Question

我已经研究了这一天，搜寻了Swift和SwiftUI文档，SO，论坛等，但似乎找不到答案。

这是问题所在

我有一个SwiftUI自定义视图，该视图对到远程资源的自定义API请求类进行一些状态确定。 View会显示加载状态和失败状态，并通过ViewBuilder传递其主体内容，这样，如果API中的状态成功并且资源数据已加载，它将显示页面的内容。

问题是，子类ObservedObject更新时，ViewBuilder内容不会重新呈现。对象会根据UI进行更新（按下按钮等时），但UI不会重新渲染/更新以反映子类ObservedObject内的更改，例如，子类ObservedObject内的数组后面的ForEach在以下情况下不会刷新数组内容更改。如果将其移出自定义视图，则ForEach会按预期工作。

我可以确认代码已编译并运行。观察者和debugPrint()的全部显示ApiObject正在正确更新状态，并且View反映了ApiState的更改绝对正确。它只是ViewBuilder的Content。我认为这是因为ViewBuilder只会被调用一次。

编辑：上面的段应该是提示，ApiState已正确更新，但是在将大量日志记录到应用程序中之后，UI并未监听子类的发布。 ObservedObject。属性在变化，状态也在变化，但是UI对此没有反应。此外，下句话证明是错误的，我在VStack中再次进行了测试，但该组件仍然没有重新渲染，这意味着我在错误的位置查找了

！

如果是这种情况，VStack和其他此类元素如何解决呢？还是因为状态更改导致我的ApiObjectView被重新渲染而导致子视图“重置”？尽管在这种情况下，我希望它可以接管新数据并按预期工作，但它永远不会重新呈现。

有问题的代码在下面的CustomDataList.swift和ApiObjectView.swift中。我留下评论指向正确的方向。

这是示例代码；

// ApiState.swift
// Stores the API state for where the request and data parse is currently at.
// This drives the ApiObjectView state UI.

import Foundation

enum ApiState: String
{
    case isIdle

    case isFetchingData
    case hasFailedToFetchData

    case isLoadingData
    case hasFailedToLoadData

    case hasUsableData
}

// ApiObject.swift
// A base class that the Controllers for the app extend from.
// These classes can make data requests to the remote resource API over the
// network to feed their internal data stores.

class ApiObject: ObservableObject
{
    @Published var apiState: ApiState = .isIdle

    let networkRequest: NetworkRequest = NetworkRequest(baseUrl: "https://api.example.com/api")

    public func apiGetJson<T: Codable>(to: String, decodeAs: T.Type, onDecode: @escaping (_ unwrappedJson: T) -> Void) -> Void
    {
        self.apiState = .isFetchingData

        self.networkRequest.send(
            to: to,
            onComplete: {
                self.apiState = .isLoadingData

                let json = self.networkRequest.decodeJsonFromResponse(decodeAs: decodeAs)

                guard let unwrappedJson = json else {
                    self.apiState = .hasFailedToLoadData
                    return
                }

                onDecode(unwrappedJson)

                self.apiState = .hasUsableData
            },
            onFail: {
                self.apiState = .hasFailedToFetchData
            }
        )
    }
}

// DataController.swift
// This is a genericised example of the production code.
// These controllers build, manage and serve their resource data.
// Subclassed from the ApiObject, inheriting ObservableObject

import Foundation
import Combine

class CustomDataController: ApiObject
{
    @Published public var customData: [CustomDataStruct] = []

    public func fetch() -> Void
    {
        self.apiGetJson(
            to: "custom-data-endpoint ",
            decodeAs: [CustomDataStruct].self,
            onDecode: { unwrappedJson in
                self.customData = unwrappedJson
            }
        )
    }
}

这是在将ForEach更改为其绑定数组属性时重新呈现其ObservedObject的视图。

// CustomDataList.swift
// This is the SwiftUI View that drives the content to the user as a list
// that displays the CustomDataController.customData.
// The ForEach in this View 

import SwiftUI

struct CustomDataList: View
{
    @ObservedObject var customDataController: CustomDataController = CustomDataController()

    var body: some View
    {
        ApiObjectView(
            apiObject: self.customDataController,
            onQuit: {}
        ) {
            List
            {
                Section(header: Text("Custom Data").padding(.top, 40))
                {
                    ForEach(self.customDataController.customData, id: \.self, content: { customData in
                        // This is the example that doesn't re-render when the
                        // customDataController updates its data. I have
                        // verified via printing at watching properties
                        // that the object is updating and pushing the
                        // change.

                        // The ObservableObject updates the array, but this ForEach
                        // is not run again when the data is changed.

                        // In the production code, there are buttons in here that
                        // change the array data held within customDataController.customData.

                        // When tapped, they update the array and the ForEach, when placed
                        // in the body directly does reflect the change when
                        // customDataController.customData updates.
                        // However, when inside the ApiObjectView, as by this example,
                        // it does not.

                        Text(customData.textProperty)
                    })
                }
            }
            .listStyle(GroupedListStyle())
        }
        .navigationBarTitle(Text("Learn"))
        .onAppear() {
            self.customDataController.fetch()
        }
    }
}

struct CustomDataList_Previews: PreviewProvider
{
    static var previews: some View
    {
        CustomDataList()
    }
}

这是有问题的自定义视图，不会重新呈现其内容。

// ApiObjectView
// This is the containing View that is designed to assist in the UI rendering of ApiObjects
// by handling the state automatically and only showing the ViewBuilder contents when
// the state is such that the data is loaded and ready, in a non errornous, ready state.
// The ViewBuilder contents loads fine when the view is rendered or the state changes,
// but the Content is never re-rendered if it changes.
// The state renders fine and is reactive to the object, the apiObjectContent
// however, is not.

import SwiftUI

struct ApiObjectView<Content: View>: View {
    @ObservedObject var apiObject: ApiObject

    let onQuit: () -> Void

    let apiObjectContent: () -> Content

    @inlinable public init(apiObject: ApiObject, onQuit: @escaping () -> Void, @ViewBuilder content: @escaping () -> Content) {
        self.apiObject = apiObject
        self.onQuit = onQuit
        self.apiObjectContent = content
    }

    func determineViewBody() -> AnyView
    {
        switch (self.apiObject.apiState) {
            case .isIdle:
                return AnyView(
                    ActivityIndicator(
                        isAnimating: .constant(true),
                        style: .large
                    )
                )

            case .isFetchingData:
                return AnyView(
                    ActivityIndicator(
                        isAnimating: .constant(true),
                        style: .large
                    )
                )

            case .isLoadingData:
                return AnyView(
                    ActivityIndicator(
                        isAnimating: .constant(true),
                        style: .large
                    )
                )

            case .hasFailedToFetchData:
                return AnyView(
                    VStack
                    {
                        Text("Failed to load data!")
                            .padding(.bottom)

                        QuitButton(action: self.onQuit)
                    }
                )

            case .hasFailedToLoadData:
                return AnyView(
                    VStack
                    {
                        Text("Failed to load data!")
                            .padding(.bottom)

                        QuitButton(action: self.onQuit)
                    }
                )

            case .hasUsableData:
                return AnyView(
                    VStack
                    {
                        self.apiObjectContent()
                    }
                )
        }
    }

    var body: some View
    {
        self.determineViewBody()
    }
}

struct ApiObjectView_Previews: PreviewProvider {
    static var previews: some View {
        ApiObjectView(
            apiObject: ApiObject(),
            onQuit: {
                print("I quit.")
            }
        ) {
            EmptyView()
        }
    }
}

现在，如果不使用ApiObjectView并将其内容直接放置在视图中，则上述所有代码都可以正常工作。

但是，这对于代码重用和体系结构来说太可怕了，这样虽然好看又整洁，但是不起作用。

还有其他方法可以解决此问题吗，例如是通过ViewModifier还是View扩展名？

对此将提供任何帮助。

正如我所说，我似乎找不到任何人遇到此问题，也找不到任何在线资源可以为我指出正确的方向来解决此问题，或者可能是由什么引起的，例如ViewBuilder文档中概述的内容。

编辑：为了添加一些有趣的东西，自那以后，我在CustomDataList中添加了倒数计时器，该计时器每1秒更新一次标签。如果由该计时器对象更新文本，则重新渲染视图，但仅（当标签上显示倒计时时间的文本更新时）。

Answer 1

拔出头发一周后就发现了它，这是ObservableObject子类化的一个未记录的问题，如本SO answer所示。

这特别令人讨厌，因为Xcode显然提示您删除该类，因为父类提供了对ObservableObject的继承，所以在我看来一切都很好。

此修复程序是在子类中，用于通过有问题的self.objectWillChange.send()或任何您需要的变量上的willSet侦听器手动触发通用状态更改@Published。

在我提供的示例中，问题中的基类ApiObject保持不变。

尽管如此，CustomDataController需要进行如下修改：

// DataController.swift
// This is a genericised example of the production code.
// These controllers build, manage and serve their resource data.

import Foundation
import Combine

class CustomDataController: ApiObject
{
    @Published public var customData: [CustomDataStruct] = [] {
        willSet {
            // This is the generic state change fire that needs to be added.
            self.objectWillChange.send()
        }
    }

    public func fetch() -> Void
    {
        self.apiGetJson(
            to: "custom-data-endpoint ",
            decodeAs: [CustomDataStruct].self,
            onDecode: { unwrappedJson in
                self.customData = unwrappedJson
            }
        )
    }
}

我添加了手动发布后，问题就解决了。

链接答案中的重要提示：请勿在子类上重新声明objectWillChange，因为这将再次导致状态无法正确更新。例如。声明默认值

let objectWillChange = PassthroughSubject<Void, Never>()

子类上的

会再次中断状态更新，这需要保留在直接从ObservableObject扩展的父类上，无论是我的手动还是自动默认定义（键入或不键入，并保留为继承的声明）

尽管您仍然可以根据需要定义任意数量的自定义PassthroughSubject声明，例如在子类上没有问题

// DataController.swift
// This is a genericised example of the production code.
// These controllers build, manage and serve their resource data.

import Foundation
import Combine

class CustomDataController: ApiObject
{
    var customDataWillUpdate = PassthroughSubject<[CustomDataStruct], Never>()

    @Published public var customData: [CustomDataStruct] = [] {
        willSet {
            // Custom state change handler.
            self.customDataWillUpdate.send(newValue)

            // This is the generic state change fire that needs to be added.
            self.objectWillChange.send()
        }
    }

    public func fetch() -> Void
    {
        self.apiGetJson(
            to: "custom-data-endpoint ",
            decodeAs: [CustomDataStruct].self,
            onDecode: { unwrappedJson in
                self.customData = unwrappedJson
            }
        )
    }
}

只要

self.objectWillChange.send()保留在子类上所需的@Published属性上
未在子类上重新声明默认的PassthroughSubject声明

它将工作并正确传播状态更改。

SwiftUI自定义视图的ViewBuilder不会在子类的ObservedObject更新上重新渲染/更新

1 个答案: