swift的String处理

import UIKit
import CoreText

class ViewController: UIViewController {

    override func viewDidLoad() {
        super.viewDidLoad()

        print(UIFont.familyNames)

        let font = UIFont(name: "Heiti SC", size: 18)
        let textStr = "Hi, I'm lilei, what's your name please? Are you Han Meimei?"

        let label = UILabel(frame: CGRect(x: 10, y: 30, width: 100, height: 400))
        self.view.addSubview(label)
        label.font = font
        label.numberOfLines = 0
        label.text = textStr

        print(getLines(text: textStr, font: font! , rect: CGRect(x: 10, y: 30, width: 100, height: 400)))

    }

    func getLines(text: String, font: UIFont, rect:CGRect) -> [String] {

        let fontName: CFString = font.fontName as CFString
        let myFont = CTFontCreateWithName(fontName, font.pointSize, nil)
        let attStr = NSMutableAttributedString(string: text)
        attStr.addAttribute(kCTFontAttributeName as NSAttributedString.Key, value: myFont, range: NSMakeRange(0, attStr.length))
        let frameSetter: CTFramesetter = CTFramesetterCreateWithAttributedString(attStr)
        let path: CGMutablePath = CGMutablePath()
        path.addRect(CGRect(x: 0, y: 0, width: rect.size.width, height: CGFloat.greatestFiniteMagnitude))
        let frame: CTFrame = CTFramesetterCreateFrame(frameSetter, CFRangeMake(0, 0), path, nil)
        let lines = CTFrameGetLines(frame) as Array
        var linesArray = [String]()

        for line in lines {
            let lineRef: CTLine = line as! CTLine
            let lineRange: CFRange = CTLineGetStringRange(lineRef)
            let range: NSRange = NSMakeRange(lineRange.location, lineRange.length)
            let subStr = text.substring(from: range.location, length: range.length)
            linesArray.append(subStr)
        }

        return linesArray
    }

}

extension String {
    func substring(from: Int?, to: Int?) -> String {
        if let start = from {
            guard start < self.characters.count else {
                return ""
            }
        }

        if let end = to {
            guard end >= 0 else {
                return ""
            }
        }

        if let start = from, let end = to {
            guard end - start >= 0 else {
                return ""
            }
        }

        let startIndex: String.Index
        if let start = from, start >= 0 {
            startIndex = self.index(self.startIndex, offsetBy: start)
        } else {
            startIndex = self.startIndex
        }

        let endIndex: String.Index
        if let end = to, end >= 0, end < self.characters.count {
            endIndex = self.index(self.startIndex, offsetBy: end + 1)
        } else {
            endIndex = self.endIndex
        }

        return String(self[startIndex ..< endIndex])
    }

    func substring(from: Int) -> String {
        return self.substring(from: from, to: nil)
    }

    func substring(to: Int) -> String {
        return self.substring(from: nil, to: to)
    }

    func substring(from: Int?, length: Int) -> String {
        guard length > 0 else {
            return ""
        }

        let end: Int
        if let start = from, start > 0 {
            end = start + length - 1
        } else {
            end = length - 1
        }

        return self.substring(from: from, to: end)
    }

    func substring(length: Int, to: Int?) -> String {
        guard let end = to, end > 0, length > 0 else {
            return ""
        }

        let start: Int
        if let end = to, end - length > 0 {
            start = end - length + 1
        } else {
            start = 0
        }

        return self.substring(from: start, to: to)
    }
}