Calcular datas de ocorrência de EKRecurrenceRule

Here’s a general approach:

Step-by-step (in Swift):

1. Create an EKEvent with a recurrence rule.


2. Use EKEventStore to fetch events in a date range, which will include the recurring ones.



Example in Swift:

import EventKit

let store = EKEventStore()
store.requestAccess(to: .event) { (granted, error) in
    if granted {
        let calendars = store.calendars(for: .event)
        let startDate = Date()
        let endDate = Calendar.current.date(byAdding: .year, value: 1, to: startDate)!

        let predicate = store.predicateForEvents(withStart: startDate, end: endDate, calendars: calendars)

        let events = store.events(matching: predicate)
        
        let recurringEvents = events.filter { $0.recurrenceRules != nil }
        
        for event in recurringEvents {
            print("Event:

Foi isso que eu criei. Não está completo (algumas das regras mais esotéricas de mês/ano precisam de algum ajuste), mas é um começo e bom o suficiente para dados de calendário mais simples.

@interface EKRecurrenceRule ()
-(NSArray<NSDate*>*)occurrencesForEventStart:(NSDate*)eventStart rangeStart:(NSDate*)rangeStart rangeEnd:(NSDate*)rangeEnd;
@end

@implementation EKRecurrenceRule ()
-(NSArray<NSDate*>*)occurrencesForEventStart:(NSDate*)eventStart rangeStart:(NSDate*)rangeStart rangeEnd:(NSDate*)rangeEnd
{
    static NSCalendar *calendar = [NSCalendar.alloc initWithCalendarIdentifier:NSCalendarIdentifierGregorian];

    //  daily every N days
    //  weekly every N weeks (on days)
    //  monthly every N months (on days of the month)
    //  monthly every N months (on the 1st/2nd/3rd/4th/5th/last day/weekday/weekend day/sun-mon-tue etc of the month)
    //  yearly every N years (in Jan/Feb/Mar etc) (on the 1st/2nd/3rd/4th/5th/last day/weekday/weekend day/sun-mon-tue etc of the month)

    //  always fix H:M:S

    NSCalendarUnit hms = NSCalendarUnitHour | NSCalendarUnitMinute | NSCalendarUnitSecond;

    //  make an array of NSDateComponents's for eg 1st and 3rd Sunday in the month

    NSMutableArray<NSDateComponents*> *componentss = NSMutableArray.array;
    switch( self.frequency )
    {
        case EKRecurrenceFrequencyDaily:
            [componentss addObject:[calendar components:hms fromDate:eventStart]];
            break;

        case EKRecurrenceFrequencyWeekly:
            if( self.daysOfTheWeek )
                for( EKRecurrenceDayOfWeek *dow in self.daysOfTheWeek )
                {
                    NSDateComponents *dc = [calendar components:hms fromDate:eventStart];
                    dc.weekday = dow.dayOfTheWeek;
                    [componentss addObject:dc];
                }
            else
                [componentss addObject:[calendar components:hms|NSCalendarUnitWeekday fromDate:eventStart]];
            break;

        case EKRecurrenceFrequencyMonthly:
            if( self.daysOfTheMonth )
            {
                for( NSNUmber *day in self.daysOfTheMonth )
                {
                    NSDateComponents *dc = [calendar components:hms|NSCalendarUnitDay fromDate:eventStart];
                    dc.day = day.integerValue;
                    [componentss addObject:dc];
                }
            }
            else if( self.daysOfTheWeek )
            {
                for( EKRecurrenceDayOfWeek *dow in self.daysOfTheWeek )
                {
                    NSDateComponents *dc = [calendar components:hms|NSCalendarUnitWeekday fromDate:eventStart];
                    dc.weekday = dow.dayOfTheWeek;
                    dc.weekdayOrdinal = dow.weekNumber;
                    [componentss addObject:dc];
                }];
            }
            else
                [componentss addObject:[calendar components:hms|NSCalendarUnitDay fromDate:eventStart]];
            break;

        case EKRecurrenceFrequencyYearly:
            [componentss addObject:[calendar components:hms|NSCalendarUnitDay|NSCalendarUnitMonth fromDate:eventStart]];
            break;
    }

    NSMutableArray<NSDate*> *occurrences = NSMutableArray.array;
    __block NSInteger interval = 1;

    //  enumerateDates seems to be buggy depending on this date, eg setting it to 1 second prior to the eventStart skips the 2nd expected occurrence
    //  I set it to eventStart - 1 day + 1 second

    NSDate *afterDate = [NSDate dateWithTimeInterval:-86400+1 sinceDate:eventStart];
    for( NSDateComponents *components in componentss )
    {
        [calendar enumerateDatesStartingAfterDate:afterDate
                                matchingComponents:components
                                           options:NSCalendarMatchStrictly
                                        usingBlock:^(NSDate *date, BOOL, BOOL *stop)
         {
             // reached end?

             if( [date compare:rangeEnd]==NSOrderedDescending
                || (self.recurrenceEnd && [date compare:self.recurrenceEnd.endDate]==NSOrderedDescending) )
             {
                 *stop = YES;
                 return;
             }

             // interval?

             if( interval==1 )
             {
                 // include this one, but only if after rangeStart

                 if( [rangeStart compare:date]==NSOrderedAscending )
                     [occurrences addObject:date];

                 // start counting down the interval

                 interval = self.interval;
             }
             else
             {
                 // don't include this one, and count down the interval

                 interval--;
             }
         }];
    }];

    //  sort dates and limit by number of occurrences

    [occurrences sortUsingSelector:@selector(compare:)];

    if( self.recurrenceEnd.occurrenceCount )
        [occurrences removeObjectsInRange:NSMakeRange(self.recurrenceEnd.occurrenceCount,
                                                      occurrences.count-self.recurrenceEnd.occurrenceCount)];
    return occurrences;
}

@end