Swift 3 String截取字符串

Swift 3中截取字符串的方法有substring(with: Range<String.Index>, substring(to: String.Index), substring(from: String.Index)这三个方法。

使用的方法如下:

let text = "This is a text string"

        let start = text.index(text.startIndex, offsetBy: 4)
        let a = text.substring(from: start)
        let b = text.substring(to: start)
        let end = text.index(text.startIndex, offsetBy: 8)
	let c = text.substring(with: Range(uncheckedBounds: (start, end)))
	print("a: \(a)")
        print("b: \(b)")
        print("c: \(c)")

输出结果如下：

a:  is a text string
b: This
c:  is 

因为String.Index类型不是我们之前用的Int类型，所以截取字符串的话会有较多的代码量

如果觉得每次获取都要写一堆index比较麻烦的话，也可以使用extension来扩展String的subscript

e.g:

extension String {
    subscript(r: Range<Int>) -> String {
        get {
            let startIndex = self.index(self.startIndex, offsetBy: r.lowerBound)
            let endIndex = self.index(self.startIndex, offsetBy: r.upperBound+1)

            return self[Range(uncheckedBounds: (startIndex, endIndex))]
        }
    }

    subscript(start: Int, end: Int) -> String {
        get {
            let startIndex = self.index(self.startIndex, offsetBy: start)
            let endIndex = self.index(self.startIndex, offsetBy: end+1)
            return self[Range(uncheckedBounds: (startIndex, endIndex))]
        }
    }
}