Infinite scroll support for Backbone.js' collection

backbone.infinitescroll.coffee

_.extend Backbone.Collection.prototype,
  options:
    infinitescroll:
      success: $.noop
      error: $.noop
      bufferPx: 40
      scrollPx: 150
      page:
        current: 0
        per: null
      state:
          isDuringAjax: false
          isDone: false
          isInvalid: false
      loading:
        wrapper: 'backbone-infinitescroll-wrapper'
        loadingId: 'backbone-infinitescroll-loading'
        loadingImg: 'loading.gif'
        loadingMsg: '<em>Loading ...</em>'
        finishedMsg: '<em>No more</em>'
        msg: null
        speed: 'fast'

  infinitescroll: (options={})->
    # NOTE: coffeescript cannot deal with nested scope!
    that = @

    _.extend(@options.infinitescroll, options.infinitescroll) if options.infinitescroll

    infinitescroll_options = @options.infinitescroll

    # where we want to place the load message in?
    infinitescroll_options.loading.wrapper = $(infinitescroll_options.loading.wrapper)
    if !infinitescroll_options.loading.msg and infinitescroll_options.loading.wrapper.size() > 0
      infinitescroll_options.loading.msg = $('<div/>', {
        id: infinitescroll_options.loading.loadingId
      }).html('<img alt="'+$(infinitescroll_options.loading.loadingMsg).text()+'" src="' + infinitescroll_options.loading.loadingImg + '" /><div>' + infinitescroll_options.loading.loadingMsg + '</div>')
      infinitescroll_options.loading.msg.appendTo(infinitescroll_options.loading.wrapper).hide()
    else
      infinitescroll_options.loading.msg = null

    fetch_options = _.omit(options, 'infinitescroll')
    infinitescroll_fetch = ()=>
      # mark the XHR request
      infinitescroll_options.state.isDuringAjax = true

      # increase page count
      infinitescroll_options.page.current++

      payload = {
        page: infinitescroll_options.page.current
      }
      payload['limit'] = infinitescroll_options.page.per if infinitescroll_options.page.per isnt null

      _.extend(fetch_options, {
        remove: false
        data: payload
      })

      if infinitescroll_options.loading.msg
        # preload loading.loadingImg
        (new Image()).src = infinitescroll_options.loading.loadingImg if infinitescroll_options.loading.loadingImg

        infinitescroll_options.loading.msg.fadeIn infinitescroll_options.loading.speed, ()->
          that.fetch(fetch_options)
          .success (data, state, jqXHR)=>
            infinitescroll_options.state.isDuringAjax = false
            infinitescroll_options.state.isDone = true if _.size(data) is 0

            infinitescroll_options.loading.msg.fadeOut infinitescroll_options.loading.speed, ()->
              infinitescroll_options.success.call(data, state, jqXHR) if _.isFunction(infinitescroll_options.success)
              return
            return
          .error (data, state, jqXHR)=>
            infinitescroll_options.state.isDuringAjax = false
            infinitescroll_options.state.isInvalid = true

            infinitescroll_options.loading.msg.fadeOut infinitescroll_options.loading.speed, ()->
              infinitescroll_options.error.call(data, state, jqXHR) if _.isFunction(infinitescroll_options.error)
              return
            return
          return

      else
        that.fetch(fetch_options)
        .success (data, state, jqXHR)=>
          infinitescroll_options.state.isDuringAjax = false
          infinitescroll_options.state.isDone = true if _.size(data) is 0

          infinitescroll_options.success.call(data, state, jqXHR) if _.isFunction(infinitescroll_options.success)
          return

        .error (data, state, jqXHR)=>
          infinitescroll_options.state.isDuringAjax = false
          infinitescroll_options.state.isInvalid = true

          infinitescroll_options.error.call(data, state, jqXHR) if _.isFunction(infinitescroll_options.error)
          return
      return

    $(document).scroll ()->
      $doc = $(document)
      isNearBottom = ()->
        bottomPx = 0 + $doc.height() - $doc.scrollTop() - $(window).height()

        # if distance remaining in the scroll (including buffer) is less than expected?
        (bottomPx - infinitescroll_options.bufferPx) < infinitescroll_options.scrollPx

      return if infinitescroll_options.state.isDuringAjax || infinitescroll_options.state.isDone || infinitescroll_options.state.isInvalid || !isNearBottom()

      infinitescroll_fetch()
      return


    infinitescroll_fetch()
    return

Usage

NOTE: You MUST pass success and error callbacks as options!

collection = new Backbone.Collection({
  url: 'path/to/your/resource'
})
collection.infinitescroll({
  infinitescroll:
    success: (collection, state, jqXHR)->
      # do something with the staff...
      return
    loading:
      wrapper: 'body'
      loadingId: 'backbone-infinitescroll-loading'
      loadingImg: '/assets/spinner.gif'
      loadingMsg: "<em>Loading...</em>"
      finishedMsg: "<em>'No more pages'</em>"
})