计算给定月份的周数 - Swift

Question

查看了一些关于 SO 的不同建议后，我无法确定为什么下面的功能不起作用。它似乎在某些月份返回 6，而在其他月份返回 5。当将星期更改为几天时，它可以完美地工作。

例如，尝试

weeksInMonth(8, forYear 2015) 

结果为 6。

我相信我对日历上的“firstWeekday”属性有什么误解，但苹果或网上没有找到充分的解释。

已尝试过 .WeekOfMonth 和 .WeekOfYear。再次找不到确切差异的解释。

非常欢迎任何建议。

func weeksInMonth(month: Int, forYear year: Int) -> Int?
{
    if (month < 1 || month > 12) { return nil }

    let dateString = String(format: "%4d/%d/01", year, month)
    let dateFormatter = NSDateFormatter()
    dateFormatter.dateFormat = "yyyy/MM/dd"
    if let date = dateFormatter.dateFromString(dateString),
       let calendar = NSCalendar(calendarIdentifier: NSCalendarIdentifierGregorian)
    {
        calendar.firstWeekday = 2 // Monday
        let weekRange = calendar.rangeOfUnit(.WeekOfMonth, inUnit: .Month, forDate: date)
        let weeksCount = weekRange.length
        return weeksCount
    }
    else
    {
        return nil
    }
}

更新：

抱歉，我的问题不清楚。我正在尝试计算一个月中有多少周，其中包括星期一。对于 8 月，这应该是 5。

Answer 1

您的代码计算发生的周数（全部或部分） 在一个月。您显然想要的是星期一的数量 在给定的月份。使用NSDateComponents，尤其是使用 weekdayOrdinal 属性，您可以计算第一个 (weekdayOrdinal=1) 和最后一个 (weekdayOrdinal=-1) 星期一 在一个月。然后计算以周为单位的差异（并加一）。

Swift 2 中的可能实现：

func numberOfMondaysInMonth(month: Int, forYear year: Int) -> Int?
{
    let calendar = NSCalendar(calendarIdentifier: NSCalendarIdentifierGregorian)!
    calendar.firstWeekday = 2 // 2 == Monday

    // First monday in month:
    let comps = NSDateComponents()
    comps.month = month
    comps.year = year
    comps.weekday = calendar.firstWeekday
    comps.weekdayOrdinal = 1
    guard let first = calendar.dateFromComponents(comps)  else {
        return nil
    }

    // Last monday in month:
    comps.weekdayOrdinal = -1
    guard let last = calendar.dateFromComponents(comps)  else {
        return nil
    }

    // Difference in weeks:
    let weeks = calendar.components(.WeekOfMonth, fromDate: first, toDate: last, options: [])
    return weeks.weekOfMonth + 1
}

注意： 从文档中看不到负数 weekdayOrdinal 从月底倒数。观察到 Determine NSDate for Memorial Day in a given year 和 confirmed by Dave DeLong)。

---

Swift 3 更新：

func numberOfMondaysInMonth(_ month: Int, forYear year: Int) -> Int? {
    var calendar = Calendar(identifier: .gregorian)
    calendar.firstWeekday = 2 // 2 == Monday

    // First monday in month:
    var comps = DateComponents(year: year, month: month,
                               weekday: calendar.firstWeekday, weekdayOrdinal: 1)
    guard let first = calendar.date(from: comps)  else {
        return nil
    }

    // Last monday in month:
    comps.weekdayOrdinal = -1
    guard let last = calendar.date(from: comps)  else {
        return nil
    }

    // Difference in weeks:
    let weeks = calendar.dateComponents([.weekOfMonth], from: first, to: last)
    return weeks.weekOfMonth! + 1
}

Answer 2

实际上您的问题是：给定月份有多少个星期一？

我的方法是计算当月的第一个星期一，这可以通过将 CalendarUnit WeekdayOrdinal 设置为 1 来完成。然后得到总天数并做一些数学运算。

斯威夫特 1.2

func mondaysInMonth(month: Int, forYear year: Int) -> Int?
{
  if 1...12 ~= month {

    let calendar = NSCalendar(calendarIdentifier: NSCalendarIdentifierGregorian)!
    let components = NSDateComponents()
    components.weekday = 2 // Monday
    components.weekdayOrdinal = 1
    components.month = month
    components.year = year

    if let date = calendar.dateFromComponents(components)  {
      let firstDay = calendar.component(.CalendarUnitDay, fromDate: date)
      let days = calendar.rangeOfUnit(.CalendarUnitDay, inUnit:.CalendarUnitMonth, forDate:date).length
      return (days - firstDay) / 7 + 1
    }
  }
  return nil
}

斯威夫特 2

func mondaysInMonth(month: Int, forYear year: Int) -> Int?
{
  guard 1...12 ~= month else { return nil }

  let calendar = NSCalendar(calendarIdentifier: NSCalendarIdentifierGregorian)!
  let components = NSDateComponents()
  components.weekday = 2 // Monday
  components.weekdayOrdinal = 1
  components.month = month
  components.year = year

  if let date = calendar.dateFromComponents(components)  {
    let firstDay = calendar.component(.Day, fromDate: date)
    let days = calendar.rangeOfUnit(.Day, inUnit:.Month, forDate:date).length
    return (days - firstDay) / 7 + 1
  }
  return nil
}

Answer 3

斯威夫特 2

以下代码计算一个月的周数。它不取决于月份的开始或结束日期。

func weeksInMonth(month: Int, year: Int) -> (Int)? {

  let calendar = NSCalendar.currentCalendar()

  let comps = NSDateComponents()
  comps.month = month+1
  comps.year = year
  comps.day = 0

  guard let last = calendar.dateFromComponents(comps) else {
    return nil
  }
  // Note: Could get other options as well
  let tag = calendar.components([.WeekOfMonth,.WeekOfYear,
       .YearForWeekOfYear,.Weekday,.Quarter],fromDate: last)

  return tag.weekOfMonth

}

// Example Usage:

if let numWeeks = weeksInMonth(8,year: 2015) {
  print(numWeeks) // Prints 6
}

if let numWeeks = weeksInMonth(12,year: 2015) {
  print(numWeeks) // Prints 5
}

Answer 4

斯威夫特 4+：

//-- Get number of weeks from calendar
func numberOfWeeksInMonth(_ date: Date) -> Int {
     var calendar = Calendar(identifier: .gregorian)
     calendar.firstWeekday = 1
     let weekRange = calendar.range(of: .weekOfMonth,
                                    in: .month,
                                    for: date)
     return weekRange!.count
}

//-- Implementation
let weekCount = numberOfWeeksInMonth(Date)

Answer 5

斯威夫特 4+：

从您的组件生成日期：

let dateComponents = DateComponents.init(year: 2019, month: 5)
let monthCurrentDayFirst = Calendar.current.date(from: dateComponents)!
let monthNextDayFirst = Calendar.current.date(byAdding: .month, value: 1, to: monthCurrentDayFirst)!
let monthCurrentDayLast = Calendar.current.date(byAdding: .day, value: -1, to: monthNextDayFirst)!

检测日期的周数：

let numberOfWeeks = Calendar.current.component(.weekOfMonth, from: monthCurrentDayLast)