SwiftUI 没有加载数据

Question

您好，我正在构建一个从 json 文件示例中获取事件的应用程序，如下所示：

[{"title": "Evening Picnic", "start": "November 10, 2018 6:00 PM", "end": "November 10, 2018 7:00 PM"}]

在解析事件后，它必须按日期对它们进行排序并按日期对它们进行分组，List Section 还检查事件之间的冲突并将行颜色更改为红色。

我做了一个简单的用户界面，一个包含动态部分的列表和内部排序的事件，但由于某种原因，应用程序编译时没有错误或崩溃，但视图是空白的，就像数据没有加载到查看。

到目前为止，这是我的代码：

Events.swift

public struct Event: Codable, IntervalProtocol, Hashable, Identifiable {
    public var id = UUID()
    var title: String
    var start: String
    var end: String
}

struct AppConstants {
    static let mockDataFilename = "mock"
    static let jsonDataFormat = "MMMM d, yyyy h:mm a"
    static let sectionDateFormat = "MMMM d, yyyy"
    static let eventDateFormat = "h:mm a"
}

extension Event {
    public static var jsonDateFormatter: DateFormatter = {
        let formatter = DateFormatter()
        formatter.dateFormat = AppConstants.jsonDataFormat
        return formatter
    }()
    // Parse dates into Struct variables.
    var startDate: Date {
        guard let date = Event.jsonDateFormatter.date(from: self.start) else {
            fatalError("Unable to parse date.")
        }
        return date
    }

    var endDate: Date {
        guard let date = Event.jsonDateFormatter.date(from: self.end) else {
            fatalError("Unable to parse date.")
        }
        return date
    }

    var interval: Interval {
        return Interval(self.startDate.timeIntervalSince1970, self.endDate.timeIntervalSince1970)
    }
}

ViewModel.swift

class ViewModel: ObservableObject {
    let queue = DispatchQueue(label: "com.elbeheiry")

    @Published var sections = [Date: [Event]]()
    @Published var sortedDays = [Date]()
    @Published var intervalTree: IntervalTree!

    func getEvents() {
        let events = Bundle.main.eventJsonData(fileName: AppConstants.mockDataFilename)
        self.intervalTree = IntervalTree(events ?? [])
        let inOrder = self.intervalTree.inOrder.map { $0 as! Event }
        self.buildSections(inOrder)
    }

    func buildSections(_ events: [Event]) {
        for event in events {
            let startTimestamp = Date(timeIntervalSince1970: event.interval.min)
            let strippedDate = Calendar.current.dateComponents([.year, .month, .day],
                                                               from: startTimestamp)

            guard let date = Calendar.current.date(from: strippedDate) else {
                fatalError("Failed to remove time from Date object")
            }
            self.sections[date, default: []].append(event)
        }
        self.sortedDays = self.sections.keys.sorted()
    }
}

public extension Bundle {
    func eventJsonData(fileName: String) -> [Event]? {
        guard let url = self.url(forResource: fileName, withExtension: "json") else {
            fatalError("File was not reachable.")
        }

        guard let data = try? Data(contentsOf: url) else {
            fatalError("File was not readable.")
        }

        return try? JSONDecoder().decode([Event].self, from: data)
    }
}

ContentView.swift

struct ContentView: View {
    @ObservedObject var viewModel: ViewModel = ViewModel()

    var body: some View {
        VStack {
            List {
                ForEach(viewModel.sortedDays, id: \.self) { header in
                    Section(header: Text(header, style: .date)) {
                        ForEach(viewModel.sections[header]!) { event in
                            EventCell(event: event)
                                .background(Color(self.viewModel.intervalTree.isConflicted(event) ? #colorLiteral(red: 0.8078431487, green: 0.02745098062, blue: 0.3333333433, alpha: 1) : #colorLiteral(red: 1.0, green: 1.0, blue: 1.0, alpha: 1.0)))
                        }
                    }
                }
            }
        }
        .onAppear(perform: {
            viewModel.getEvents()
        })
    }
}

Answer 1

这是因为您正在使用Codable，并且它正在尝试从 JSON 中解码 id。 id 不在您的 JSON 中，我可以看到您希望每个新实例都创建自己的 ID。

如果您在 catch 语句中打印错误，使用 do-try-catch 而不是 try?，您会看到以下内容：

keyNotFound(CodingKeys(stringValue: "id", intValue: nil), Swift.DecodingError.Context(codingPath: [_JSONKey(stringValue: "Index 0", intValue: 0)], debugDescription: "没有与键关联的值CodingKeys(stringValue: "id", intValue: nil) ("id").", underlyingError: nil))

基本上，它们没有与键 id 关联的值，因为键不在 JSON 中。

您需要创建自定义CodingKeys 以排除id 被编码/解码：

public struct Event: Codable, Hashable, Identifiable {
    private enum CodingKeys: CodingKey {
        case title
        case start
        case end
    }

    public var id = UUID()
    var title: String
    var start: String
    var end: String
}