DateComponentsIterator.swift

import Foundation

private extension DateComponents {
  func scaled(by: Int) -> DateComponents {
    let s: (Int?)->Int? = { $0.map { $0 * by } }
    return DateComponents(calendar: calendar,
                          timeZone: timeZone,
                          era: s(era),
                          year: s(year), month: s(month), day: s(day),
                          hour: s(hour), minute: s(minute), second: s(second), nanosecond: s(nanosecond),
                          weekday: s(weekday), weekdayOrdinal: s(weekdayOrdinal), quarter: s(quarter),
                          weekOfMonth: s(weekOfMonth), weekOfYear: s(weekOfYear), yearForWeekOfYear: s(yearForWeekOfYear))
  }
}

extension Calendar {

  func makeIterator(components: DateComponents, from date: Date, until: Date?) -> Calendar.DateComponentsIterator {
    return DateComponentsIterator(calendar: self, startDate: date, cutoff: until, components: components, count: 1)
  }

  func makeIterator(every component: Component, stride: Int = 1, from date: Date, until: Date?) -> Calendar.DateComponentsIterator {
    var components = DateComponents(); components.setValue(stride, for: component)
    return makeIterator(components: components, from: date, until: until)
  }

  struct DateComponentsIterator: IteratorProtocol {
    let calendar: Calendar
    let startDate: Date
    let cutoff: Date?
    let components: DateComponents
    var count: Optional<Int> = 1

    mutating func next() -> Date? {
      guard let count = self.count else { return nil } // Ended.
      guard let nextDate = calendar.date(byAdding: components.scaled(by: count), to: startDate) else {
        self.count = nil; return nil
      }
      if let cutoff = self.cutoff, nextDate > cutoff {
        self.count = nil; return nil
      }
      self.count = count + 1
      return nextDate
    }
  }
}

var iter = Calendar.current.makeIterator(every: .day, from: Date(), until: Date().addingTimeInterval(10_000_000))

IteratorSequence(iter).forEach { print($0) }