Get the Week Number in iOS SDK

ObjectiveC:

 NSCalendar *calendar = [NSCalendar currentCalendar];
 NSDateComponents *dateComponent = [calendar components:(NSWeekOfYearCalendarUnit | NSDayCalendarUnit | NSMonthCalendarUnit | NSYearCalendarUnit) fromDate:[NSDate date]];
 NSLog(@"%@",dateComponent);

Update

 NSCalendar *calendar = [NSCalendar currentCalendar];
    NSDateComponents *dateComponent = [calendar components:(NSCalendarUnitWeekOfYear | NSCalendarUnitDay | NSCalendarUnitMonth | NSCalendarUnitYear) fromDate:date];
    NSLog(@"%@",dateComponent);

Swift:

let calendar = NSCalendar.currentCalendar()
let dateComponent = calendar.components([.WeekOfYear, .Day, .Month, .Year], fromDate: NSDate(timeIntervalSinceNow: 0))
print("weekOfYear \(dateComponent.weekOfYear)")

Swift 3 - Swift 5:

let component = Calendar.current.component(.weekOfYear, from: Date())
print("Week of Year \(component)")

[NSCalendar currentCalendar] gives you a Gregorian Calendar as default. You can use

NSCalendar *calendar = [[NSCalendar alloc] initWithCalendarIdentifier:NSISO8601Calendar];

to get an ISO 08601 conform calendar, which has week number 37 for your date.