（本文代码已升级至swift3）

Swift语言的类扩展是一个强大的工具，我们可以通过类扩展完成如下事情：

1，给已有的类添加计算属性和计算静态属性

2，定义新的实例方法和类方法

3，提供新的构造器

4，定义下标脚本

5，是一个已有的类型符合某个协议

（注意：扩展只能添加新的计算型属性，不能添加存储型属性，也不能添加新的属性监视器）

示例1：给字符串String类添加下标脚本，支持索引访问

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extension String {      subscript(start: Int , length: Int ) -> String          {          get {              let index1 = self .index( self .startIndex, offsetBy: start)              let index2 = self .index(index1, offsetBy: length)              let range = Range (uncheckedBounds: (lower: index1, upper: index2))              return self .substring(with: range)          }          set {              let tmp = self              var s = ""              var e = ""              for (idx, item) in tmp.characters.enumerated() {                  if (idx < start)                  {                      s += "\(item)"                  }                  if (idx >= start + length)                  {                      e += "\(item)"                  }              }              self = s + newValue + e          }      }      subscript(index: Int ) -> String          {          get {              return String ( self [ self .index( self .startIndex, offsetBy: index)])          }          set {              let tmp = self              self = ""              for (idx, item) in tmp.characters.enumerated() {                  if idx == index {                      self += "\(newValue)"                  } else {                      self += "\(item)"                  }              }          }      } }   var str = "hangge.com" print (str[7,3]) print (str[7])   str[7,3] = "COM" str[0] = "H"   print (str[0,10])

运行结果如下： 

示例2：给Double增加mm，cm等方法，进行进制转换

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extension Double {      func mm()-> String      {          return "\(self/1)mm"      }      func cm()-> String      {         return "\(self/10)cm"      }            func dm()-> String {          return "\(self/100)dm"      }            func m()-> String      {          return "\(self/1000)m"      }      func km()-> String      {          return "\(self/(1000*1000))km"      } }   let value = 2000000000.0 print (value.mm()) print (value.cm()) print (value.dm()) print (value.m()) print (value.km())

运行结果如下：