let date = Date()

var date: [Int] = []
let calendar: Calendar = Calendar(identifier: .gregorian)
var comps: DateComponents = DateComponents()
comps = calendar.dateComponents([.year, .month, .day], from: Date())
date.append(comps.year!)
date.append(comps.month!)
date.append(comps.day!)

  func getCountOfDaysInMonth(year: Int, month: Int) -> (count: Int, week: Int) {
    let dateFormatter = DateFormatter()
    dateFormatter.dateFormat = "yyyy-MM"
    let date = dateFormatter.date(from: String(year)+"-"+String(month))
    let calendar: Calendar = Calendar(identifier: .gregorian)
    
    let range = calendar.range(of: .day, in: .month, for: date!)
    let week = calendar.component(.weekday, from: date!)
    return ((range?.count)!, week)
  }

EEEE: 代表一天的全名,比如Monday.使用1-3个E就代表简写,比如Mon.
MMMM: 代表一个月的全名,比如July.使用1-3个M就代表简写,比如Jul.
dd: 代表一个月里的几号,比如07或者30.
yyyy: 代表4个数字表示的年份,比如2016.
HH: 代表2个数字表示的小时,比如08或17.
mm: 代表2个数字表示的分钟,比如01或59.
ss: 代表2个数字表示的秒,比如2016.
zzz: 代表3个字母表示的时区,比如GTM(格林尼治标准时间,GMT+8为北京所在的时区,俗称东八区)
GGG: BC或者AD, 即公元前或者公元

extension ViewController: UICollectionViewDelegate, UICollectionViewDataSource {
  // 返回Section的数量
  func numberOfSections(in collectionView: UICollectionView) -> Int {
    return 0
  }
  // 返回Item的数量
  func collectionView(_ collectionView: UICollectionView, numberOfItemsInSection section: Int) -> Int {
    return 0
  }
  // 返回Cell
  func collectionView(_ collectionView: UICollectionView, cellForItemAt indexPath: IndexPath) -> UICollectionViewCell {
    let cell = collectionView.dequeueReusableCell(withReuseIdentifier: "dateItem", for: indexPath) as! dateCollectionViewCell
    return cell
  } 
}

override func viewDidLoad() {
    super.viewDidLoad()
    // Do any additional setup after loading the view, typically from a nib.
    // 这两句话很重要！！！
    CalendarCollectionView.dataSource = self
    CalendarCollectionView.delegate = self
  }

extension ViewController: UICollectionViewDelegate, UICollectionViewDataSource {
  // 返回Section的数量
  func numberOfSections(in collectionView: UICollectionView) -> Int {
    return 2
  }
  // 返回Item的数量
  func collectionView(_ collectionView: UICollectionView, numberOfItemsInSection section: Int) -> Int {
    if section == 0 {
      return weekArray.count
    } else {
      return 42
    }
  }
  // 返回Cell
  func collectionView(_ collectionView: UICollectionView, cellForItemAt indexPath: IndexPath) -> UICollectionViewCell {
    let cell = collectionView.dequeueReusableCell(withReuseIdentifier: "dateItem", for: indexPath) as! dateCollectionViewCell
    if indexPath.section == 0 {
      cell.textLabel.text = weekArray[indexPath.row]
    } else {
      var daysArray: [String] = []
      // 第一天之前的空白区域
      for number in 0..<firstDayOfMonth-1 {
        daysArray.append("")
      }
      for number in firstDayOfMonth-1...firstDayOfMonth+numberOfTheMonth-2 {
        daysArray.append(String(number-firstDayOfMonth+2))
      }
      // 最后一天后的空白区域
      for number in firstDayOfMonth+numberOfTheMonth-2...41 {
        daysArray.append("")
      }
      cell.textLabel.text = daysArray[indexPath.row]
    }
    return cell
  }
}

@IBAction func lastMonth(_ sender: UIButton) {
    if month == 1 {
      year -= 1
      month = 12
    }else {
      month -= 1
    }
    dateDisplayLabel.text = String(year)+"-"+String(month)
    firstDayOfMonth = date.getCountOfDaysInMonth(year: year, month: month).week
    numberOfTheMonth = date.getCountOfDaysInMonth(year: year, month: month).count
    CalendarCollectionView.reloadData()
  }