distance(from：to :)’不可用：任何String视图索引转换都可能在Swift 4中失败;请打开可选索引

我试图将我的应用程序迁移到 Swift 4,Xcode 9.我收到此错误.它来自第三方框架.

distance(from:to:)’ is unavailable: Any String view index conversion can fail in Swift 4; please unwrap the optional indices

func nsRange(from range: Range<String.Index>) -> NSRange {
    let utf16view = self.utf16
    let from = range.lowerBound.samePosition(in: utf16view)
    let to = range.upperBound.samePosition(in: utf16view)
    return NSMakeRange(utf16view.distance(from: utf16view.startIndex,to: from),// Error: distance(from:to:)' is unavailable: Any String view index conversion can fail in Swift 4; please unwrap the optional indices
                       utf16view.distance(from: from,to: to))// Error: distance(from:to:)' is unavailable: Any String view index conversion can fail in Swift 4; please unwrap the optional indices
}

你可以简单地打开这样的可选索引：

func nsRange(from range: Range<String.Index>) -> NSRange? {
    let utf16view = self.utf16
    if let from = range.lowerBound.samePosition(in: utf16view),let to = range.upperBound.samePosition(in: utf16view) {
       return NSMakeRange(utf16view.distance(from: utf16view.startIndex,utf16view.distance(from: from,to: to))
    }
    return nil
}