Code sample for displaying sharing bar when certain percentage scrolled

share_bar.erb

<div class="share-bar">
  <%= @post.tweet_button -%>
  <%= @post.share_button -%>
  <%# No point showing this on mobiles since they can't easily copy/paste
      the URL into an RSS app. %>
  <%= @post.rss_button unless mobile_device?  -%>
</div>

showWhenScrolled.coffee

# Show an element when we are scrolled down past a certain point on the page
# but hide it otherwise.
jQuery.fn.showWhenScrolled = (options = {}) ->
  $window = $(window)
  $element = @
  elementVisible = false

  if options.percentToShowBar
    # Calculate the point as a percentage of the height of the page.
    pointToShowBar = options.percentToShowBar * 0.01 * $(document).height()
  else
    pointToShowBar = options.pointToShowBar || 600

  checkPosition = ->
    if $window.scrollTop() > pointToShowBar and !elementVisible
      $element.slideDown 'fast', -> elementVisible = true
    else if $window.scrollTop() < pointToShowBar and elementVisible
      $element.slideUp 'fast', -> elementVisible = false

  # Using setInterval because listenting to the 'scroll' event causes a severe
  # preformance hit: http://ejohn.org/blog/learning-from-twitter/
  setInterval(checkPosition, 500)

jQuery ->
  $('.share-bar').showWhenScrolled(percentToShowBar: 30)