获取当天所在周的周一是什么时候

这个仅仅是为了 获取当天所在周的周一是什么时间

 - (NSString *)getMonthBeginAndEndWith:(NSDate *)newDate{
    if (newDate == nil) {
        newDate = [NSDate date];
    }
    double interval = 0;
    
    NSDate *beginDate = nil;
    
    NSDate *endDate = nil;
    
    NSCalendar *calendar = [NSCalendar currentCalendar];
    
    [calendar setFirstWeekday:2];//设定周一为周首日
    ////NSCalendarUnitWeekOfMonth     周
    BOOL ok = [calendar rangeOfUnit:NSCalendarUnitWeekOfMonth startDate:&beginDate interval:&interval forDate:newDate];
    
    //分别修改为 NSCalendarUnitDay NSWeekCalendarUnit NSYearCalendarUnit
    //NSCalendarUnitWeekOfMonth or NSCalendarUnitWeekOfYear,
    if (ok) {
        
        endDate = [beginDate dateByAddingTimeInterval:interval-1];
        
    }else {
        
        return nil;
        
    }
    
    NSDateFormatter *myDateFormatter = [[NSDateFormatter alloc] init];
    
    [myDateFormatter setDateFormat:@"yyyyMMdd"];
    
    NSString *beginString = [myDateFormatter stringFromDate:beginDate];

    return beginString;
    
    
}