快速获取一个月的第一天

Question

我正在寻找一种方法来获取一个月的第一天（在 Swift 中）。 我想知道是星期一，星期二等...通过返回相应的数字。

我尝试了许多解决方案，例如获取 NSCalendar 组件 .weekDay 但没有一个工作。

Example :

print(getTheFirstDate("2016-2-18"))

// Should return : 0 (because the first day of February 2016 is Monday).

任何帮助将不胜感激。

Answer 1

首先您需要解析您的日期字符串，然后您可以使用 Calendar 方法 dateComponents 从该日期获取日历、年份和月份组件，并从这些组件创建一个新日期。然后您可以从中提取工作日日期组件：

Xcode 11.5 • Swift 5.2

extension Date {
    var weekday: Int { Calendar.current.component(.weekday, from: self) }
    var firstDayOfTheMonth: Date {
        Calendar.current.dateComponents([.calendar, .year,.month], from: self).date!
    }
}

---

extension String {
    static var dateFormatter: DateFormatter = {
        let formatter = DateFormatter()
        formatter.dateFormat = "yyyy-MM-dd"
        return formatter
    }()
    
    var date: Date? {
       String.dateFormatter.date(from: self)
    }
}

"2016-2-18".date?.firstDayOfTheMonth.weekday   // 2 = Monday (Sunday-Saturday 1-7)

Answer 2

extension Date {
   var startOfWeek: Date {
      let gregorian = Calendar(identifier: .gregorian)
      let sunday = gregorian.date(from: gregorian.dateComponents([.yearForWeekOfYear, .weekOfYear], from: self))
      return gregorian.date(byAdding: .day, value: 1, to: sunday!)!
   }

   var endOfWeek: Date {
      let gregorian = Calendar(identifier: .gregorian)
      let sunday = gregorian.date(from: gregorian.dateComponents([.yearForWeekOfYear, .weekOfYear], from: self))
      return gregorian.date(byAdding: .day, value: 7, to: sunday!)!
   }

   var startOfPreviousWeek: Date {
      let gregorian = Calendar(identifier: .gregorian)
      let sunday = gregorian.date(from: gregorian.dateComponents([.yearForWeekOfYear, .weekOfYear], from: self))
      return gregorian.date(byAdding: .day, value: -6, to: sunday!)!
   }

   var endOfPreviousWeek: Date {
      let gregorian = Calendar(identifier: .gregorian)
      let sunday = gregorian.date(from: gregorian.dateComponents([.yearForWeekOfYear, .weekOfYear], from: self))
      return gregorian.date(byAdding: .day, value: 0, to: sunday!)!
   }

   var startDateOfMonth: Date {
      return Calendar.current.date(from: Calendar.current.dateComponents([.year, .month], from: Calendar.current.startOfDay(for: self)))!
   }

   var endDateOfMonth: Date {
      return Calendar.current.date(byAdding: DateComponents(month: 1, day: -1), to: self.startDateOfMonth)!
   }

   var getPreviousMonthDate: Date {
      return Calendar.current.date(byAdding: .month, value: -1, to: self)!
   }

   var startDateOfPreviousMonth: Date {
      return getPreviousMonthDate.startDateOfMonth
   }

   var endOfPreviousMonth: Date {
      return getPreviousMonthDate.endDateOfMonth
   }
}

Answer 3

那么得到对应的星期几可以用：

var weekday = calendar!.component(NSCalendarUnit.Weekday, fromDate: yourNSDate)
var weekdayName = self.getDayOfWeek(weekday)

然后就可以调用我创建的这个函数，根据传入getDayOfWeek()的数字来获取星期几的对应名称

func getDayOfWeek(weekday:Int) -> String {
        if(weekday == 1) {
            return "Sunday"
        }
        else if(weekday == 2) {
            return "Monday"
        }
        else if(weekday == 3) {
            return "Tuesday"
        }
        else if(weekday == 4) {
            return "Wednesday"
        }
        else if(weekday == 5) {
            return "Thursday"
        }
        else if(weekday == 6) {
            return "Friday"
        }
        else {
            return "Saturday"
        }

    }

如果您使用字符串作为开始日期，您可以将其更改为 NSDate，如下所示：

let dateFormatter = NSDateFormatter()
dateFormatter.dateFormat = /*place date format here*/
let date = dateFormatter.dateFromString(/*your_date_string*/)

Answer 4

我喜欢 Leo 的回答，但如果您想将逻辑封装到单个函数中或不想使用扩展。

func getDayOfWeekForFirstDayOfMonthFromDateString(dateString:String) -> String? {

    let formatter = NSDateFormatter()
    formatter.dateFormat = "yyyy-MM-dd"

    if let date = formatter.dateFromString(dateString) {
        if let firstOfMonth = NSCalendar.currentCalendar().dateFromComponents(NSCalendar.currentCalendar().components([.Year, .Month], fromDate: date)) {
            formatter.dateFormat = "EEEE"
            return formatter.stringFromDate(firstOfMonth)
        }
    }
    return nil
}

编辑：调整代码以利用 Leo 的伟大建议，并借用他更简洁的创建月份对象的创建。谢谢，狮子座！