iOS: 在Swift中优雅的实现Substring

在Swift中，当我们想要截取某个字符串时，方法如下：

let carNumber = "沪A12345"
let startIndex = advance(userCar.carPlateNumber.startIndex, 0)
let endIndex = advance(startIndex, 1)
let range = startIndex..<endIndex
println(carNumber[range]) // 输出沪

呃，好怀念substring方法啊，那我们就自己实现一个吧。

extension String {
    subscript (r: Range<Int>) -> String {
        get {
            let startIndex = advance(self.startIndex, r.startIndex)
            let endIndex = advance(startIndex, r.endIndex - r.startIndex)
            
            return self[Range(start: startIndex, end: endIndex)]
        }
    }
}

调用很简单，示例如下：

var s = "Hello, playground"
println(s[0...5]) // ==> "Hello,"
println(s[0..<5]) // ==> "Hello"

感谢Rob Napier http://stackoverflow.com/questions/24044851/how-do-you-use-string-substringwithrange-or-how-do-ranges-work-in-swift