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Perpetual Scroll

This is a jQuery plugin that will facilitate the loading of additional results by simply scrolling to the bottom of an element, in a perpetual fashion.

Example Usage

Below is an example of using Perpetual Scroll in a Rails app.

Controller

class UsersController < ApplicationController
  respond_to :html, :js
  @@result_set_size = 50

  def index
    offset = params[:result_count].try(:to_i) || 0
    @users = User.order('last_name asc').limit(@@result_set_size).offset(offset)

    if request.xhr?
      render @users, :layout => false and return
    end
  end
end

Views

app/views/users/index.html.haml:

#results{:style => "height: 250px; overflow: auto"}
  = render @users

:javascript
  $(function() {
    $('#results').perpetualScroll();
  });

app/views/users/_user.html.haml:

.result
  = "#{user.first_name} #{user.last_name}"

How it works

When a user scrolls near the bottom of the element containing the list of results, the plugin will make an Ajax GET request to the current URL to load more results. A request parameter named result_count will be sent that contains the current number of results on the page. This is calculated by counting the number of elements that match the resultSelector option (defaults to .result). You can then use this parameter in your code to offset your results during subsequent loads.

Loading more results from the server

A callback function can be given to perpetual-scroll which determines whether or not more results can be retrieved from the server. The xhr object is passed to the callback in the event that it needs to be used to determine this.

One way to do this could be for the server to return a response header indicating whether or not more results can be retrieved. The callback would then query the xhr object for this header:

$('#results-area').perpetualScroll({
  url: '/search',
  appendTo: $('#results-area ul'),
  beforeSend: showSpinner,
  complete: hideSpinner,
  moreResultsCheck: function(xhr) {

    // If X-EOR header is set, it means we are at
    // the end of the results.
    return xhr.getResponseHeader('X-EOR') == null;
  }
});

Options

appendTo

This is the element to which to append the results from the Ajax call. Defaults to $(this).

beforeSend

A pre-request callback function. Use this to show a status indicator, for example.

complete

A function to be called when the request finishes, regardless of the return status code.

data

This can either be a function or the raw data to be sent to the server. If a function, it must return the data in the form of a url-encoded string or a Javascript object.

moreResultsCheck

Callback function that will be used to notify perpetual-scroll if more results can be retrieved from the server. This is used by perpetual-scroll to determined whether or not to request more results when the scroll bar reaches the bottom the next time. The xhr object from the Ajax call is passed to this function. Must return either true or false.

resultSelector

This can be any valid CSS selector that is used to identify an element that wraps an individual result. Defaults to .result.

triggerMargin

This is the number of pixels from the bottom of the scrollable results element where the Ajax request will be triggered. Defaults to 30. Set it higher to make the request farther away from the bottom of the results list.

type

The type of request to make ("POST" or "GET"), default is "GET".

Note: Other HTTP request methods, such as PUT and DELETE, can also be used here, but they are not supported by all browsers.

url

This is the url to use in the request. It can include a query string. perpetual-scroll is smart enough to deal with them. Defaults to window.location.href.