Xcode - Set week of month, then get the week of year

Question

Currently trying to use NSDateFormatter and NSDateComponents to set the week number for a month, then find the week of year that would be.

Let’s say we have a date: 20/05/2020, then set the week number to 0, or 1, or 2, etc.. then find out the week of year from that.

So far with NSDateComponents I’ve tried doing:

NSDate *date = self.date; // 20/05/2020
NSCalendar *calendar = [NSCalendar currentCalendar];
NSDateComponents *components = [calendar components:NSCalendarUnitMonth | NSCalendarUnitYear fromDate:date];

[components setWeekOfMonth:1];
NSDate *newDate = [calendar dateFromComponents:components];

NSInteger weekOfYear = [calendar components:NSCalendarUnitWeekOfYear fromDate:newDate].weekOfYear;

I'm always getting the output 18.

Expected output:

Change week of month to: 1 | output: 19 (4th)
Change week of month to: 2 | output: 20 (11th)
Change week of month to: 3 | output: 21 (18th)
Change week of month to: 4 | output: 22 (25th)

Answer 1

The problem is setting setWeekOfMonth in the date components has no effect.

Instead set the weekday and weekdayOrdinal

NSDate *date = self.date; // 20/05/2020
NSCalendar *calendar = [NSCalendar currentCalendar];
NSDateComponents *components = [calendar components:NSCalendarUnitMonth | NSCalendarUnitYear fromDate:date];
components.weekday = 2;
components.weekdayOrdinal = 3;
NSDate *newDate = [calendar dateFromComponents:components];

NSInteger weekOfYear = [calendar components:NSCalendarUnitWeekOfYear fromDate:newDate].weekOfYear;

---

This is a different approach:

It calculates the first monday in the current month, then it enumerates the next mondays until the end of the month.

- (NSArray *)yearOfWeeksInCurrentMonth
{
    NSCalendar *calendar = [NSCalendar currentCalendar];
    NSDateComponents *components = [calendar components: NSCalendarUnitMonth | NSCalendarUnitYear fromDate:[NSDate date]];
    components.weekday = 2;
    components.weekdayOrdinal = 1;

    NSDate *firstMonday = [calendar dateFromComponents:components];
    NSInteger weekOfYear = [calendar component:NSCalendarUnitWeekOfYear fromDate:firstMonday];

    __block NSMutableArray *result = [NSMutableArray array];
    [result addObject:@[firstMonday, @(weekOfYear)]];
    NSDateComponents *weekComponents = [calendar components: NSCalendarUnitWeekday fromDate:firstMonday];
    [calendar enumerateDatesStartingAfterDate:firstMonday matchingComponents:weekComponents options:NSCalendarMatchNextTime usingBlock:^(NSDate * _Nullable date, BOOL exactMatch, BOOL * _Nonnull stop) {
        if (date != nil) {
            if ( [calendar component:NSCalendarUnitMonth fromDate:date] > components.month ) {
                *stop = YES;
            } else {
                NSInteger weekOfYear = [calendar component:NSCalendarUnitWeekOfYear fromDate:date];
                [result addObject:@[date, @(weekOfYear)]];
            }
        }
    }];
    return [result copy];
}

The return value is an array containing the date (NSDate) and the corresponding weekOfYear (NSNumber)

(
        (
        "2020-05-03 22:00:00 +0000",
        19
    ),
        (
        "2020-05-10 22:00:00 +0000",
        20
    ),
        (
        "2020-05-17 22:00:00 +0000",
        21
    ),
        (
        "2020-05-24 22:00:00 +0000",
        22
    )
)