如何计算本周的第一个 NSDate？

Question

即：

  NSDate *firstDayOfWeek = [[NSDate date] firstDayOfWeek];

例如，今天是 8 月 19 日，我想从上面的代码行中获取 NSDate 或 2010-08-15 12:00am。谢谢！

Answer 1

对于 Swift 4.1：

extension Date {
  var startOfWeek: Date? {
        let calendar = Calendar(identifier: .gregorian)
        return calendar.date(from: calendar.dateComponents([.yearForWeekOfYear, .weekOfYear], from: self))
    }
}

Answer 2

为什么这么复杂？ :)

func firstDateOfWeekWithDate(date: NSDate) -> NSDate {

    var beginningOfWeek: NSDate?

    calendar.rangeOfUnit(.WeekOfYear, startDate: &beginningOfWeek, interval: nil, forDate: date)

    return beginningOfWeek!

}

Answer 3

这已经被回答了，但是作为Wolfgang has pointed out，在接受的答案中有一个错误。如果将第一个工作日设置为大于相关NSDate 的日期，则结果将不正确。这是一个适用于所有可能的firstWeekday 值的完整解决方案（示例使用星期一）。

- (NSDate *)weekOfDate:(NSDate *)originalDate
{
    NSCalendar *currentCal = [NSCalendar currentCalendar];
    [currentCal setFirstWeekday:2]; // Sun = 1, Mon = 2, etc.

    NSDateComponents *weekdayComponents = [currentCal components:NSCalendarUnitWeekday fromDate:originalDate];

    // Calculate the number of days to subtract and create NSDateComponents with them.
    NSDateComponents *componentsToSubtract = [[NSDateComponents alloc] init];
    NSInteger daysToSubtract = (([weekdayComponents weekday] - [currentCal firstWeekday]) + 7) % 7;
    [componentsToSubtract setDay:-1 * daysToSubtract];

    return [currentCal dateByAddingComponents:componentsToSubtract toDate:originalDate options:0];
}

Answer 4

最简单的解决方案：

func firstDayOfWeek(date: NSDate) -> NSDate {
  let calendar = NSCalendar.currentCalendar()
  var dateComponents = calendar.components(.CalendarUnitYear | .CalendarUnitMonth | .CalendarUnitWeekOfMonth, fromDate: date)
  dateComponents.weekday = 1
  return calendar.dateFromComponents(dateComponents)!
}

Answer 5

- (NSDate *)firstDateOfWeekWithDate:(NSDate *)date {

    NSCalendar * calendar = [NSCalendar currentCalendar];

    NSDateComponents *weekdayComponents = [calendar components:(NSDayCalendarUnit | NSWeekdayCalendarUnit) fromDate:date];

    NSDateComponents *components = [calendar components:(NSYearCalendarUnit | NSMonthCalendarUnit | NSDayCalendarUnit | NSWeekdayCalendarUnit) fromDate:date];
    [components setDay:-((([weekdayComponents weekday] - [calendar firstWeekday]) + 5 ) % 7)];

    return [calendar dateFromComponents:components];
}

Answer 6

如果星期一 (2) 设置为第一个工作日并且您正在检查的日期是星期日，则 naixn 的解决方案将产生错误的结果。在这种情况下，您将获得下周星期一的结果。

正确的方法是按如下方式计算减法分量：

[componentsToSubtract setDay: - ((([weekdayComponents weekday] - [gregorian firstWeekday])
                                  + 7 ) % 7)];

假设您每周总是有 7 天。

Answer 7

我认为此线程响应了您正在寻找的内容：http://www.cocoabuilder.com/archive/cocoa/211648-nsdatecomponents-question.html#211826

但请注意，它不会将星期一作为一周的第一天处理，因此您可能需要通过减去 [gregorian firstWeekday] 而不仅仅是 1 来稍微调整一下。另外，我将其修改为使用-currentCalendar，但这取决于你:-)

NSDate *today = [NSDate date];
NSCalendar *gregorian = [NSCalendar currentCalendar];

// Get the weekday component of the current date
NSDateComponents *weekdayComponents = [gregorian components:NSWeekdayCalendarUnit fromDate:today];
/*
Create a date components to represent the number of days to subtract
from the current date.
The weekday value for Sunday in the Gregorian calendar is 1, so
subtract 1 from the number
of days to subtract from the date in question.  (If today's Sunday,
subtract 0 days.)
*/
NSDateComponents *componentsToSubtract = [[NSDateComponents alloc] init];
/* Substract [gregorian firstWeekday] to handle first day of the week being something else than Sunday */
[componentsToSubtract setDay: - ([weekdayComponents weekday] - [gregorian firstWeekday])];
NSDate *beginningOfWeek = [gregorian dateByAddingComponents:componentsToSubtract toDate:today options:0];

/*
Optional step:
beginningOfWeek now has the same hour, minute, and second as the
original date (today).
To normalize to midnight, extract the year, month, and day components
and create a new date from those components.
*/
NSDateComponents *components = [gregorian components: (NSYearCalendarUnit | NSMonthCalendarUnit | NSDayCalendarUnit)
                                            fromDate: beginningOfWeek];
beginningOfWeek = [gregorian dateFromComponents: components];

Answer 8

使用NSDateComponents 并将weekday 设置为1。