Swift - 类扩展（extension）

（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：