Date.FormatStyle の出力サンプル

ContentView.swift

import SwiftUI

struct ContentView: View {
    @State private var date = ISO8601DateFormatter().date(from: "2025-01-01T00:00:00+09:00")!
    @State private var code: LanguageCode = .japanese
    @State private var timeZoneID: TimeZoneID = .jst
    @State private var calendarName: CalendarName = .gregorian

    var body: some View {
        NavigationStack {
            List {
                Section("出力") {
                    DateCell($date, style: style1)
                    DateCell($date, style: style2)
                }
                Section("暦法") {
                    ForEach(CalendarName.allCases, id: \.self) {
                        SelectionCell($calendarName, value: $0)
                    }
                }
                Section("ロケール") {
                    ForEach(LanguageCode.allCases, id: \.self) {
                        SelectionCell($code, value: $0)
                    }
                }
                Section("タイムゾーン") {
                    ForEach(TimeZoneID.allCases, id: \.self) {
                        SelectionCell($timeZoneID, value: $0)
                    }
                }
            }
            .contentTransition(.numericText(countsDown: false))
            .animation(.default, value: date)
            .animation(.default, value: code)
            .animation(.default, value: timeZoneID)
            .animation(.default, value: calendarName)
            .environment(\.locale, Locale(identifier: code.rawValue))
            .environment(\.timeZone, TimeZone(timeZoneID))
            .environment(\.calendar, Calendar(identifier: calendarName.identifier))
        }
    }

    private var style1: Date.FormatStyle {
        .dateTime.year().month().day().hour().minute().second()
    }

    private var style2: Date.FormatStyle {
        .dateTime.year().month(.twoDigits).day(.twoDigits)
    }
}

/// 言語コード
enum LanguageCode: String, Hashable, CaseIterable, CustomStringConvertible {
    case japanese = "ja_JP"
    case english = "en_US"
    case th_TH = "th_TH"
    case hi_IN = "hi_IN"

    var description: String { rawValue }
}

/// タイムゾーン
enum TimeZoneID: String, Hashable, CaseIterable, CustomStringConvertible {
    case jst = "JST"
    case utc = "UTC"

    var description: String { rawValue }
}

/// 暦法
enum CalendarName: String, Hashable, CaseIterable, CustomStringConvertible {
    case gregorian = "グレゴリオ暦"
    case japanese = "和暦"
    case chinese = "中国暦"
    case buddhist = "仏暦"
    case indian = "インド暦"

    var identifier: Calendar.Identifier {
        switch self {
        case .gregorian:
            return .gregorian
        case .japanese:
            return .japanese
        case .chinese:
            return .chinese
        case .buddhist:
            return .buddhist
        case .indian:
            return .indian
        }
    }

    var description: String { rawValue }
}

extension TimeZone {
    init(_ id: TimeZoneID) {
        self.init(identifier: id.rawValue)!
    }
}

struct DateCell: View {
    @Binding private var date: Date
    @EnvironmentFormat private var style: Date.FormatStyle

    init(_ date: Binding<Date>, style: Date.FormatStyle) {
        self._date = date
        self._style = .init(wrappedValue: style)
    }

    var body: some View {
        Text(date.formatted(style))
            .font(.body.monospacedDigit().bold())
    }
}

@propertyWrapper
struct EnvironmentFormat: DynamicProperty {
    @Environment(\.locale) private var locale
    @Environment(\.timeZone) private var timeZone
    @Environment(\.calendar) private var calendar

    private var value: Date.FormatStyle

    init(wrappedValue value: Date.FormatStyle) {
        self.value = value
    }

    var wrappedValue: Date.FormatStyle {
        var value = value
        value.timeZone = timeZone
        value.locale = locale
        value.calendar = calendar
        return value
    }
}

struct SelectionCell<T: Equatable>: View {
    @Binding private var selection: T
    private let value: T

    init(_ selection: Binding<T>, value: T) {
        _selection = selection
        self.value = value
    }

    var body: some View {
        Button {
            selection = value
        } label: {
            HStack {
                Text("\(value)")
                Spacer()
                Image(systemName: "checkmark")
                    .opacity(selection == value ? 1 : 0)
            }
        }
    }
}

#Preview {
    ContentView()
}

ja_JP

グレゴリオ暦	和暦	中国歴	仏暦	インド暦

en_US

グレゴリオ暦	和暦	中国歴	仏暦	インド暦