Convert HTML to NSAttributedString in iOS

Question

I am using a instance of UIWebView to process some text and color it correctly  it gives the result as HTML but rather than displaying it in the UIWebView I want to display it using Core Text with a NSAttributedString    I am able to create and draw the NSAttributedString but I am unsure how I can convert and map the HTML into the attributed string   I understand that under Mac OS X NSAttributedString has a initWithHTML  method but this was a Mac only addition and is not available for iOS   I also know that there is a similar question to this but it had no answers  I though I would try again and see whether anyone has created a way to do this and if so  if they could share it

User · Answer

Swift 3  Try this     extension String       func htmlAttributedString   - gt  NSAttributedString            guard let data   self data using  String Encoding utf16  allowLossyConversion  false  else   return nil           guard let html   try  NSMutableAttributedString              data  data              options   NSDocumentTypeDocumentAttribute  NSHTMLTextDocumentType               documentAttributes  nil  else   return nil           return html             And for using     let str     lt h1 gt Hello bro lt  h1 gt  lt h2 gt Come On lt  h2 gt  lt h3 gt Go sis lt  h3 gt  lt ul gt  lt li gt ME 1 lt  li gt  lt li gt ME 2 lt  li gt  lt  ul gt   lt p gt It is me bro   remember please lt  p gt    self contentLabel attributedText   str htmlAttributedString

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honoring font family  dynamic font I ve concocted this abomination  extension NSAttributedString       convenience fileprivate init  html  String  font  UIFont    Font dynamic style   subheadline                 guard let data   html data using  String Encoding utf8  allowLossyConversion  true  else           var totalString   html                     https   stackoverflow com questions 32660748 how-to-use-apples-new-san-francisco-font-on-a-webpage              AppleSystemUIFont I get in font familyName does not work          while -apple-system does                      var ffamily    quot -apple-system quot          if let font   font               let lLDBsucks   font familyName             if  lLDBsucks hasPrefix  quot  appleSystem quot                     ffamily   font familyName                           totalString    quot  lt style gt  nhtml    font-family    ffamily   important   n             lt  style gt  n quot    html                   guard let data   totalString data using  String Encoding utf8  allowLossyConversion  true  else               return nil                   assert Thread isMainThread          guard let attributedText   try   NSAttributedString data  data  options    documentType  NSAttributedString DocumentType html   characterEncoding  String Encoding utf8 rawValue   documentAttributes  nil  else               return nil                   let mutable   NSMutableAttributedString attributedString  attributedText          if let font   font           do               var found   false             mutable beginEditing               mutable enumerateAttribute NSAttributedString Key font  in  NSMakeRange 0  attributedText length   options  NSAttributedString EnumerationOptions rawValue  0      value  range  stop  in                     if let oldFont   value as  UIFont                           let newsize   oldFont pointSize   15   Font scaleHeruistic   12                         let newFont   oldFont withSize newsize                          mutable addAttribute NSAttributedString Key font  value  newFont  range  range                          found   true                                                         if  found                          No font was found - do something else                                 mutable endEditing                              mutable addAttribute  font  value  font  range  NSRange location  0  length  mutable length                     self init attributedString  mutable            alternatively you can use the versions this was derived from and set font on UILabel after setting attributedString this will clobber the size and boldness encapsulated in the attributestring though kudos for reading through all the answers up to here  You are a very patient man woman or child

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Made some modification on Andrew s solution and update the code to Swift 3   This code now use UITextView as self and able to inherit its original font  font size and text color  Note  toHexString   is extension from here  extension UITextView       func setAttributedStringFromHTML   htmlCode  String  completionBlock   escaping  NSAttributedString   - gt               let inputText      htmlCode  lt style gt body   font-family      self font  fontName      font-size    self font  pointSize   px  color     self textColor   toHexString       lt  style gt            guard let data   inputText data using  String Encoding utf16  else               print  Unable to decode data from html string    self                return completionBlock nil                     DispatchQueue main async               if let attributedString   try  NSAttributedString data  data  options   NSDocumentTypeDocumentAttribute  NSHTMLTextDocumentType   documentAttributes  nil                    self attributedText   attributedString                 completionBlock attributedString                else                   print  Unable to create attributed string from html string    self                    completionBlock nil                                    Example usage   mainTextView setAttributedStringFromHTML   lt i gt Hello world  lt  i gt        in

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This is a String extension written in Swift to return a HTML string as NSAttributedString   extension String       func htmlAttributedString   - gt  NSAttributedString            guard let data   self dataUsingEncoding NSUTF16StringEncoding  allowLossyConversion  false  else   return nil           guard let html   try  NSMutableAttributedString data  data  options   NSDocumentTypeDocumentAttribute  NSHTMLTextDocumentType   documentAttributes  nil  else   return nil           return html           To use    label attributedText     lt b gt Hello lt  b gt   u 2022  babe  htmlAttributedString     In the above  I have purposely added a unicode  u2022 to show that it renders unicode correctly    A trivial  The default encoding that NSAttributedString uses is NSUTF16StringEncoding  not UTF8

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Swift 3 0 Xcode 8 Version  func htmlAttributedString   - gt  NSAttributedString        guard let data   self data using  String Encoding utf16  allowLossyConversion  false  else   return nil       guard let html   try  NSMutableAttributedString data  data  options   NSDocumentTypeDocumentAttribute  NSHTMLTextDocumentType   documentAttributes  nil  else   return nil       return html

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Swift initializer extension on NSAttributedString  My inclination was to add this as an extension to NSAttributedString rather than String  I tried it as a static extension and an initializer  I prefer the initializer which is what I ve included below   Swift 4  internal convenience init  html  String        guard let data   html data using  String Encoding utf16  allowLossyConversion  false  else           return nil            guard let attributedString   try   NSAttributedString data  data  options    documentType  NSAttributedString DocumentType html   characterEncoding  String Encoding utf8 rawValue   documentAttributes  nil  else           return nil            self init attributedString  attributedString      Swift 3  extension NSAttributedString    internal convenience init  html  String        guard let data   html data using  String Encoding utf16  allowLossyConversion  false  else           return nil            guard let attributedString   try  NSMutableAttributedString data  data  options   NSAttributedString DocumentReadingOptionKey documentType  NSAttributedString DocumentType html   documentAttributes  nil  else           return nil            self init attributedString  attributedString        Example  let html     lt b gt Hello World  lt  b gt   let attributedString   NSAttributedString html  html

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Here is the Swift 5 version of Mobile Dan s answer  public extension NSAttributedString       convenience init    html  String            guard let data   html data using   unicode  else                   return nil                    try  self init data  data  options    documentType  NSAttributedString DocumentType html   documentAttributes  nil

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In iOS 7  UIKit added an initWithData options documentAttributes error  method which can initialize an NSAttributedString using HTML  eg     NSAttributedString alloc  initWithData  htmlString dataUsingEncoding NSUTF8StringEncoding                                    options   NSDocumentTypeDocumentAttribute  NSHTMLTextDocumentType                                             NSCharacterEncodingDocumentAttribute    NSUTF8StringEncoding                          documentAttributes nil error nil     In Swift   let htmlData   NSString string  details  data using  String Encoding unicode rawValue  let options    NSAttributedString DocumentReadingOptionKey documentType          NSAttributedString DocumentType html  let attributedString   try  NSMutableAttributedString data  htmlData    Data                                                              options  options                                                            documentAttributes  nil

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Using of NSHTMLTextDocumentType is slow and it is hard to control styles  I suggest you to try my library which is called Atributika  It has its own very fast HTML parser  Also you can have any tag names and define any style for them   Example   let str     lt strong gt Hello lt  strong gt  World   style tags      Style  strong   font  boldSystemFont ofSize  15    attributedString  label attributedText   str   You can find it here https   github com psharanda Atributika

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The only solution you have right now is to parse the HTML  build up some nodes with given point font etc attributes  then combine them together into an NSAttributedString  It s a lot of work  but if done correctly  can be reusable in the future

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The above solution is correct      NSAttributedString alloc  initWithData  htmlString dataUsingEncoding NSUTF8StringEncoding                                    options   NSDocumentTypeDocumentAttribute  NSHTMLTextDocumentType                                             NSCharacterEncodingDocumentAttribute    NSUTF8StringEncoding                          documentAttributes nil error nil     But the app wioll crash if you are running it on ios 8 1 2 or 3   To avoid the crash what you can do is   run this in a queue  So that it always be on main thread

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Helpful Extensions  Inspired by this thread  a pod  and Erica Sadun s ObjC example in iOS Gourmet Cookbook p 80  I wrote an extension on String and on NSAttributedString to go back and forth between HTML plain-strings and NSAttributedStrings and vice versa -- on GitHub here  which I have found helpful   The signatures are  again  full code in a Gist  link above    extension NSAttributedString       func encodedString ext  DocEXT  - gt  String      static func fromEncodedString   eString  String  ext  DocEXT  - gt  NSAttributedString       static func fromHTML   html  String  - gt  NSAttributedString     same as above  where ext    html    extension String       func attributedString ext  DocEXT  - gt  NSAttributedString     enum DocEXT  String   case rtfd  rtf  htm  html  txt

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Swift 4     NSAttributedString convenience initializer Without extra guards throws error     extension NSAttributedString        convenience init htmlString html  String  throws           try self init data  Data html utf8   options                 documentType  NSAttributedString DocumentType html               characterEncoding  String Encoding utf8 rawValue            documentAttributes  nil             Usage  UILabel attributedText   try  NSAttributedString htmlString    lt strong gt Hello lt  strong gt  World

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Creating an NSAttributedString from HTML must be done on the main thread   Update  It turns out that NSAttributedString HTML rendering depends on WebKit under the hood  and must be run on the main thread or it will occasionally crash the app with a SIGTRAP   New Relic crash log     Below is an updated thread-safe Swift 2 String extension   extension String       func attributedStringFromHTML completionBlock NSAttributedString  - gt               guard let data   dataUsingEncoding NSUTF8StringEncoding  else               print  Unable to decode data from html string    self                return completionBlock nil                     let options    NSDocumentTypeDocumentAttribute   NSHTMLTextDocumentType                     NSCharacterEncodingDocumentAttribute  NSNumber unsignedInteger NSUTF8StringEncoding            dispatch async dispatch get main queue                  if let attributedString   try  NSAttributedString data  data  options  options  documentAttributes  nil                    completionBlock attributedString                else                   print  Unable to create attributed string from html string    self                    completionBlock nil                                    Usage   let html     lt center gt Here is some  lt b gt HTML lt  b gt  lt  center gt   html attributedStringFromHTML   attString in     self bodyLabel attributedText   attString     Output

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The built in conversion always sets the text color to UIColor black  even if you pass an attributes dictionary with  forgroundColor set to something else  To support DARK mode on iOS 13  try this version of the extension on NSAttributedString   extension NSAttributedString       internal convenience init  html  String                               guard              let data   html data using  String Encoding utf16  allowLossyConversion  false  else   return nil            let options    DocumentReadingOptionKey   Any                   documentType  NSAttributedString DocumentType html               characterEncoding  String Encoding utf8 rawValue                    guard             let string   try  NSMutableAttributedString data  data  options  options                                                   documentAttributes  nil  else   return nil            if  available iOS 13                   let colour    NSAttributedString Key foregroundColor  UIColor label              string addAttributes colour  range  NSRange location  0  length  string length                      self init attributedString  string

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There is a work-in-progress open source addition to NSAttributedString by Oliver Drobnik at Github  It uses NSScanner for HTML parsing

[iphone] Convert HTML to NSAttributedString in iOS

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