（本文代码已升级至swift3）
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]) |
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 = 2000000000.0print(value.mm())print(value.cm())print(value.dm())print(value.m())print(value.km()) |
