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:

Swift 3.0

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)
    }

mutating func hiddenTelphone() {
        
        let start = self.index(self.startIndex, offsetBy: 3)
        let end = self.index(self.endIndex, offsetBy: -4)
        
        let range = Range.init(uncheckedBounds: (lower: start, upper: end))
        
        self.replaceSubrange(range, with: "****")
//        self.replaceSubrange(3...6, with: "****")
    }

}

Swift 4.0+

//
//  LQString.swift
//  LQProjectorSwift
//
//  Created by LiuQiqiang on 2018/6/30.
//  Copyright © 2018年 LiuQiqiang. All rights reserved.
//

import UIKit

extension String {
    
    /// 字符串的第一个字符
    var firstString: String {
        
        return self.substringTo(0)
    }
    
    /// 字符串的最后一个字符
    var lastString: String {
        
        return self.substringFrom(self.count - 1)
    }
    
    /// 字符串开始到第index
    ///
    /// - Parameter index: 结束索引
    /// - Returns: 子字符串
    func substringTo(_ index: Int) -> String {
        
        guard index < self.count else {
            assertionFailure("index beyound the length of the string")
            return ""
        }
        
        let theIndex = self.index(self.startIndex, offsetBy: index)
        
        return String(self[startIndex...theIndex])
    }
    
    /// 从第index个开始到结尾的字符
    ///
    /// - Parameter index: 开始索引
    /// - Returns: 子字符串
    func substringFrom(_ index: Int) -> String {
        
        guard index < self.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.count)
        
        return String(self[theIndex..<endIndex])
    }
    
    /// 某个闭区间内的字符
    ///
    /// - Parameter range: 闭区间，例如：1...6
    /// - Returns: 子字符串
    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.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)
        
        return String(self[start..<end])
    }
    
    /// 手机号字符串隐藏中间部分数字
    mutating func hiddenTelphone() {
        
        if self.count != 11 {
            return ;
        }
        
        let start = self.index(self.startIndex, offsetBy: 3)
        let end = self.index(self.endIndex, offsetBy: -4)
        
        let range = Range.init(uncheckedBounds: (lower: start, upper: end))
        
        self.replaceSubrange(range, with: "****")
        //        self.replaceSubrange(3...6, with: "****")
    }
    
}

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?