确定给定年份的阵亡将士纪念日NSDate [英] Determine NSDate for Memorial Day in a given year

问题描述

是否有更好的方法来确定给定年份的阵亡将士纪念日(5月的最后一个星期一)?

Is there a better way to determine the NSDate for Memorial Day (the last Monday in May) in a given year?

NSInteger aGivenYear = 2013 ;

NSCalendar* calendar = [NSCalendar currentCalendar] ;
NSDateComponents* firstMondayInJuneComponents = [NSDateComponents new] ;
firstMondayInJuneComponents.month = 6 ;
// Thanks, Martin R., for pointing out that `weekOfMonth` is wrong for returning the first Monday in June.
firstMondayInJuneComponents.weekOfMonth = 1 ;
firstMondayInJuneComponents.weekday = 2 ; //Monday
firstMondayInJuneComponents.year = aGivenYear ;
NSDate* firstMondayInJune = [calendar dateFromComponents:firstMondayInJuneComponents] ;

NSDateComponents* subtractAWeekComponents = [NSDateComponents new] ;
subtractAWeekComponents.week = 0 ;
NSDate* memorialDay = [calendar dateByAddingComponents:subtractAWeekComponents toDate:firstMondayInJune options:0] ;

编辑

我现在看到上述示例中的firstMondayInJune并不能长期有效；它返回2012年5月28日.

I see now that firstMondayInJune in the above example doesn't work for all years; it returns May 28 for the year 2012.

谢谢，马丁R . weekdayOrdinal完全按照我的期望去做，它以少于三行代码的形式返回了阵亡将士纪念日:

Thanks, Martin R. weekdayOrdinal does exactly what I hoped, and it returns Memorial Day with 3 fewer lines of code:

NSInteger aGivenYear = 2013 ;

NSDateComponents* memorialDayComponents = [NSDateComponents new] ;
memorialDayComponents.year = aGivenYear ;
memorialDayComponents.month = 5 ;
memorialDayComponents.weekday = 2 ; //Monday
memorialDayComponents.weekdayOrdinal = -1 ; //The last instance of the specified weekday in the specified month & year.
NSDate* memorialDay = [calendar dateFromComponents:memorialDayComponents] ;

推荐答案

要获取一个月中的第一个星期一，请设置weekdayOrdinal = 1而不是weekOfMonth = 1:

To get the first Monday in a month, set weekdayOrdinal = 1 instead of weekOfMonth = 1:

NSInteger aGivenYear = 2012 ;

NSCalendar* calendar = [NSCalendar currentCalendar] ;
NSDateComponents* firstMondayInJuneComponents = [NSDateComponents new] ;
firstMondayInJuneComponents.month = 6 ;
firstMondayInJuneComponents.weekdayOrdinal = 1 ;
firstMondayInJuneComponents.weekday = 2 ; //Monday
firstMondayInJuneComponents.year = aGivenYear ;
NSDate* firstMondayInJune = [calendar dateFromComponents:firstMondayInJuneComponents] ;
// --> 2012-06-04

NSDateComponents* subtractAWeekComponents = [NSDateComponents new] ;
subtractAWeekComponents.week = -1 ;
NSDate* memorialDay = [calendar dateByAddingComponents:subtractAWeekComponents toDate:firstMondayInJune options:0] ;
// --> 2012-05-28

从NSDateComponents文档中:

工作日的序数单位代表工作日在广告中的位置 下一个较大的日历单位，例如月份.例如， 2 是 每月 秒 的工作日序数单位.

Weekday ordinal units represent the position of the weekday within the next larger calendar unit, such as the month. For example, 2 is the weekday ordinal unit for the second Friday of the month.

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