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

swift">

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swift plain">extension swift color1">String swift plain">{ swift spaces">     swift plain">subscript(start: swift color1">Int swift plain">, length: swift color1">Int swift plain">) -> swift color1">String swift spaces">         swift plain">{ swift spaces">         swift keyword">get swift plain">{ swift spaces">             swift keyword">let swift plain">index1 = swift keyword">self swift plain">.index( swift keyword">self swift plain">.startIndex, offsetBy: start) swift spaces">             swift keyword">let swift plain">index2 = swift keyword">self swift plain">.index(index1, offsetBy: length) swift spaces">             swift keyword">let swift plain">range = swift color2">Range swift plain">(uncheckedBounds: (lower: index1, upper: index2)) swift spaces">             swift keyword">return swift keyword">self swift plain">.substring(with: range) swift spaces">         swift plain">} swift spaces">         swift keyword">set swift plain">{ swift spaces">             swift keyword">let swift plain">tmp = swift keyword">self swift spaces">             swift keyword">var swift plain">s = swift string">"" swift spaces">             swift keyword">var swift plain">e = swift string">"" swift spaces">             swift keyword">for swift plain">(idx, item) swift keyword">in swift plain">tmp.characters.enumerated() { swift spaces">                 swift keyword">if swift plain">(idx < start) swift spaces">                 swift plain">{ swift spaces">                     swift plain">s += swift string">"\(item)" swift spaces">                 swift plain">} swift spaces">                 swift keyword">if swift plain">(idx >= start + length) swift spaces">                 swift plain">{ swift spaces">                     swift plain">e += swift string">"\(item)" swift spaces">                 swift plain">} swift spaces">             swift plain">} swift spaces">             swift keyword">self swift plain">= s + newValue + e swift spaces">         swift plain">} swift spaces">     swift plain">} swift spaces">     swift plain">subscript(index: swift color1">Int swift plain">) -> swift color1">String swift spaces">         swift plain">{ swift spaces">         swift keyword">get swift plain">{ swift spaces">             swift keyword">return swift color1">String swift plain">( swift keyword">self swift plain">[ swift keyword">self swift plain">.index( swift keyword">self swift plain">.startIndex, offsetBy: index)]) swift spaces">         swift plain">} swift spaces">         swift keyword">set swift plain">{ swift spaces">             swift keyword">let swift plain">tmp = swift keyword">self swift spaces">             swift keyword">self swift plain">= swift string">"" swift spaces">             swift keyword">for swift plain">(idx, item) swift keyword">in swift plain">tmp.characters.enumerated() { swift spaces">                 swift keyword">if swift plain">idx == index { swift spaces">                     swift keyword">self swift plain">+= swift string">"\(newValue)" swift spaces">                 swift plain">} swift keyword">else swift plain">{ swift spaces">                     swift keyword">self swift plain">+= swift string">"\(item)" swift spaces">                 swift plain">} swift spaces">             swift plain">} swift spaces">         swift plain">} swift spaces">     swift plain">} swift plain">}   swift keyword">var swift plain">str = swift string">"hangge.com" swift functions">print swift plain">(str[7,3]) swift functions">print swift plain">(str[7])   swift plain">str[7,3] = swift string">"COM" swift plain">str[0] = swift string">"H"   swift functions">print swift plain">(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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swift plain">extension swift color1">Double swift plain">{ swift spaces">     swift keyword">func swift plain">mm()-> swift color1">String swift spaces">     swift plain">{ swift spaces">         swift keyword">return swift string">"\(self/1)mm" swift spaces">     swift plain">} swift spaces">     swift keyword">func swift plain">cm()-> swift color1">String swift spaces">     swift plain">{ swift spaces">        swift keyword">return swift string">"\(self/10)cm" swift spaces">     swift plain">} swift spaces">      swift spaces">     swift keyword">func swift plain">dm()-> swift color1">String swift plain">{ swift spaces">         swift keyword">return swift string">"\(self/100)dm" swift spaces">     swift plain">} swift spaces">      swift spaces">     swift keyword">func swift plain">m()-> swift color1">String swift spaces">     swift plain">{ swift spaces">         swift keyword">return swift string">"\(self/1000)m" swift spaces">     swift plain">} swift spaces">     swift keyword">func swift plain">km()-> swift color1">String swift spaces">     swift plain">{ swift spaces">         swift keyword">return swift string">"\(self/(1000*1000))km" swift spaces">     swift plain">} swift plain">}   swift keyword">let swift plain">value = 2000000000.0 swift functions">print swift plain">(value.mm()) swift functions">print swift plain">(value.cm()) swift functions">print swift plain">(value.dm()) swift functions">print swift plain">(value.m()) swift functions">print swift plain">(value.km())

运行结果如下：