DateTimePicker.dart

// ref. https://medium.com/@gauthamns/date-time-input-fields-in-flutter-ae74ffbf4ef2
class DateTimePicker extends StatefulWidget {
  const DateTimePicker({
    Key key,
    this.labelText,
    @required
    this.selectedDate,
    @required
    this.selectedTime,
    this.onSelectDate,
    this.onSelectTime,
  }) : super(key: key);

  final String labelText;
  final DateTime selectedDate;
  final TimeOfDay selectedTime;
  final ValueChanged<DateTime> onSelectDate;
  final ValueChanged<TimeOfDay> onSelectTime;

  @override
  State<DateTimePicker> createState() => _DateTimePickerState();
}

class _DateTimePickerState extends State<DateTimePicker> {
  DateTime _selectedDate;

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

    initialize();
  }

  void initialize() {
    _selectedDate = widget.selectedDate;
    _selectedDate = _selectedDate.copyWith(
      hour: widget.selectedTime.hour,
      minute: widget.selectedTime.minute,
    );
  }

  @override
  void didUpdateWidget(DateTimePicker oldWidget) {
    if (oldWidget.selectedDate != widget.selectedDate
        || oldWidget.selectedTime != widget.selectedTime) {
      initialize();
    }
    super.didUpdateWidget(oldWidget);
  }


  Future<void> _selectDate(BuildContext context) async {
    final DateTime picked = await showDatePicker(
        context: context,
        initialDate: _selectedDate,
        firstDate: DateTime(1970, 8),
        lastDate: DateTime.now()
    );
    if (picked != null && picked != _selectedDate) {
      setState(() {
        _selectedDate = picked;
      });
      widget.onSelectDate?.call(picked);
    }
  }

  Future<void> _selectTime(BuildContext context) async {
    final TimeOfDay picked = await showTimePicker(
        context: context,
        initialTime: _selectedDate.toTimeOfDay()
    );
    if (picked != null && picked != _selectedDate.toTimeOfDay()) {
      setState(() {
        _selectedDate = _selectedDate.copyWith(
          hour: picked.hour,
          minute: picked.minute,
        );
      });
      widget.onSelectTime?.call(picked);
    }
  }

  @override
  Widget build(BuildContext context) {
    final TextStyle valueStyle = Theme.of(context).textTheme.body1;
    return Row(
      crossAxisAlignment: CrossAxisAlignment.end,
      children: <Widget>[
        Expanded(
          flex: 4,
          child: _InputDropdown(
            labelText: widget.labelText,
            valueText: intl.DateFormat.yMMMd().format(_selectedDate),
            valueStyle: valueStyle,
            onPressed: () {
              _selectDate(context);
            },
          ),
        ),
        const SizedBox(width: 12.0),
        Expanded(
          flex: 3,
          child: _InputDropdown(
            valueText: _selectedDate.toTimeOfDay().format(context),
            valueStyle: valueStyle,
            onPressed: () {
              _selectTime(context);
            },
          ),
        ),
      ],
    );
  }
}

class _InputDropdown extends StatelessWidget {
  const _InputDropdown(
      {Key key,
        this.child,
        this.labelText,
        this.valueText,
        this.valueStyle,
        this.onPressed})
      : super(key: key);

  final String labelText;
  final String valueText;
  final TextStyle valueStyle;
  final VoidCallback onPressed;
  final Widget child;

  @override
  Widget build(BuildContext context) {
    return InkWell(
      onTap: onPressed,
      child: InputDecorator(
        decoration: InputDecoration(
          labelText: labelText,
        ),
        baseStyle: valueStyle,
        child: Row(
          mainAxisAlignment: MainAxisAlignment.spaceBetween,
          mainAxisSize: MainAxisSize.min,
          children: <Widget>[
            Text(valueText, style: valueStyle),
            Icon(Icons.arrow_drop_down,
                color: Theme.of(context).brightness == Brightness.light
                    ? Colors.grey.shade700
                    : Colors.white70),
          ],
        ),
      ),
    );
  }
}