A couple of week ago I blogged about using Pygments to do live syntax highlighting in the browser using Silverlight.

A major problem with the sample was that it did the pygmentizing on the UI thread which caused most browsers to become unresponsive. Today I wanted to fix that by using the BackgroundWorker to do the pygmentizing in a background thread.

Firstly I refactored the pygmentizing into a method that didn't interact with the UI.

def pygmentize_text(self, text, language):

    # attempt to pygmentize input with current language

        from pygments import highlight
        from pygments.lexers import get_lexer_by_name
        from pygments.formatters import HtmlFormatter

        lexer = get_lexer_by_name( language, stripall=True)
        formatter = HtmlFormatter(linenos=False, cssclass="source")
        markup = highlight(text, lexer, formatter)
        return markup


        return "Error Generating Markup"

I then added a method that could be passed into a DoWorkEventHandler. It gets it arguments as a tuple from the event arguments and then sets the event argument result with the marked up HTML. The lack of explicit typing and use of tuples is good example of how some python idioms can be used when working with the .NET framework.

def worker(self, sender, e):

    # do work off UI thread.
    e.Result = self.pygmentize_text(e.Argument[0],e.Argument[1])

The required BackgroundWorker and DoWorkEventHandler can be simply imported from the System.ComponentModel namespace.

from System.ComponentModel import BackgroundWorker, DoWorkEventHandler

The BackgroundWorker can then be setup and started. Again it's syntactically nice how the tuple can be created and passed as a RunWorkerAsync parameter.

def start_pygmentize(self):

    # update application state
    self.input_changed = False
    self.pygmentizing = True

    # get paremters
    input = self.input.GetProperty("value")
    language = self.language.value

    # setup background worker
    worker = BackgroundWorker()
    worker.DoWork += DoWorkEventHandler(self.worker)
    worker.RunWorkerCompleted += self.complete

    # start the worker
    worker.RunWorkerAsync( (input,language) )

The completed event handler is the responsible for taking the markup generated by the BackgroundWorker and updating the DOM. It also fires off another worker if the source has changed since the last worker started.

def complete(self, sender, e):

    if e.Error:

        # handle errors/exceptions in worker


        # show the result

    if self.input_changed:

        # input has changed, starty pygmentize again


        # no work queued
        self.pygmentizing = False

The update has made the sample much more responsive, however it appears downloading the Silverlight application is still causing some browers to become a little unresponsive which is annoying. I will be interested to find out if this effect can be mitigated.

The actually pygmentizing processing could possibly be made a little faster by reusing the BackgroundWorker and only doing the Pygment imports once but the responsiveness of the browser has improved the sample enormously.

Check out the updated demo here.