[Swift 3.0]String的Extension--字符串的截取

在Swift中进行字符串的截取的时候, 不像使用NSString那么方便, 这是因为String的截取方法substring(from: <#T##String.Index#>)所需要的参数不是Int类型, 这在使用的时候就很不方便, 这里我对String写了一个Extension, 用法十分简单:

extension String {    // 字符串的第一个字符    var first: String {        return self.substringTo(0)    }    // 字符串的最后一个字符    var last: String {        return self.substringFrom(self.characters.count - 1)    }    // 字符串开始到第index    func substringTo(_ index: Int) -> String {        guard index < self.characters.count else {            assertionFailure("index beyound the length of the string")            return ""        }        let theIndex = self.index(self.startIndex, offsetBy: index + 1)        return self.substring(to: theIndex)    }    // 从第index个开始到结尾的字符    func substringFrom(_ index: Int) -> String {        guard index < self.characters.count else {            assertionFailure("index beyound the length of the string")            return ""        }        guard index >= 0 else {            assertionFailure("index can't be lower than 0")            return ""        }        let theIndex = self.index(self.endIndex, offsetBy: index - self.characters.count)        return self.substring(from: theIndex)    }    // 某个闭区间内的字符    func substringInRange(_ range: CountableClosedRange<Int>) -> String {        guard range.lowerBound >= 0 else {            assertionFailure("lowerBound of the Range can't be lower than 0")            return ""        }        guard range.upperBound < self.characters.count else {            assertionFailure("upperBound of the Range beyound the length of the string")            return ""        }        let start = self.index(self.startIndex, offsetBy: range.lowerBound)        let end = self.index(self.startIndex, offsetBy: range.upperBound + 1)        let strRange = Range.init(uncheckedBounds: (lower: start, upper: end))        return self.substring(with: strRange)    }}

使用:

let str = "我爱写BUG,写BUG使我快乐"        print("前部分字符: \(str.substringTo(3))")        print("后部分字符: \(str.substringFrom(11))")        print("区域: \(str.substringInRange(10...13))")        print("第一个字符串: \(str.first)")        print("最好一个字符: \(str.last)")

输出:

前部分字符: 我爱写B后部分字符: 使我快乐区域: G使我快第一个字符串: 我最好一个字符: 乐

是不是很简单呢?