，我一直试图解决此问题，但没有成功。也许某人在SwiftUI / Combine中有最高级的知识可以帮助我，谢谢!

编辑

这是有效的解决方案:

struct ContentView: View {

    @State private var viewModel = "test"
    let textContainer: ReactiveContainer<String>

    var body: some View {
        Text(viewModel).onReceive(textContainer.$containedValue) { (newContainedValue) in
            self.viewModel = newContainedValue ?? ""
        }
    }

    init(textContainer: ReactiveContainer<String>) {
        self.textContainer = textContainer
    }
}

最佳答案

我宁愿使用下面的ObservableObject/ObservedObject模式，但也可以使用其他变体(进一步提供)

所有均通过Xcode 11.2 / iOS 13.2测试

final class ReactiveContainer<T: Equatable>: ObservableObject {
    @Published var containedValue: T?
}

struct TestSwiftUIView: View {

    @ObservedObject var vm: ReactiveContainer<String>

    var body: some View {
        Text("\(vm.containedValue ?? "<none>")")
    }

    init(textContainer: ReactiveContainer<String>) {
        self._vm = ObservedObject(initialValue: textContainer)
    }
}

备用:

以下内容可解决您的问题(如果您不存储订户，发布者将立即被取消)

private var subscriber: AnyCancellable?
init(textContainer: ReactiveContainer<String>) {

    subscriber = textContainer.$containedValue.compactMap {
        print("compact map \($0)")
        return $0
    }.assign(to: \.viewModel, on: self)
}

请注意，视图的状态仅在视图层次结构中链接，就像在Playground中一样，它仅保留初始值。

另一种可能更适合SwiftUI层次结构的方法是

struct TestSwiftUIView: View {

    @State private var viewModel: String = "test"

    var body: some View {
        Text("\(viewModel)")
            .onReceive(publisher) { value in
                self.viewModel = value
            }
    }

    let publisher: AnyPublisher<String, Never>
    init(textContainer: ReactiveContainer<String>) {

        publisher = textContainer.$containedValue.compactMap {
            print("compact map \($0)")
            return $0
        }.eraseToAnyPublisher()
    }
}

关于ios - @Published和.assign对值更新没有反应，我们在Stack Overflow上找到一个类似的问题：https://stackoverflow.com/questions/60667976/