Support for year pickers

[hamishjohnson]

Thanks for the awesome package, it's exactly what I needed.

The only extra I'd like is YearPicker.

At the moment I'm using the _YearPicker from flutter's CalendarDatePicker, and it mostly works fine but the styling is slightly inconsistent. Here's the code, with some minor changes I made.

class _YearPicker extends StatefulWidget {
  /// Creates a year picker.
  ///
  /// The [currentDate, [firstDate], [lastDate], [selectedDate], and [onChanged]
  /// arguments must be non-null. The [lastDate] must be after the [firstDate].
  _YearPicker({
    Key key,
    @required this.currentDate,
    @required this.firstDate,
    @required this.lastDate,
    @required this.initialDate,
    @required this.selectedDate,
    @required this.onChanged,
  })  : assert(currentDate != null),
        assert(firstDate != null),
        assert(lastDate != null),
        assert(initialDate != null),
        assert(selectedDate != null),
        assert(onChanged != null),
        assert(!firstDate.isAfter(lastDate)),
        super(key: key);

  /// The current date.
  ///
  /// This date is subtly highlighted in the picker.
  final DateTime currentDate;

  /// 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 initial date to center the year display around.
  final DateTime initialDate;

  /// 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;

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

class _YearPickerState extends State<_YearPicker> {
  ScrollController scrollController;

  // The approximate number of years necessary to fill the available space.
  static const int minYears = 18;

  @override
  void initState() {
    super.initState();

    // Set the scroll position to approximately center the initial year.
    final int initialYearIndex =
        widget.selectedDate.year - widget.firstDate.year;
    final int initialYearRow = initialYearIndex ~/ _yearPickerColumnCount;
    // Move the offset down by 2 rows to approximately center it.
    final int centeredYearRow = initialYearRow - 2;
    final double scrollOffset =
        _itemCount < minYears ? 0 : centeredYearRow * _yearPickerRowHeight;
    scrollController = ScrollController(initialScrollOffset: scrollOffset);
  }

  Widget _buildYearItem(BuildContext context, int index) {
    final ColorScheme colorScheme = Theme.of(context).colorScheme;
    final TextTheme textTheme = Theme.of(context).textTheme;

    // Backfill the _YearPicker with disabled years if necessary.
    final int offset = _itemCount < minYears ? (minYears - _itemCount) ~/ 2 : 0;
    final int year = widget.firstDate.year + index - offset;
    final bool isSelected = year == widget.selectedDate.year;
    final bool isCurrentYear = year == widget.currentDate.year;
    final bool isDisabled =
        year < widget.firstDate.year || year > widget.lastDate.year;
    const double decorationHeight = 36.0;
    const double decorationWidth = 72.0;

    Color textColor;
    if (isSelected) {
      textColor = colorScheme.onPrimary;
    } else if (isDisabled) {
      textColor = colorScheme.onSurface.withOpacity(0.38);
    } else if (isCurrentYear) {
      textColor = colorScheme.primary;
    } else {
      textColor = colorScheme.onSurface.withOpacity(0.87);
    }
    final TextStyle itemStyle = textTheme.bodyText1?.apply(color: textColor);

    BoxDecoration decoration;
    if (isSelected) {
      decoration = BoxDecoration(
        color: colorScheme.primary,
        borderRadius: BorderRadius.circular(decorationHeight / 2),
        shape: BoxShape.rectangle,
      );
    } else if (isCurrentYear && !isDisabled) {
      decoration = BoxDecoration(
        border: Border.all(
          color: colorScheme.primary,
          width: 1,
        ),
        borderRadius: BorderRadius.circular(decorationHeight / 2),
        shape: BoxShape.rectangle,
      );
    }

    Widget yearItem = Center(
      child: Container(
        decoration: decoration,
        height: decorationHeight,
        width: decorationWidth,
        child: Center(
          child: Semantics(
            selected: isSelected,
            child: Text(year.toString(), style: itemStyle),
          ),
        ),
      ),
    );

    if (isDisabled) {
      yearItem = ExcludeSemantics(
        child: yearItem,
      );
    } else {
      yearItem = InkWell(
        key: ValueKey<int>(year),
        onTap: () {
          widget.onChanged(
            DateTime(
              year,
              widget.initialDate.month,
              widget.initialDate.day,
            ),
          );
        },
        child: yearItem,
      );
    }

    return yearItem;
  }

  int get _itemCount {
    return widget.lastDate.year - widget.firstDate.year + 1;
  }

  @override
  Widget build(BuildContext context) {
    return Column(
      children: <Widget>[
        Expanded(
          child: GridView.builder(
            controller: scrollController,
            gridDelegate: _yearPickerGridDelegate,
            itemBuilder: _buildYearItem,
            itemCount: math.max(_itemCount, minYears),
            padding: const EdgeInsets.symmetric(horizontal: _yearPickerPadding),
          ),
        ),
      ],
    );
  }
}

class _YearPickerGridDelegate extends SliverGridDelegate {
  const _YearPickerGridDelegate();

  @override
  SliverGridLayout getLayout(SliverConstraints constraints) {
    final double tileWidth = (constraints.crossAxisExtent -
            (_yearPickerColumnCount - 1) * _yearPickerRowSpacing) /
        _yearPickerColumnCount;
    return SliverGridRegularTileLayout(
      childCrossAxisExtent: tileWidth,
      childMainAxisExtent: _yearPickerRowHeight,
      crossAxisCount: _yearPickerColumnCount,
      crossAxisStride: tileWidth + _yearPickerRowSpacing,
      mainAxisStride: _yearPickerRowHeight,
      reverseCrossAxis: axisDirectionIsReversed(constraints.crossAxisDirection),
    );
  }

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

const _YearPickerGridDelegate _yearPickerGridDelegate =
    _YearPickerGridDelegate();

[MariaMelnik]

@magpiedigital thank you for your suggestion. I had thoughts about adding year picker here but for previous flutter version YearPicker was available to use. I think I will add YearPicker with options to some customization in future.