问如何从DateTime Dart对象中获取一年中的第几天、第几周

EN

/// Calculates week number from a date as per https://en.wikipedia.org/wiki/ISO_week_date#Calculation
int weekNumber(DateTime date) {
  int dayOfYear = int.parse(DateFormat("D").format(date));
  return ((dayOfYear - date.weekday + 10) / 7).floor();
}

/// Calculates number of weeks for a given year as per https://en.wikipedia.org/wiki/ISO_week_date#Weeks_per_year
int numOfWeeks(int year) {
  DateTime dec28 = DateTime(year, 12, 28);
  int dayOfDec28 = int.parse(DateFormat("D").format(dec28));
  return ((dayOfDec28 - dec28.weekday + 10) / 7).floor();
}

/// Calculates week number from a date as per https://en.wikipedia.org/wiki/ISO_week_date#Calculation
int weekNumber(DateTime date) {
  int dayOfYear = int.parse(DateFormat("D").format(date));
  int woy =  ((dayOfYear - date.weekday + 10) / 7).floor();
  if (woy < 1) {
    woy = numOfWeeks(date.year - 1);
  } else if (woy > numOfWeeks(date.year)) {
    woy = 1;
  }
  return woy;
}

final date = someDate;
final diff = now.difference(new DateTime(date.year, 1, 1, 0, 0));
final diffInDays = diff.inDays;

final date = someDate;
final startOfYear = new DateTime(date.year, 1, 1, 0, 0);
final firstMonday = startOfYear.weekday;
final daysInFirstWeek = 8 - firstMonday;
final diff = date.difference(startOfYear);
var weeks = ((diff.inDays - daysInFirstWeek) / 7).ceil();
// It might differ how you want to treat the first week
if(daysInFirstWeek > 3) {
  weeks += 1;
}

final monthOfYear = new DateTime.now().month;

// This will return the day of year from now
Jiffy().dayOfYear; // 295

// You can also pass in a dateTime object
Jiffy(DateTime(2019, 1, 3)).dayOfYear; // 3

Jiffy().week; // 43

// You can also pass in an Array or Map
Jiffy([2019, 1, 3]).week; // 1

Jiffy().month; // 10

Jiffy({
    "year": 2019,
    "month": 1,
    "day": 3
  }).month; // 1