Cannot convert value of type NSAttributedString.DocumentAttributeKey to .DocumentReadingOptionKey

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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

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