Very slow HTML rendering in NSAttributedString

Question

I have UITableView with dynamic sizing cells that displays list of comments in HTML format and I faced with the problem that NSAttributedString renders HTML content extremely slow!

Here is snapshot from profiler.

I tried to put the NSAttributedString initialization to separate thread, but still slow and user sees empty cells while HTML is being rendered and finally when it finished rendering cell is not layout properly.

    dispatch_async(GlobalQueue, {
       let html = NSAttributedString(
                    data: self.comment.htmlBody.dataUsingEncoding(NSUnicodeStringEncoding, allowLossyConversion: false)!,
                    options: [NSDocumentTypeDocumentAttribute: NSHTMLTextDocumentType],
                    documentAttributes: nil,
                    error: nil)

        dispatch_async(MainQueue, {
            self.commentLabel.attributedText = html
            self.commentLabel.font = UIFont(name: "HelveticaNeue-Light", size: 14.0)!

            if let writer = self.comment.author {
                self.authorLabel.text = writer.name
            }

            self.layoutIfNeeded()
      })
   })

Looks following

Please advice how to speed up rendering and fix cell layout.

Thanks!

UPDATE:

Solved with cell delegate and flag indicating that attributed string is initialized. Maybe would help somebody:

// TicketCell    
private var isContentInitialized = false
private var commentAttributedString:NSAttributedString?

var delegate: TicketCommentCellDelegate?
var indexPath: NSIndexPath!
var comment: TicketComment! {
    willSet {
        if newValue != self.comment {
            self.isContentInitialized = false
        }
    }
    didSet{
        self.configure()
    }
}

...
private func configure() {        
    if isContentInitialized {
        // here might be activity indicator stop
        ...
        if let writer = self.comment.author {
            self.authorLabel.text = writer.name
        }
    }
    else {
         // here might be activity indicator start

         dispatch_async(GlobalQueue, {
            self.commentAttributedString = NSAttributedString(
                                data: self.comment.htmlBody.dataUsingEncoding(NSUnicodeStringEncoding, allowLossyConversion: false)!,
                                options: [NSDocumentTypeDocumentAttribute: NSHTMLTextDocumentType],
                                documentAttributes: nil,
                                error: nil)                        

            self.isContentInitialized = true

            // here might be spinner stop
            dispatch_async(MainQueue, {
                self.delegate?.ticketCommentCellDidRenderCommentHTML(self)
            })
        })
    }
}

... 
protocol TicketCommentCellDelegate {
    func ticketCommentCellDidRenderCommentHTML(cell: TicketCommentCell)
}


// TableViewDataSource

func tableView(tableView: UITableView, cellForRowAtIndexPath indexPath: NSIndexPath) -> UITableViewCell {
    let cell = tableView.dequeueReusableCellWithIdentifier(kTicketCommentCellIdentifier, forIndexPath: indexPath) as! TicketCommentCell

    cell.indexPath = indexPath
    cell.delegate = self
    cell.comment = self.rows[indexPath.section][indexPath.row]

    return cell
}

// MARK: - TicketCommentCellDelegate

func ticketCommentCellDidRenderCommentHTML(cell: TicketCommentCell) {
    self.tableView.reloadRowsAtIndexPaths([cell.indexPath], withRowAnimation: UITableViewRowAnimation.Automatic)
}

// MARK: UITableViewDelegate

func tableView(tableView: UITableView, heightForRowAtIndexPath indexPath: NSIndexPath) -> CGFloat {        
   var cell = self.commentCell

   cell.comment = self.rows[indexPath.section][indexPath.row]
   cell.setNeedsDisplay()
   cell.setNeedsLayout()

   let height = cell.contentView.systemLayoutSizeFittingSize(UILayoutFittingCompressedSize).height + 1

    return height
}

Answer 1

ABout the slow parsing of the HTML into a string: The first time you create an attributed string of HTML, iOS creates all sorts of extra threads needed to parse the string, among which JavascriptCore engine.

Before parsing the first NSAttributedString from HTML:

And immediately after:

So you can imagine it takes almost a second sometimes to start this all up. Subsequent calls are much faster. My workaround was to parse HTML in the application:didFinishingLaunchingWithOptions: function in the Appdelegate, so that I had all necessary frameworks in memory when needed (Objective-C):

- (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions {
    NSMutableAttributedString *attrStringFromHtml = [[NSMutableAttributedString alloc]
                                                     initWithData: [@"<span>html enabled</span>" dataUsingEncoding:NSUnicodeStringEncoding
                                                                                allowLossyConversion:NO]
                                                     options:@{NSDocumentTypeDocumentAttribute: NSHTMLTextDocumentType}
                                                     documentAttributes:nil error:nil];
    NSLog(@"%@",[attrStringFromHtml string]);

    return YES;
}

Also see this answer.