可可得到一周的第一天

如何获取约会一周的第一天

这似乎更容易，因为：

• 当一周从星期日开始时，我需要取回星期日日期
• 如果从星期一开始，我需要获取星期一的日期

输入日期是一周内的任何日期...我尝试了几种方法，但边缘情况使其变得困难

我做了一个函数，但它不能 100% 工作（不确定 [components setDay: -weekday + 2];）

- (NSDate *)firstDateOfWeek {

    NSCalendar * calendar = [NSCalendar currentCalendar];

    NSDateComponents *weekdayComponents = [calendar components:(NSDayCalendarUnit | NSWeekdayCalendarUnit) fromDate:self];
    // because sunday is 0, we need it to be 6
    NSInteger weekday = (([weekdayComponents weekday] + 6) % 7);

    NSDateComponents *components = [calendar components:(NSYearCalendarUnit | NSMonthCalendarUnit | NSDayCalendarUnit | NSWeekdayCalendarUnit) fromDate:self];
    [components setDay: -weekday + 2];

    return [calendar dateFromComponents:components];

}

使用起来更容易rangeOfUnitcalendar 方法，根据当前区域设置正确处理“一周的开始”：

NSDate *date = your date;
NSCalendar *calendar = [NSCalendar currentCalendar];
NSDate *startOfWeek;
[calendar rangeOfUnit:NSWeekOfYearCalendarUnit
            startDate:&startOfWeek
             interval:NULL
              forDate:date];

Using NSDateComponents，其工作原理如下（假设一周有 7 天）：

NSDate *date = your date;
NSDateComponents *comp = [calendar components:NSYearCalendarUnit|NSMonthCalendarUnit|NSDayCalendarUnit|NSWeekdayCalendarUnit
                                     fromDate:date];
NSDate *startOfDay = [calendar dateFromComponents:comp];
NSInteger diff = (NSInteger)[calendar firstWeekday] - (NSInteger)[comp weekday];
if (diff > 0)
    diff -= 7;
NSDateComponents *subtract = [[NSDateComponents alloc] init];
[subtract setDay:diff];
NSDate *startOfWeek = [calendar dateByAddingComponents:subtract toDate:startOfDay options:0];