Sometimes, String's Range<String.Index> and NSString's NSRange is not compatible. Here is an example.

String+NSRange.swift

import Foundation

// Because NSString using UTF-16, so the following answer is suitable.
// Update for Swift 4 (Xcode 9):
extension String {

    /// 将 Range 转为 NSRange
    ///
    /// - Parameter range: 要转换的range
    /// - Returns: 转换后的结果（如果range合法，那么nsRange就不会是nil）
    func nsRange(from range: Range<String.Index>) -> NSRange? {
        guard let from = range.lowerBound.samePosition(in: utf16), let to = range.upperBound.samePosition(in: utf16) else {
            return nil
        }
        return NSRange(location: utf16.distance(from: utf16.startIndex, to: from), length: utf16.distance(from: from, to: to))
    }


    /// 将 NSRange 转换为 Range
    ///
    /// - Parameter nsRange: 要转换的 NSRange
    /// - Returns: 转换后的结果
    func range(from nsRange: NSRange) -> Range<String.Index>? {
        guard let from16 = utf16.index(utf16.startIndex, offsetBy: nsRange.location, limitedBy: utf16.endIndex)
            , let to16 = utf16.index(from16, offsetBy: nsRange.length, limitedBy: utf16.endIndex)
            , let from = String.Index(from16, within: self)
            , let to = String.Index(to16, within: self)
            else { return nil }
        return from ..< to
    }
}


let cafe = "cafe\u{0301}"
cafe.count
cafe.utf16.count
for i in 0 ..< cafe.utf16.count {
    let nsRange = NSRange(location: i, length: 1)
    if let r = cafe.range(from: nsRange) { // "e" is not a substring, which is different from NSString
        let sub = String(cafe[r])
        print(cafe[r], "e".contains(sub), r)
    } else {
        let sub = (cafe as NSString).substring(with: nsRange)
        print(sub, "e".contains(sub), i)
    }
}

print()

for i in cafe {
    print(i)
}

1. "e" is not a substring, which is different from NSString;
2. when coloring "e" using NSAttributedString, "é" is colored.

总之，String 的 Range<String.Index> 和 NSString 的 NSRange 在能力上不是等价的。主要体现在取子串(substring)和属性字符串(attributedString)这两项能力上。