Cannot convert value of type NSAttributedString.DocumentAttributeKey to .DocumentReadingOptionKey

You need to pass one of the available NSAttributedString DocumentType options:

---

Hypertext Markup Language (HTML) document.

static let html: NSAttributedString.DocumentType

---

Plain text document.

static let plain: NSAttributedString.DocumentType

---

Rich text format document.

static let rtf: NSAttributedString.DocumentType

---

Rich text format with attachments document.

static let rtfd: NSAttributedString.DocumentType

---

In this case you will need to pass the first one (html) NSAttributedString.DocumentType.html

So the extension updated to Swift 4 should look like this:

extension NSAttributedString {
    convenience init(data: Data, documentType: DocumentType, encoding: String.Encoding = .utf8) throws {
        try self.init(data: data,
                      options: [.documentType: documentType,
                                .characterEncoding: encoding.rawValue],
                      documentAttributes: nil)
    }
    convenience init(html data: Data) throws {
        try self.init(data: data, documentType: .html)
    }
    convenience init(txt data: Data) throws {
        try self.init(data: data, documentType: .plain)
    }
    convenience init(rtf data: Data) throws {
        try self.init(data: data, documentType: .rtf)
    }
    convenience init(rtfd data: Data) throws {
        try self.init(data: data, documentType: .rtfd)
    }
}

---

extension StringProtocol {
    var data: Data { return Data(utf8) }
    var htmlToAttributedString: NSAttributedString? {
        do {
            return try .init(html: data)
        } catch {
            print("html error:", error)
            return nil
        }
    }
    var htmlDataToString: String? {
        return htmlToAttributedString?.string
    }
}

---

extension Data {
    var htmlToAttributedString: NSAttributedString? {
        do {
            return try .init(html: self)
        } catch {
            print("html error:", error)
            return nil
        }

    }
}

---

Playground Testing

let htmlString = "<style type=\"text/css\">#red{color:#F00}#green{color:#0F0}#blue{color: #00F; font-weight: Bold; font-size: 32}</style><span id=\"red\" >Red</span><span id=\"green\" > Green </span><span id=\"blue\">Blue</span>"

let htmlData = Data(htmlString.utf8)

htmlString.htmlToAttributedString
htmlData.htmlToAttributedString

Discussion The HTML importer should not be called from a background thread (that is, the options dictionary includes documentType with a value of html). It will try to synchronize with the main thread, fail, and time out. Calling it from the main thread works (but can still time out if the HTML contains references to external resources, which should be avoided at all costs). The HTML import mechanism is meant for implementing something like markdown (that is, text styles, colors, and so on), not for general HTML import

Had this after automatic conversion to Swift 4. Was fixed by changing from:

NSMutableAttributedString(data: data, 
   options: [NSAttributedString.DocumentAttributeKey.documentType : NSAttributedString.DocumentType.html], 
   documentAttributes: nil)

to:

NSMutableAttributedString(data: data,
   options: [.documentType : NSAttributedString.DocumentType.html],
   documentAttributes: nil) {

This works for me:

let attrStr = try! NSAttributedString(
    data: modifiedFont.data(using: .unicode, allowLossyConversion: true)!,
    options:[.documentType: NSAttributedString.DocumentType.html,
    .characterEncoding: String.Encoding.utf8.rawValue],
    documentAttributes: nil)

If you don’t add

.characterEncoding: String.Encoding.utf8.rawValue

the app will crash.