// Copyright 2015 The Chromium Authors. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.

import 'dart:async';
import 'dart:math' as math;

import 'package:flutter/foundation.dart';
import 'package:flutter/rendering.dart';
import 'package:flutter/services.dart';
import 'package:flutter/widgets.dart';

import 'button.dart';
import 'button_bar.dart';
import 'colors.dart';
import 'debug.dart';
import 'dialog.dart';
import 'feedback.dart';
import 'flat_button.dart';
import 'icon_button.dart';
import 'icons.dart';
import 'ink_well.dart';
import 'material.dart';
import 'material_localizations.dart';
import 'theme.dart';
import 'typography.dart';

/// Initial display mode of the date picker dialog.
///
/// Date picker UI mode for either showing a list of available years or a
/// monthly calendar initially in the dialog shown by calling [showDatePicker].
///
/// Also see:
///
///  * <https://material.io/guidelines/components/pickers.html#pickers-date-pickers>
enum DatePickerMode {
  /// Show a date picker UI for choosing a month and day.
  day,

  /// Show a date picker UI for choosing a year.
  year,
}

const double _kDatePickerHeaderPortraitHeight = 100.0;
const double _kDatePickerHeaderLandscapeWidth = 168.0;

const Duration _kMonthScrollDuration = const Duration(milliseconds: 200);
const double _kDayPickerRowHeight = 42.0;
const int _kMaxDayPickerRowCount = 6; // A 31 day month that starts on Saturday.
// Two extra rows: one for the day-of-week header and one for the month header.
const double _kMaxDayPickerHeight = _kDayPickerRowHeight * (_kMaxDayPickerRowCount + 2);

const double _kMonthPickerPortraitWidth = 330.0;
const double _kMonthPickerLandscapeWidth = 344.0;

const double _kDialogActionBarHeight = 52.0;
const double _kDatePickerLandscapeHeight = _kMaxDayPickerHeight + _kDialogActionBarHeight;

// Shows the selected date in large font and toggles between year and day mode
class _DatePickerHeader extends StatelessWidget {
  const _DatePickerHeader({
    Key key,
    @required this.selectedDate,
    @required this.mode,
    @required this.onModeChanged,
    @required this.orientation,
  }) : assert(selectedDate != null),
       assert(mode != null),
       assert(orientation != null),
       super(key: key);

  final DateTime selectedDate;
  final DatePickerMode mode;
  final ValueChanged<DatePickerMode> onModeChanged;
  final Orientation orientation;

  void _handleChangeMode(DatePickerMode value) {
    if (value != mode)
      onModeChanged(value);
  }

  @override
  Widget build(BuildContext context) {
    final MaterialLocalizations localizations = MaterialLocalizations.of(context);
    final ThemeData themeData = Theme.of(context);
    final TextTheme headerTextTheme = themeData.primaryTextTheme;
    Color dayColor;
    Color yearColor;
    switch (themeData.primaryColorBrightness) {
      case Brightness.light:
        dayColor = mode == DatePickerMode.day ? Colors.black87 : Colors.black54;
        yearColor = mode == DatePickerMode.year ? Colors.black87 : Colors.black54;
        break;
      case Brightness.dark:
        dayColor = mode == DatePickerMode.day ? Colors.white : Colors.white70;
        yearColor = mode == DatePickerMode.year ? Colors.white : Colors.white70;
        break;
    }
    final TextStyle dayStyle = headerTextTheme.display1.copyWith(color: dayColor, height: 1.4);
    final TextStyle yearStyle = headerTextTheme.subhead.copyWith(color: yearColor, height: 1.4);

    Color backgroundColor;
    switch (themeData.brightness) {
      case Brightness.light:
        backgroundColor = themeData.primaryColor;
        break;
      case Brightness.dark:
        backgroundColor = themeData.backgroundColor;
        break;
    }

    double width;
    double height;
    EdgeInsets padding;
    MainAxisAlignment mainAxisAlignment;
    switch (orientation) {
      case Orientation.portrait:
        height = _kDatePickerHeaderPortraitHeight;
        padding = const EdgeInsets.symmetric(horizontal: 16.0);
        mainAxisAlignment = MainAxisAlignment.center;
        break;
      case Orientation.landscape:
        width = _kDatePickerHeaderLandscapeWidth;
        padding = const EdgeInsets.all(8.0);
        mainAxisAlignment = MainAxisAlignment.start;
        break;
    }

    final Widget yearButton = new IgnorePointer(
      ignoring: mode != DatePickerMode.day,
      ignoringSemantics: false,
      child: new _DateHeaderButton(
        color: backgroundColor,
        onTap: Feedback.wrapForTap(() => _handleChangeMode(DatePickerMode.year), context),
        child: new Semantics(
          selected: mode == DatePickerMode.year,
          child: new Text(localizations.formatYear(selectedDate), style: yearStyle),
        ),
      ),
    );

    final Widget dayButton = new IgnorePointer(
      ignoring: mode == DatePickerMode.day,
      ignoringSemantics: false,
      child: new _DateHeaderButton(
        color: backgroundColor,
        onTap: Feedback.wrapForTap(() => _handleChangeMode(DatePickerMode.day), context),
        child: new Semantics(
          selected: mode == DatePickerMode.day,
          child: new Text(localizations.formatMediumDate(selectedDate), style: dayStyle),
        ),
      ),
    );

    return new Container(
      width: width,
      height: height,
      padding: padding,
      color: backgroundColor,
      child: new Column(
        mainAxisAlignment: mainAxisAlignment,
        crossAxisAlignment: CrossAxisAlignment.start,
        children: <Widget>[yearButton, dayButton],
      ),
    );
  }
}

class _DateHeaderButton extends StatelessWidget {
  const _DateHeaderButton({
    Key key,
    this.onTap,
    this.color,
    this.child,
  }) : super(key: key);

  final VoidCallback onTap;
  final Color color;
  final Widget child;

  @override
  Widget build(BuildContext context) {
    final ThemeData theme = Theme.of(context);

    return new Material(
      type: MaterialType.button,
      color: color,
      child: new InkWell(
        borderRadius: kMaterialEdges[MaterialType.button],
        highlightColor: theme.highlightColor,
        splashColor: theme.splashColor,
        onTap: onTap,
        child: new Container(
          padding: const EdgeInsets.symmetric(horizontal: 8.0),
          child: child,
        ),
      ),
    );
  }
}

class _DayPickerGridDelegate extends SliverGridDelegate {
  const _DayPickerGridDelegate();

  @override
  SliverGridLayout getLayout(SliverConstraints constraints) {
    final int columnCount = DateTime.DAYS_PER_WEEK;
    final double tileWidth = constraints.crossAxisExtent / columnCount;
    final double tileHeight = math.min(_kDayPickerRowHeight, constraints.viewportMainAxisExtent / (_kMaxDayPickerRowCount + 1));
    return new SliverGridRegularTileLayout(
      crossAxisCount: columnCount,
      mainAxisStride: tileHeight,
      crossAxisStride: tileWidth,
      childMainAxisExtent: tileHeight,
      childCrossAxisExtent: tileWidth,
      reverseCrossAxis: axisDirectionIsReversed(constraints.crossAxisDirection),
    );
  }

  @override
  bool shouldRelayout(_DayPickerGridDelegate oldDelegate) => false;
}

const _DayPickerGridDelegate _kDayPickerGridDelegate = const _DayPickerGridDelegate();

/// Displays the days of a given month and allows choosing a day.
///
/// The days are arranged in a rectangular grid with one column for each day of
/// the week.
///
/// The day picker widget is rarely used directly. Instead, consider using
/// [showDatePicker], which creates a date picker dialog.
///
/// See also:
///
///  * [showDatePicker].
///  * <https://material.google.com/components/pickers.html#pickers-date-pickers>
class DayPicker extends StatelessWidget {
  /// Creates a day picker.
  ///
  /// Rarely used directly. Instead, typically used as part of a [MonthPicker].
  DayPicker({
    Key key,
    @required this.selectedDate,
    @required this.currentDate,
    @required this.onChanged,
    @required this.firstDate,
    @required this.lastDate,
    @required this.displayedMonth,
    this.selectableDayPredicate,
  }) : assert(selectedDate != null),
       assert(currentDate != null),
       assert(onChanged != null),
       assert(displayedMonth != null),
       assert(!firstDate.isAfter(lastDate)),
       assert(selectedDate.isAfter(firstDate) || selectedDate.isAtSameMomentAs(firstDate)),
       super(key: key);

  /// The currently selected date.
  ///
  /// This date is highlighted in the picker.
  final DateTime selectedDate;

  /// The current date at the time the picker is displayed.
  final DateTime currentDate;

  /// Called when the user picks a day.
  final ValueChanged<DateTime> onChanged;

  /// The earliest date the user is permitted to pick.
  final DateTime firstDate;

  /// The latest date the user is permitted to pick.
  final DateTime lastDate;

  /// The month whose days are displayed by this picker.
  final DateTime displayedMonth;

  /// Optional user supplied predicate function to customize selectable days.
  final SelectableDayPredicate selectableDayPredicate;

  /// Builds widgets showing abbreviated days of week. The first widget in the
  /// returned list corresponds to the first day of week for the current locale.
  ///
  /// Examples:
  ///
  /// ```
  /// ┌ Sunday is the first day of week in the US (en_US)
  /// |
  /// S M T W T F S  <-- the returned list contains these widgets
  /// _ _ _ _ _ 1 2
  /// 3 4 5 6 7 8 9
  ///
  /// ┌ But it's Monday in the UK (en_GB)
  /// |
  /// M T W T F S S  <-- the returned list contains these widgets
  /// _ _ _ _ 1 2 3
  /// 4 5 6 7 8 9 10
  /// ```
  List<Widget> _getDayHeaders(TextStyle headerStyle, MaterialLocalizations localizations) {
    final List<Widget> result = <Widget>[];
    for (int i = localizations.firstDayOfWeekIndex; true; i = (i + 1) % 7) {
      final String weekday = localizations.narrowWeekdays[i];
      result.add(new ExcludeSemantics(
        child: new Center(child: new Text(weekday, style: headerStyle)),
      ));
      if (i == (localizations.firstDayOfWeekIndex - 1) % 7)
        break;
    }
    return result;
  }

  // Do not use this directly - call getDaysInMonth instead.
  static const List<int> _kDaysInMonth = const <int>[31, -1, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31];

  /// Returns the number of days in a month, according to the proleptic
  /// Gregorian calendar.
  ///
  /// This applies the leap year logic introduced by the Gregorian reforms of
  /// 1582. It will not give valid results for dates prior to that time.
  static int getDaysInMonth(int year, int month) {
    if (month == DateTime.FEBRUARY) {
      final bool isLeapYear = (year % 4 == 0) && (year % 100 != 0) || (year % 400 == 0);
      if (isLeapYear)
        return 29;
      return 28;
    }
    return _kDaysInMonth[month - 1];
  }

  /// Computes the offset from the first day of week that the first day of the
  /// [month] falls on.
  ///
  /// For example, September 1, 2017 falls on a Friday, which in the calendar
  /// localized for United States English appears as:
  ///
  /// ```
  /// S M T W T F S
  /// _ _ _ _ _ 1 2
  /// ```
  ///
  /// The offset for the first day of the months is the number of leading blanks
  /// in the calendar, i.e. 5.
  ///
  /// The same date localized for the Russian calendar has a different offset,
  /// because the first day of week is Monday rather than Sunday:
  ///
  /// ```
  /// M T W T F S S
  /// _ _ _ _ 1 2 3
  /// ```
  ///
  /// So the offset is 4, rather than 5.
  ///
  /// This code consolidates the following:
  ///
  /// - [DateTime.weekday] provides a 1-based index into days of week, with 1
  ///   falling on Monday.
  /// - [MaterialLocalizations.firstDayOfWeekIndex] provides a 0-based index
  ///   into the [MaterialLocalizations.narrowWeekdays] list.
  /// - [MaterialLocalizations.narrowWeekdays] list provides localized names of
  ///   days of week, always starting with Sunday and ending with Saturday.
  int _computeFirstDayOffset(int year, int month, MaterialLocalizations localizations) {
    // 0-based day of week, with 0 representing Monday.
    final int weekdayFromMonday = new DateTime(year, month).weekday - 1;
    // 0-based day of week, with 0 representing Sunday.
    final int firstDayOfWeekFromSunday = localizations.firstDayOfWeekIndex;
    // firstDayOfWeekFromSunday recomputed to be Monday-based
    final int firstDayOfWeekFromMonday = (firstDayOfWeekFromSunday - 1) % 7;
    // Number of days between the first day of week appearing on the calendar,
    // and the day corresponding to the 1-st of the month.
    return (weekdayFromMonday - firstDayOfWeekFromMonday) % 7;
  }

  @override
  Widget build(BuildContext context) {
    final ThemeData themeData = Theme.of(context);
    final MaterialLocalizations localizations = MaterialLocalizations.of(context);
    final int year = displayedMonth.year;
    final int month = displayedMonth.month;
    final int daysInMonth = getDaysInMonth(year, month);
    final int firstDayOffset = _computeFirstDayOffset(year, month, localizations);
    final List<Widget> labels = <Widget>[];
    labels.addAll(_getDayHeaders(themeData.textTheme.caption, localizations));
    for (int i = 0; true; i += 1) {
      // 1-based day of month, e.g. 1-31 for January, and 1-29 for February on
      // a leap year.
      final int day = i - firstDayOffset + 1;
      if (day > daysInMonth)
        break;
      if (day < 1) {
        labels.add(new Container());
      } else {
        final DateTime dayToBuild = new DateTime(year, month, day);
        final bool disabled = dayToBuild.isAfter(lastDate)
            || dayToBuild.isBefore(firstDate)
            || (selectableDayPredicate != null && !selectableDayPredicate(dayToBuild));

        BoxDecoration decoration;
        TextStyle itemStyle = themeData.textTheme.body1;

        final bool isSelectedDay = selectedDate.year == year && selectedDate.month == month && selectedDate.day == day;
        if (isSelectedDay) {
          // The selected day gets a circle background highlight, and a contrasting text color.
          itemStyle = themeData.accentTextTheme.body2;
          decoration = new BoxDecoration(
            color: themeData.accentColor,
            shape: BoxShape.circle
          );
        } else if (disabled) {
          itemStyle = themeData.textTheme.body1.copyWith(color: themeData.disabledColor);
        } else if (currentDate.year == year && currentDate.month == month && currentDate.day == day) {
          // The current day gets a different text color.
          itemStyle = themeData.textTheme.body2.copyWith(color: themeData.accentColor);
        }

        Widget dayWidget = new Container(
          decoration: decoration,
          child: new Center(
            child: new Semantics(
              // We want the day of month to be spoken first irrespective of the
              // locale-specific preferences or TextDirection. This is because
              // an accessibility user is more likely to be interested in the
              // day of month before the rest of the date, as they are looking
              // for the day of month. To do that we prepend day of month to the
              // formatted full date.
              label: '${localizations.formatDecimal(day)}, ${localizations.formatFullDate(dayToBuild)}',
              selected: isSelectedDay,
              child: new ExcludeSemantics(
                child: new Text(localizations.formatDecimal(day), style: itemStyle),
              ),
            ),
          ),
        );

        if (!disabled) {
          dayWidget = new GestureDetector(
            behavior: HitTestBehavior.opaque,
            onTap: () {
              onChanged(dayToBuild);
            },
            child: dayWidget,
          );
        }

        labels.add(dayWidget);
      }
    }

    return new Padding(
      padding: const EdgeInsets.symmetric(horizontal: 8.0),
      child: new Column(
        children: <Widget>[
          new Container(
            height: _kDayPickerRowHeight,
            child: new Center(
              child: new ExcludeSemantics(
                child: new Text(
                  localizations.formatMonthYear(displayedMonth),
                  style: themeData.textTheme.subhead,
                ),
              ),
            ),
          ),
          new Flexible(
            child: new GridView.custom(
              gridDelegate: _kDayPickerGridDelegate,
              childrenDelegate: new SliverChildListDelegate(labels, addRepaintBoundaries: false),
            ),
          ),
        ],
      ),
    );
  }
}

/// A scrollable list of months to allow picking a month.
///
/// Shows the days of each month in a rectangular grid with one column for each
/// day of the week.
///
/// The month picker widget is rarely used directly. Instead, consider using
/// [showDatePicker], which creates a date picker dialog.
///
/// See also:
///
///  * [showDatePicker]
///  * <https://material.google.com/components/pickers.html#pickers-date-pickers>
class MonthPicker extends StatefulWidget {
  /// Creates a month picker.
  ///
  /// Rarely used directly. Instead, typically used as part of the dialog shown
  /// by [showDatePicker].
  MonthPicker({
    Key key,
    @required this.selectedDate,
    @required this.onChanged,
    @required this.firstDate,
    @required this.lastDate,
    this.selectableDayPredicate,
  }) : assert(selectedDate != null),
       assert(onChanged != null),
       assert(!firstDate.isAfter(lastDate)),
       assert(selectedDate.isAfter(firstDate) || selectedDate.isAtSameMomentAs(firstDate)),
       super(key: key);

  /// The currently selected date.
  ///
  /// This date is highlighted in the picker.
  final DateTime selectedDate;

  /// Called when the user picks a month.
  final ValueChanged<DateTime> onChanged;

  /// The earliest date the user is permitted to pick.
  final DateTime firstDate;

  /// The latest date the user is permitted to pick.
  final DateTime lastDate;

  /// Optional user supplied predicate function to customize selectable days.
  final SelectableDayPredicate selectableDayPredicate;

  @override
  _MonthPickerState createState() => new _MonthPickerState();
}

class _MonthPickerState extends State<MonthPicker> {
  @override
  void initState() {
    super.initState();
    // Initially display the pre-selected date.
    final int monthPage = _monthDelta(widget.firstDate, widget.selectedDate);
    _dayPickerController = new PageController(initialPage: monthPage);
    _handleMonthPageChanged(monthPage);
    _updateCurrentDate();
  }

  @override
  void didUpdateWidget(MonthPicker oldWidget) {
    super.didUpdateWidget(oldWidget);
    if (widget.selectedDate != oldWidget.selectedDate) {
      final int monthPage = _monthDelta(widget.firstDate, widget.selectedDate);
      _dayPickerController = new PageController(initialPage: monthPage);
      _handleMonthPageChanged(monthPage);
    }
  }

  MaterialLocalizations localizations;
  TextDirection textDirection;

  @override
  void didChangeDependencies() {
    super.didChangeDependencies();
    localizations = MaterialLocalizations.of(context);
    textDirection = Directionality.of(context);
  }

  DateTime _todayDate;
  DateTime _currentDisplayedMonthDate;
  Timer _timer;
  PageController _dayPickerController;

  void _updateCurrentDate() {
    _todayDate = new DateTime.now();
    final DateTime tomorrow = new DateTime(_todayDate.year, _todayDate.month, _todayDate.day + 1);
    Duration timeUntilTomorrow = tomorrow.difference(_todayDate);
    timeUntilTomorrow += const Duration(seconds: 1);  // so we don't miss it by rounding
    _timer?.cancel();
    _timer = new Timer(timeUntilTomorrow, () {
      setState(() {
        _updateCurrentDate();
      });
    });
  }

  static int _monthDelta(DateTime startDate, DateTime endDate) {
    return (endDate.year - startDate.year) * 12 + endDate.month - startDate.month;
  }

  /// Add months to a month truncated date.
  DateTime _addMonthsToMonthDate(DateTime monthDate, int monthsToAdd) {
    return new DateTime(monthDate.year + monthsToAdd ~/ 12, monthDate.month + monthsToAdd % 12);
  }

  Widget _buildItems(BuildContext context, int index) {
    final DateTime month = _addMonthsToMonthDate(widget.firstDate, index);
    return new DayPicker(
      key: new ValueKey<DateTime>(month),
      selectedDate: widget.selectedDate,
      currentDate: _todayDate,
      onChanged: widget.onChanged,
      firstDate: widget.firstDate,
      lastDate: widget.lastDate,
      displayedMonth: month,
      selectableDayPredicate: widget.selectableDayPredicate,
    );
  }

  void _handleNextMonth() {
    if (!_isDisplayingLastMonth) {
      SemanticsService.announce(localizations.formatMonthYear(_nextMonthDate), textDirection);
      _dayPickerController.nextPage(duration: _kMonthScrollDuration, curve: Curves.ease);
    }
  }

  void _handlePreviousMonth() {
    if (!_isDisplayingFirstMonth) {
      SemanticsService.announce(localizations.formatMonthYear(_previousMonthDate), textDirection);
      _dayPickerController.previousPage(duration: _kMonthScrollDuration, curve: Curves.ease);
    }
  }

  /// True if the earliest allowable month is displayed.
  bool get _isDisplayingFirstMonth {
    return !_currentDisplayedMonthDate.isAfter(
        new DateTime(widget.firstDate.year, widget.firstDate.month));
  }

  /// True if the latest allowable month is displayed.
  bool get _isDisplayingLastMonth {
    return !_currentDisplayedMonthDate.isBefore(
        new DateTime(widget.lastDate.year, widget.lastDate.month));
  }

  DateTime _previousMonthDate;
  DateTime _nextMonthDate;

  void _handleMonthPageChanged(int monthPage) {
    setState(() {
      _previousMonthDate = _addMonthsToMonthDate(widget.firstDate, monthPage - 1);
      _currentDisplayedMonthDate = _addMonthsToMonthDate(widget.firstDate, monthPage);
      _nextMonthDate = _addMonthsToMonthDate(widget.firstDate, monthPage + 1);
    });
  }

  @override
  Widget build(BuildContext context) {
    return new SizedBox(
      width: _kMonthPickerPortraitWidth,
      height: _kMaxDayPickerHeight,
      child: new Stack(
        children: <Widget>[
          new PageView.builder(
            key: new ValueKey<DateTime>(widget.selectedDate),
            controller: _dayPickerController,
            scrollDirection: Axis.horizontal,
            itemCount: _monthDelta(widget.firstDate, widget.lastDate) + 1,
            itemBuilder: _buildItems,
            onPageChanged: _handleMonthPageChanged,
          ),
          new PositionedDirectional(
            top: 0.0,
            start: 8.0,
            child: new IconButton(
              icon: const Icon(Icons.chevron_left),
              tooltip: _isDisplayingFirstMonth ? null : '${localizations.previousMonthTooltip} ${localizations.formatMonthYear(_previousMonthDate)}',
              onPressed: _isDisplayingFirstMonth ? null : _handlePreviousMonth,
            ),
          ),
          new PositionedDirectional(
            top: 0.0,
            end: 8.0,
            child: new IconButton(
              icon: const Icon(Icons.chevron_right),
              tooltip: _isDisplayingLastMonth ? null : '${localizations.nextMonthTooltip} ${localizations.formatMonthYear(_nextMonthDate)}',
              onPressed: _isDisplayingLastMonth ? null : _handleNextMonth,
            ),
          ),
        ],
      ),
    );
  }

  @override
  void dispose() {
    _timer?.cancel();
    _dayPickerController?.dispose();
    super.dispose();
  }
}

/// A scrollable list of years to allow picking a year.
///
/// The year picker widget is rarely used directly. Instead, consider using
/// [showDatePicker], which creates a date picker dialog.
///
/// Requires one of its ancestors to be a [Material] widget.
///
/// See also:
///
///  * [showDatePicker]
///  * <https://material.google.com/components/pickers.html#pickers-date-pickers>
class YearPicker extends StatefulWidget {
  /// Creates a year picker.
  ///
  /// The [selectedDate] and [onChanged] arguments must not be null. The
  /// [lastDate] must be after the [firstDate].
  ///
  /// Rarely used directly. Instead, typically used as part of the dialog shown
  /// by [showDatePicker].
  YearPicker({
    Key key,
    @required this.selectedDate,
    @required this.onChanged,
    @required this.firstDate,
    @required this.lastDate,
  }) : assert(selectedDate != null),
       assert(onChanged != null),
       assert(!firstDate.isAfter(lastDate)),
       super(key: key);

  /// The currently selected date.
  ///
  /// This date is highlighted in the picker.
  final DateTime selectedDate;

  /// Called when the user picks a year.
  final ValueChanged<DateTime> onChanged;

  /// The earliest date the user is permitted to pick.
  final DateTime firstDate;

  /// The latest date the user is permitted to pick.
  final DateTime lastDate;

  @override
  _YearPickerState createState() => new _YearPickerState();
}

class _YearPickerState extends State<YearPicker> {
  static const double _itemExtent = 50.0;
  ScrollController scrollController;

  @override
  void initState() {
    super.initState();
    scrollController = new ScrollController(
      // Move the initial scroll position to the currently selected date's year.
      initialScrollOffset: (widget.selectedDate.year - widget.firstDate.year) * _itemExtent,
    );
  }

  @override
  Widget build(BuildContext context) {
    assert(debugCheckHasMaterial(context));
    final ThemeData themeData = Theme.of(context);
    final TextStyle style = themeData.textTheme.body1;
    return new ListView.builder(
      controller: scrollController,
      itemExtent: _itemExtent,
      itemCount: widget.lastDate.year - widget.firstDate.year + 1,
      itemBuilder: (BuildContext context, int index) {
        final int year = widget.firstDate.year + index;
        final bool isSelected = year == widget.selectedDate.year;
        final TextStyle itemStyle = isSelected
          ? themeData.textTheme.headline.copyWith(color: themeData.accentColor)
          : style;
        return new InkWell(
          key: new ValueKey<int>(year),
          onTap: () {
            widget.onChanged(new DateTime(year, widget.selectedDate.month, widget.selectedDate.day));
          },
          child: new Center(
            child: new Semantics(
              selected: isSelected,
              child: new Text(year.toString(), style: itemStyle),
            ),
          ),
        );
      },
    );
  }
}

class _DatePickerDialog extends StatefulWidget {
  const _DatePickerDialog({
    Key key,
    this.initialDate,
    this.firstDate,
    this.lastDate,
    this.selectableDayPredicate,
    this.initialDatePickerMode,
  }) : super(key: key);

  final DateTime initialDate;
  final DateTime firstDate;
  final DateTime lastDate;
  final SelectableDayPredicate selectableDayPredicate;
  final DatePickerMode initialDatePickerMode;

  @override
  _DatePickerDialogState createState() => new _DatePickerDialogState();
}

class _DatePickerDialogState extends State<_DatePickerDialog> {
  @override
  void initState() {
    super.initState();
    _selectedDate = widget.initialDate;
    _mode = widget.initialDatePickerMode;
  }

  bool _announcedInitialDate = false;

  MaterialLocalizations localizations;
  TextDirection textDirection;

  @override
  void didChangeDependencies() {
    super.didChangeDependencies();
    localizations = MaterialLocalizations.of(context);
    textDirection = Directionality.of(context);
    if (!_announcedInitialDate) {
      _announcedInitialDate = true;
      SemanticsService.announce(
        localizations.formatFullDate(_selectedDate),
        textDirection,
      );
    }
  }

  DateTime _selectedDate;
  DatePickerMode _mode;
  final GlobalKey _pickerKey = new GlobalKey();

  void _vibrate() {
    switch (Theme.of(context).platform) {
      case TargetPlatform.android:
      case TargetPlatform.fuchsia:
        HapticFeedback.vibrate();
        break;
      case TargetPlatform.iOS:
        break;
    }
  }

  void _handleModeChanged(DatePickerMode mode) {
    _vibrate();
    setState(() {
      _mode = mode;
      if (_mode == DatePickerMode.day) {
        SemanticsService.announce(localizations.formatMonthYear(_selectedDate), textDirection);
      } else {
        SemanticsService.announce(localizations.formatYear(_selectedDate), textDirection);
      }
    });
  }

  void _handleYearChanged(DateTime value) {
    _vibrate();
    setState(() {
      _mode = DatePickerMode.day;
      _selectedDate = value;
    });
  }

  void _handleDayChanged(DateTime value) {
    _vibrate();
    setState(() {
      _selectedDate = value;
    });
  }

  void _handleCancel() {
    Navigator.pop(context);
  }

  void _handleOk() {
    Navigator.pop(context, _selectedDate);
  }

  Widget _buildPicker() {
    assert(_mode != null);
    switch (_mode) {
      case DatePickerMode.day:
        return new MonthPicker(
          key: _pickerKey,
          selectedDate: _selectedDate,
          onChanged: _handleDayChanged,
          firstDate: widget.firstDate,
          lastDate: widget.lastDate,
          selectableDayPredicate: widget.selectableDayPredicate,
        );
      case DatePickerMode.year:
        return new YearPicker(
          key: _pickerKey,
          selectedDate: _selectedDate,
          onChanged: _handleYearChanged,
          firstDate: widget.firstDate,
          lastDate: widget.lastDate,
        );
    }
    return null;
  }

  @override
  Widget build(BuildContext context) {
    final Widget picker = new Flexible(
      child: new SizedBox(
        height: _kMaxDayPickerHeight,
        child: _buildPicker(),
      ),
    );
    final Widget actions = new ButtonTheme.bar(
      child: new ButtonBar(
        children: <Widget>[
          new FlatButton(
            child: new Text(localizations.cancelButtonLabel),
            onPressed: _handleCancel,
          ),
          new FlatButton(
            child: new Text(localizations.okButtonLabel),
            onPressed: _handleOk,
          ),
        ],
      ),
    );
    return new Dialog(
      child: new OrientationBuilder(
        builder: (BuildContext context, Orientation orientation) {
          assert(orientation != null);
          final Widget header = new _DatePickerHeader(
            selectedDate: _selectedDate,
            mode: _mode,
            onModeChanged: _handleModeChanged,
            orientation: orientation,
          );
          switch (orientation) {
            case Orientation.portrait:
              return new SizedBox(
                width: _kMonthPickerPortraitWidth,
                child: new Column(
                  mainAxisSize: MainAxisSize.min,
                  crossAxisAlignment: CrossAxisAlignment.stretch,
                  children: <Widget>[header, picker, actions],
                ),
              );
            case Orientation.landscape:
              return new SizedBox(
                height: _kDatePickerLandscapeHeight,
                child: new Row(
                  mainAxisSize: MainAxisSize.min,
                  crossAxisAlignment: CrossAxisAlignment.stretch,
                  children: <Widget>[
                    header,
                    new Flexible(
                      child: new SizedBox(
                        width: _kMonthPickerLandscapeWidth,
                        child: new Column(
                          mainAxisSize: MainAxisSize.min,
                          crossAxisAlignment: CrossAxisAlignment.stretch,
                          children: <Widget>[picker, actions],
                        ),
                      ),
                    ),
                  ],
                ),
              );
          }
          return null;
        }
      )
    );
  }
}

/// Signature for predicating dates for enabled date selections.
///
/// See [showDatePicker].
typedef bool SelectableDayPredicate(DateTime day);

/// Shows a dialog containing a material design date picker.
///
/// The returned [Future] resolves to the date selected by the user when the
/// user closes the dialog. If the user cancels the dialog, null is returned.
///
/// An optional [selectableDayPredicate] function can be passed in to customize
/// the days to enable for selection. If provided, only the days that
/// [selectableDayPredicate] returned true for will be selectable.
///
/// An optional [initialDatePickerMode] argument can be used to display the
/// date picker initially in the year or month+day picker mode. It defaults
/// to month+day, and must not be null.
///
/// An optional [locale] argument can be used to set the locale for the date
/// picker. It defaults to the ambient locale provided by [Localizations].
///
/// An optional [textDirection] argument can be used to set the text direction
/// (RTL or LTR) for the date picker. It defaults to the ambient text direction
/// provided by [Directionality]. If both [locale] and [textDirection] are not
/// null, [textDirection] overrides the direction chosen for the [locale].
///
/// The `context` argument is passed to [showDialog], the documentation for
/// which discusses how it is used.
///
/// See also:
///
///  * [showTimePicker]
///  * <https://material.google.com/components/pickers.html#pickers-date-pickers>
Future<DateTime> showDatePicker({
  @required BuildContext context,
  @required DateTime initialDate,
  @required DateTime firstDate,
  @required DateTime lastDate,
  SelectableDayPredicate selectableDayPredicate,
  DatePickerMode initialDatePickerMode: DatePickerMode.day,
  Locale locale,
  TextDirection textDirection,
}) async {
  assert(!initialDate.isBefore(firstDate), 'initialDate must be on or after firstDate');
  assert(!initialDate.isAfter(lastDate), 'initialDate must be on or before lastDate');
  assert(!firstDate.isAfter(lastDate), 'lastDate must be on or after firstDate');
  assert(
    selectableDayPredicate == null || selectableDayPredicate(initialDate),
    'Provided initialDate must satisfy provided selectableDayPredicate'
  );
  assert(initialDatePickerMode != null, 'initialDatePickerMode must not be null');

  Widget child = new _DatePickerDialog(
    initialDate: initialDate,
    firstDate: firstDate,
    lastDate: lastDate,
    selectableDayPredicate: selectableDayPredicate,
    initialDatePickerMode: initialDatePickerMode,
  );

  if (textDirection != null) {
    child = new Directionality(
      textDirection: textDirection,
      child: child,
    );
  }

  if (locale != null) {
    child = new Localizations.override(
      context: context,
      locale: locale,
      child: child,
    );
  }

  return await showDialog<DateTime>(
    context: context,
    child: child,
  );
}
