@Published 和 .assign 对值更新没有反应

@Published and .assign not reacting to value update

这里是 SwiftUI 和 Combine noob,我在操场上隔离了我遇到的问题。这里是游乐场

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 \([=10=])")
            return [=10=]
        }.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)
}

当我运行游乐场时,这是正在发生的事情:

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,存在两个问题:

过去几个小时我一直在尝试解决这个问题,但没有成功。也许在 SwiftUI/Combine 方面具有最高知识的人可以帮助我,thx !

编辑

这是可行的解决方案:

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
    }
}

我更愿意使用下面的 ObservableObject/ObservedObject 模式,但也可以使用其他变体(如进一步提供)

全部测试 Xcode 11.2 / iOS 13.2

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)
    }
}

替补:

以下解决了您的问题(如果您不存储订阅者,发布者将立即取消)

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

    subscriber = textContainer.$containedValue.compactMap {
        print("compact map \([=11=])")
        return [=11=]
    }.assign(to: \.viewModel, on: self)
}

请注意,视图的状态仅在视图层次结构中链接,在 Playground 中,就像您所做的那样,它仅包含初始值。

另一种更适合 SwiftUI 层次结构的可能方法是

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 \([=12=])")
            return [=12=]
        }.eraseToAnyPublisher()
    }
}

我会保存对 AppContainer 的引用。

struct TestSwiftUIView: View {

    @State private var viewModel = "test"

    ///I just added this
    var textContainer: AnyCancellable?

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

    init(textContainer: ReactiveContainer<String>) {

        self.textContainer = textContainer.$containedValue.compactMap {
            print("compact map \(String(describing: [=10=]))")
            return [=10=]
        }.assign(to: \.viewModel, on: self)
    }
}

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

我们不使用 Combine 在视图之间移动数据,SwiftUI 已经内置了对此的支持。主要问题是您将 TestSwiftUIView 视为 class 但它是一个结构,即一个值。最好将视图简单地视为要显示的数据。当数据发生变化时,SwiftUI 会一遍又一遍地创建这些数据结构。所以解决方案很简单:

struct ContentView: View {

    let text: String

    var body: some View { // only called if text is different from last time ContentView was created in a parent View's body.
        Text(text)
    }
}

父体方法可以一遍又一遍地调用ContentView(text:"Test"),但是ContentView体方法只有在let text与上次不同时才会被SwiftUI调用,例如ContentView(text:"Test2")。我认为这是您尝试使用 Combine 重新创建的内容,但这是不必要的,因为 SwiftUI 已经做到了。