参考

Raw strings

SE-0200增加了 # 符号，使得写字符串更加简单。

在字符串中包含 " 时不必再加 \

//before
let rain = "The \"rain\" in \"Spain\" falls mainly on the Spaniards." 
//after
let rain = #"The "rain" in "Spain" falls mainly on the Spaniards."#

包含 \ 反斜杠也不需要再加转义符

// before
let keypaths = "Swift keypaths such as \\Person.name hold uninvoked references to properties."
// after
let keypaths = #"Swift keypaths such as \Person.name hold uninvoked references to properties."#

由于反斜杠作为字符串中的字符，所以在插入值的时候需要在后面再加个 #

  //before
 let answer = 42
 let dontpanic = "The answer to life, the universe, and everything is \(answer)"
 // after
 let answer = 42
 let dontpanic = #"The answer to life, the universe, and everything is \#(answer)"#

当字符串包含 # 时，前后应用 ## 包裹字符串
```
let str = ##"My dog said "woof"#gooddog"##
```

用 #""" 开头 """#结尾来表示多行字符串

  let multiline = #"""
  The answer to life,
  the universe,
  and everything is \#(answer).
  """#

由于不用反斜杠转义使得正则表达式更加简洁明了

 //before
 let regex1 = "\\\\[A-Z]+[A-Za-z]+\\.[a-z]+"
//after
 let regex2 = #"\\[A-Z]+[A-Za-z]+\.[a-z]+"#

Handling future enum cases

SE-0192 在枚举新增加一个 @unknown 修饰 default 分支,这样使得将来 enum 再增加一个 case 的时候，编译器会在该枚举的switch 代码里生成一个警告
比如

    enum PasswordError: Error {
          case short
          case obvious
          case simple
    }

    func showOld(error: PasswordError){
             switch error {
             case .short:
                      print("Your password was too short.")
             case .obvious:
                      print("Your password was too obvious.")
             default:
                      print("Your password was too simple.")
             }
    }

上面代码假如我们再加个 case old,执行代码时它会自动进入到default分支，可这并不是我们想要的结果，因为这个密码是一个旧密码而不是密码太简单，这时候可以用 @unknown，如下

  enum PasswordError: Error {
            case short
            case obvious
            case simple
            case old
   }

   func showNew(error: PasswordError) {
        switch error {
        case .short:
            print("Your password was too short.")
        case .obvious:
            print("Your password was too obvious.")
        @unknown default:
            print("Your password wasn't suitable.")
        }
    }

这时会产生一个警告 ,因为新增了case old ，switch没有明确地处理每一个分支。

Checking for integer muliples

SE-0225 integers 新增加了一个 isMultiple(of:) 方法
来检测一个数是否是另一个数的倍数

  let rowNumber = 4

  /*相当于 if rowNumber %  2 == 0
     改成方法调用更易懂，更方便调用（有代码补全）
 */
  if rowNumber.isMultiple(of:2) {
      print("Even")
  } else {
      print("Odd")
  }

Counting matching items in a sequence

SE-0220
Sequence 新增加了一个方法 count(where:) 相当于 filter() + count 的结果，但是它更加简洁一步到位

//before
let scores = [100,80,85]
let passCount = scores.filter{($0 >= 85)}.count

//after 避免生成一个数组
let scores = [100,80,85]
let passCount = scores.count { $0 >= 85}

这个方法适用于遵循了 Sequence 的所有类型，所以也可以用在集合和字典里。

Transforming and unwrapping dictionary values with compactMapValues()

SE-0218 为字典新增了一个方法 compactMapValues(),正如数组的compactMap函数一样，可以过滤nil,类型转换

    let times = [
        "Hudson": "38",
        "Clarke": "42",
        "Robinson": "35",
        "Hartis": "DNF"
    ]
    //通过compactMapValues 将值转换成integer类型，并且将DNF过滤掉
     let finishers1 = times.compactMapValues { Int($0) }
    //也可以这样写
     let finishers2 = times.compactMapValues(Int.init)
  
     // 过滤nil
     let people = [
        "Paul": 38,
        "Sophie": 8,
        "Charlotte": 5,
        "William": nil
      ]
     let knownAges = people.compactMapValues { $0 }

结尾

Swift的每次更新都使得写起代码越来越简单高效。
附上一个网站 What's new in Swift

Swift 5.0 新特性