How to Build a Period Tracker App in SwiftUI

import SwiftData
import Foundation

enum FlowIntensity: String, Codable, CaseIterable {
    case light  = "Light"
    case medium = "Medium"
    case heavy  = "Heavy"
}

@Model
final class CycleEntry {
    var startDate: Date
    var endDate: Date?
    var flowIntensity: FlowIntensity
    var symptoms: [String]
    var notes: String

    init(startDate: Date, flowIntensity: FlowIntensity = .medium) {
        self.startDate     = startDate
        self.flowIntensity = flowIntensity
        self.symptoms      = []
        self.notes         = ""
    }
}

struct CycleHomeView: View {
    @Query(sort: \CycleEntry.startDate, order: .reverse)
    private var cycles: [CycleEntry]
    @State private var showLogSheet = false

    var nextPeriod: Date? { CyclePredictionEngine.predict(from: cycles) }

    var body: some View {
        NavigationStack {
            ScrollView {
                VStack(spacing: 24) {
                    PredictionRingView(nextDate: nextPeriod)
                        .frame(height: 200)
                    if let next = nextPeriod {
                        Text("Next period \(next, style: .relative)")
                            .font(.headline).foregroundStyle(.pink)
                    }
                    CycleHistoryChart(cycles: Array(cycles.prefix(6)))
                        .frame(height: 160).padding(.horizontal)
                }
            }
            .navigationTitle("My Cycle")
            .toolbar {
                Button("Log Period", systemImage: "plus.circle.fill") {
                    showLogSheet = true
                }
            }
            .sheet(isPresented: $showLogSheet) { LogEntryView() }
        }
    }
}

struct CyclePredictionEngine {
    static func predict(from cycles: [CycleEntry]) -> Date? {
        let sorted = cycles.sorted { $0.startDate < $1.startDate }
        guard !sorted.isEmpty else { return nil }
        guard sorted.count >= 2 else {
            return Calendar.current.date(byAdding: .day, value: 28, to: sorted[0].startDate)
        }
        let lengths: [Int] = zip(sorted, sorted.dropFirst()).map {
            Calendar.current.dateComponents([.day], from: $0.startDate, to: $1.startDate).day ?? 28
        }
        let avg = lengths.reduce(0, +) / lengths.count
        return Calendar.current.date(byAdding: .day, value: avg, to: sorted.last!.startDate)
    }

    static func scheduleReminder(for date: Date) async throws {
        let center = UNUserNotificationCenter.current()
        try await center.requestAuthorization(options: [.alert, .sound])
        let content      = UNMutableNotificationContent()
        content.title    = "Period due soon"
        content.body     = "Your period is predicted to arrive in 2 days."
        let fireDate     = Calendar.current.date(byAdding: .day, value: -2, to: date)!
        let comps        = Calendar.current.dateComponents([.year,.month,.day,.hour], from: fireDate)
        let trigger      = UNCalendarNotificationTrigger(dateMatching: comps, repeats: false)
        try await center.add(UNNotificationRequest(identifier: UUID().uuidString, content: content, trigger: trigger))
    }
}

<?xml version="1.0" encoding="UTF-8"?>
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<plist version="1.0"><dict>
  <key>NSPrivacyTracking</key><false/>
  <key>NSPrivacyTrackingDomains</key><array/>
  <key>NSPrivacyCollectedDataTypes</key>
  <array>
    <dict>
      <key>NSPrivacyCollectedDataType</key>
      <string>NSPrivacyCollectedDataTypeHealthAndFitness</string>
      <key>NSPrivacyCollectedDataTypeLinked</key><false/>
      <key>NSPrivacyCollectedDataTypeTracking</key><false/>
      <key>NSPrivacyCollectedDataTypePurposes</key>
      <array>
        <string>NSPrivacyCollectedDataTypePurposeAppFunctionality</string>
      </array>
    </dict>
  </array>
  <key>NSPrivacyAccessedAPITypes</key><array/>
</dict></plist>