3

我已经研究了几天，搜索了 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，它对远程资源的自定义 API 请求类进行一些状态确定。View 处理显示加载状态和失败状态，以及通过 ViewBuilder 传递的主体内容，因此如果来自 API 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，当子类 ObservedObject 更新时，ViewBuilder 内容不会重新呈现。对象会根据 UI 进行更新（按下按钮时等），但 UI 从不重新渲染/更新以反映子类 ObservedObject 中的更改，例如，子类 ObservedObject 中数组后面的 ForEach 不会刷新数组内容改变。如果我将其移出自定义视图，则 ForEach 将按预期工作。

我可以确认代码编译并运行。观察者和debugPrint()'s 自始至终表明，ApiObject正在正确更新状态，并且 View 反映了ApiState变化。它只是ContentViewBuilder 的。我假设是因为 ViewBuilder 只会被调用一次。

编辑：上面的段落应该是提示，ApiState更新正确，但是在将大量日志记录到应用程序之后，UI 没有监听子类 ObservedObject 的发布。属性在变化，状态也在变化，但 UI 并没有对此做出反应。另外，下一句结果是假的，我在 VStack 中再次测试，组件仍然没有重新渲染，这意味着我找错地方了！

如果是这种情况，VStack其他此类元素如何解决这个问题？或者是因为我ApiObjectView的状态变化被重新渲染，导致子视图“重置”？尽管在这种情况下，我希望它能够接收新数据并按预期工作，但它永远不会重新渲染。

有问题的代码在CustomDataList.swift下面ApiObjectView.swift。我留下评论指出正确的方向。

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
            }
        )
    }
}

ForEach这是在ObservedObject更改其绑定数组属性时重新呈现其问题的视图。

// 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ApiObjectView不使用 并且将内容直接放在 View 中，上述所有代码都可以正常工作。

但是，这对于代码重用和架构来说是可怕的，这样它既漂亮又整洁，但不起作用。

有没有其他方法可以解决这个问题，例如通过 aViewModifier或View扩展？

对此的任何帮助将不胜感激。

正如我所说，我似乎找不到任何有此问题的人或任何在线资源可以为我指出解决此问题的正确方向，或者可能导致它的原因，例如 ViewBuilder 文档中概述的。

编辑：为了加入一些有趣的东西，我已经添加了一个倒数计时器CustomDataList，它每 1 秒更新一个标签。如果该计时器对象更新了文本，则重新呈现视图，但仅当显示倒计时时间的标签上的文本更新时。

4

1 回答 1

7

在拔掉我的头发一周后想通了，这是一个子类化的无证问题ObservableObject，如this SO answer所示。

这特别烦人，因为 Xcode 显然会提示您删除该类，因为父类提供了对 的继承ObservableObject，所以在我看来一切都很好。

修复方法是在子类中通过相关变量或任何您需要的变量上self.objectWillChange.send()的willSet侦听器手动触发通用状态更改。@Published

在我提供的示例ApiObject中，问题中的基类保持不变。

虽然，CustomDataController需要修改如下：

// 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
            }
        )
    }
}

一旦我添加了手动发布，问题就解决了。

链接答案的重要说明：不要objectWillChange在子类上重新声明，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>()

在子类上将再次中断状态更新，这需要保留在ObservableObject直接扩展的父类上，无论是我的手动或自动默认定义（输入或不输入并保留为继承声明）。

尽管您仍然可以根据需要定义任意数量的自定义PassthroughSubject声明，而不会在子类上出现问题，例如

// 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

  • 子类上您需要的属性的self.objectWillChange.send()剩余部分@Published
  • 默认PassthroughSubject声明不在子类上重新声明

它将正常工作并正确传播状态更改。

于 2020-02-11T10:56:46.373 回答