在 Objective-C 中获取一年中的第一天

Question

我正在尝试使用 DateTools 查找周的第一天，如下所示：

for (NSInteger week = 46; week <= 53; week++) {
    NSDate *tempDate = [NSDate dateWithString:[NSString stringWithFormat:@"2015-%d", (int)week] formatString:@"Y-w"];

    NSLog(@"INFO: tempDate: %@, day: %.2d, week: %d", tempDate, (int)[tempDate day], (int)week);
}

for (NSInteger week = 1; week <= 5; week++) {
    NSDate *tempDate = [NSDate dateWithString:[NSString stringWithFormat:@"2016-%d", (int)week] formatString:@"Y-w"];

    NSLog(@"INFO: tempDate: %@, day: %.2d, week: %d", tempDate, (int)[tempDate day], (int)week);
}

我得到了这个输出：

INFO: tempDate: 2015-11-07 22:00:00 +0000, day: 08, week: 46
INFO: tempDate: 2015-11-14 22:00:00 +0000, day: 15, week: 47
INFO: tempDate: 2015-11-21 22:00:00 +0000, day: 22, week: 48
INFO: tempDate: 2015-11-28 22:00:00 +0000, day: 29, week: 49
INFO: tempDate: 2015-12-05 22:00:00 +0000, day: 06, week: 50
INFO: tempDate: 2015-12-12 22:00:00 +0000, day: 13, week: 51
INFO: tempDate: 2015-12-19 22:00:00 +0000, day: 20, week: 52
INFO: tempDate: 2015-12-26 22:00:00 +0000, day: 27, week: 53
INFO: tempDate: 2015-12-26 22:00:00 +0000, day: 27, week: 1
INFO: tempDate: 2016-01-02 22:00:00 +0000, day: 03, week: 2
INFO: tempDate: 2016-01-09 22:00:00 +0000, day: 10, week: 3
INFO: tempDate: 2016-01-16 22:00:00 +0000, day: 17, week: 4
INFO: tempDate: 2016-01-23 22:00:00 +0000, day: 24, week: 5

如您所见，2015 的第 53 周与 2016 的第 1 周在同一天 (This site告诉我2015年有53周).

实际上，从 2016 开始的 1 周是从 04.01.2016 开始的。

另外，请注意 dateWithString:formatString: 给出了一周第一天的前一天。这是为什么？我可以简单地使用 dateByAddingDays:1 但我不知道它是否有问题并且应该在其他地方解决问题。

我尝试使用 NSDateComponents 作为@DarkDust 提到的，但无济于事。所以这个：

for (NSInteger week = 1; week <= 5; week++) {
    NSDate *tempDate = [NSDate dateWithString:[NSString stringWithFormat:@"2016-%d", (int)week] formatString:@"Y-w"];

    NSDateComponents *dateComponents = [NSDateComponents new];

    dateComponents.year = 2016;
    dateComponents.weekOfYear = week;

    NSLog(@"INFO: tempDate: %@, day: %.2d, week: %d = %@", tempDate, (int)[tempDate day], (int)week, [calendar dateFromComponents:dateComponents]);
}

给我这个：

INFO: tempDate: 2015-12-26 22:00:00 +0000, day: 27, week: 1 = 2015-12-31 22:00:00 +0000
INFO: tempDate: 2016-01-02 22:00:00 +0000, day: 03, week: 2 = 2015-12-31 22:00:00 +0000
INFO: tempDate: 2016-01-09 22:00:00 +0000, day: 10, week: 3 = 2015-12-31 22:00:00 +0000
INFO: tempDate: 2016-01-16 22:00:00 +0000, day: 17, week: 4 = 2015-12-31 22:00:00 +0000
INFO: tempDate: 2016-01-23 22:00:00 +0000, day: 24, week: 5 = 2015-12-31 22:00:00 +0000

这是 DateTools' dateWithString:formatString: 的实现：

+ (NSDate *)dateWithString:(NSString *)dateString formatString:(NSString *)formatString {

    return [self dateWithString:dateString formatString:formatString timeZone:[NSTimeZone systemTimeZone]];
}

+ (NSDate *)dateWithString:(NSString *)dateString formatString:(NSString *)formatString timeZone:(NSTimeZone *)timeZone {

    static NSDateFormatter *parser = nil;
    static dispatch_once_t onceToken;
    dispatch_once(&onceToken, ^{
        parser = [[NSDateFormatter alloc] init];
    });

    parser.dateStyle = NSDateFormatterNoStyle;
    parser.timeStyle = NSDateFormatterNoStyle;
    parser.timeZone = timeZone;
    parser.dateFormat = formatString;

    return [parser dateFromString:dateString];
}

关于重复报告：

The top 3 are wrong, 2 of them with downvotes, and they don't even do what I need at all.

Answer 1

你的做法很糟糕。您必须了解日期是一个时间戳，可以根据您使用的时区和日历进行不同的解释。正如评论中已经提到的那样，您应该为您的解决方案使用日期组件。请注意，打印日期可能会使用不同的格式，并且结果可能不是预期的。

接下来一年中的第一个工作日取决于定义。如果我没记错的话，某些标准（或所有标准）将根据一周中的哪一天是第一天来对待一年中的第一周。换句话说，如果 1.1 是星期日，那么一年的第一周就是十二月，但如果是星期二，那么就是一月。

所以如果你想找到给定年份的第一个星期一的开始，代码应该看起来像这样（我没有测试它）：

+ (NSDate *)firstWeekInYear:(NSInteger)year {

        NSCalendar *calendar = [NSCalendar autoupdatingCurrentCalendar];

        NSDate *toReturn = nil;
        NSDateComponents *components = [[NSDateComponents alloc] init];
        components.year = year;
        NSDate *beginningOfTheYear = [calendar dateFromComponents:components];
        NSInteger day = [calendar component:NSCalendarUnitWeekday fromDate:beginningOfTheYear];
        NSInteger daysToAdd = (8-day)%7;

        toReturn = [calendar dateByAddingUnit:NSCalendarUnitDay value:daysToAdd toDate:beginningOfTheYear options:kNilOptions];

        return toReturn;
    }

因此您需要选择日历，创建具有目标年份的日期组件，从具有日历的组件中获取日期以获得年初。然后找出是星期几，然后增加这样的天数，结果就是一年中第一个星期一的开始。

Answer 2

我终于明白了。 @Matic 的解决方案在 iOS 7 中不起作用，而且很难理解，所以我设法找到了一个更简单的解决方案，也适用于 iOS 7：

+ (NSDate *)getFirstDayInWeek:(NSInteger)week ofYear:(NSInteger)year {

    /* create the calendar only once */

    static NSCalendar *calendar = nil;
    static dispatch_once_t onceToken;

    dispatch_once(&onceToken, ^{
        calendar = [[NSCalendar alloc] initWithCalendarIdentifier:NSCalendarIdentifierGregorian];

        calendar.firstWeekday = 2;
        calendar.minimumDaysInFirstWeek = 4;
    });

    NSDateComponents *components = [NSDateComponents new]; // create an empty date components object

    components.year = year;
    components.weekOfYear = week; // set the number of the week in the year
    components.weekday = calendar.firstWeekday; // important! set the result date's day of week

    return [calendar dateFromComponents:components];
}

在 Objective-C 中获取一年中的第一天

Get first day of week of year in Objective-C

objective-c

nsdate

ios