如何在午夜自动更新SwiftUI ContentView中的变量？

1. 将常量改为可观察状态变量 #

原来的let是初始化时固定的常量，无法动态更新，需要改成@State变量（在View中使用），让SwiftUI能监听变量变化并自动刷新视图：

// 在ContentView中
@State private var doy = getDayOfYear()
@State private var drNow = daysinyear()
@State private var wn = weekNumber()
@State private var dn = dayNumber()

2. 选择更新触发方式 #

有两种可靠的方式触发午夜更新：

方式一：监听系统时间变化通知

系统会在午夜、时区变更等时间显著变化时发送UIApplication.significantTimeChangeNotification通知，用Combine监听更安全：

import Combine

struct ContentView: View {
    @State private var doy = getDayOfYear()
    @State private var drNow = daysinyear()
    @State private var wn = weekNumber()
    @State private var dn = dayNumber()
    
    private var cancellable: AnyCancellable?
    
    var body: some View {
        // 你的视图布局代码，示例如下
        VStack {
            Text("Day of Year: \(doy)")
            Text("Days Remaining: \(drNow)")
            Text("Week Number: \(wn)")
            Text("Day Number: \(dn)")
        }
    }
    .onAppear {
        // 监听时间变化通知
        cancellable = NotificationCenter.default.publisher(for: UIApplication.significantTimeChangeNotification)
            .receive(on: DispatchQueue.main)
            .sink { _ in
                // 更新所有变量
                doy = getDayOfYear()
                drNow = daysinyear()
                wn = weekNumber()
                dn = dayNumber()
            }
    }
}

方式二：精准调度午夜定时器

计算下一次午夜的时间，设置一次性定时器，触发后再调度下一次，保证精准触发：

struct ContentView: View {
    @State private var doy = getDayOfYear()
    @State private var drNow = daysinyear()
    @State private var wn = weekNumber()
    @State private var dn = dayNumber()
    
    private var updateTimer: Timer?
    
    var body: some View {
        VStack {
            Text("Day of Year: \(doy)")
            Text("Days Remaining: \(drNow)")
            Text("Week Number: \(wn)")
            Text("Day Number: \(dn)")
        }
    }
    .onAppear {
        scheduleMidnightUpdate()
    }
    .onDisappear {
        updateTimer?.invalidate()
    }
    
    // 计算下一次午夜时间
    private func nextMidnight() -> Date {
        let cal = Calendar.current
        let tomorrow = cal.date(byAdding: .day, value: 1, to: Date())!
        return cal.startOfDay(for: tomorrow)
    }
    
    // 调度午夜更新定时器
    private func scheduleMidnightUpdate() {
        let timeInterval = nextMidnight().timeIntervalSinceNow
        updateTimer = Timer.scheduledTimer(withTimeInterval: timeInterval, repeats: false) { _ in
            // 更新变量
            doy = getDayOfYear()
            drNow = daysinyear()
            wn = weekNumber()
            dn = dayNumber()
            // 重新调度下一次午夜更新
            scheduleMidnightUpdate()
        }
    }
}

3. 优化日期函数的安全性 #

原getDayOfYear()里的强制解包(!)存在崩溃风险，建议改成可选绑定：

func getDayOfYear() -> Int {
    let date = Date()
    let cal = Calendar.current
    guard let dayOfYear = cal.ordinality(of: .day, in: .year, for: date) else {
        return 0 // 可根据需求设置合适的默认值
    }
    return dayOfYear
}

内容的提问来源于stack exchange，提问作者user15154955