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Implementing Dynamic Scroll with Ajax, JavaScript, and XML

  • June 6, 2007
  • By Vlad Kofman
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In this article, I will discuss implementing an Ajax-enabled scrollable table, in which rows are dynamically fetched from the server, as the user scrolls, without the whole page refreshing. This technique is unique and very convenient for implementing pages with a lot of dynamic content in a readable form; for instance, a live news feed. As new items are added to the table, the vertical scroll bar will resize and the never-ending scroll will continue. The asynchronous fetching of the data is done with AJAX (Asynchronous JavaScript and XML). The parsing and dynamic row insertions are done in JavaScript, the requests are done with the XMLHttpRequest object, and the table appearance is controlled by the CSS.

The implementation of the dynamic scroll is possible mostly because of the introduction of the XMLHttpRequest object in the modern browsers. This JavaScript object can send requests asynchronously and receive responses from the server, without page refresh. Many web sites currently participating in the Web 2.0 design methodology often use a lot of Ajax to offer rich client interfaces, the XMLHttpRequest is extensively used on such sites and content of the page changes dynamically based on the user action.

The scrollable table is a good example of a rich client interface, which uses the Web 2.0 approach to the web content usability. The dynamic scroll is also possible in part because of the new CSS2 standards, which are supported by the main browsers as well. In the sample code, I will show how to implement scrollable table that occupies some part of the screen, but if CSS were not supported and only Ajax were, the dynamic table would just resize (and scroll) the whole page, and not part of it.

Here is the example of a dynamic table on the Google reader site. Note that there are 100 items in the table on the first screenshot and, as the vertical scroll bar is moved down more rows appear, the number changes to 140, and the scroll bar also resizes. All other elements remain in the same positions on the page.

Figure 1: A dynamic table showing 100 items.

Figure 2: The same table after 40 items have been added. Note the position of the scroll.

Presentation of the Dynamic Table

To create the scroll behavior of any HTML element (in this case a table), it needs to be enclosed in a scrollable area (or viewport) defined by an outer element (in this case a DIV), which should be smaller then the total table height. If there is no outer element (no viewport), the inner element would just grow on the page in place, and after a while the whole page would resize and start to scroll (within the browser's page viewport). For example, a div element with height of 100 pixels containing table with height of 200px would achieve this table/viewport encapsulation behavior.

<div style="height:100px; width:50px;
            overflow:auto; overflow-x:hidden;">
   <table style="height:200px;">

Figure 3: Example code to generate scroll behavior

Note: If you set the same properties on these two elements separately, only the div would scroll, but not the table.

Here is an example code and what looks like in the browser.

<table style="height:50px; width:100px;
   <tr><td><p>vlad 1</p></td></tr>
   <tr><td><p>vlad 2</p></td></tr>
   <tr><td><p>vlad 3</p></td></tr>
   <tr><td><p>vlad 4</p></td></tr>
   <tr><td><p>vlad 5</p></td></tr>


<div style="height:100px; width:100px;
   <p>Vlad 1 </p>
   <p>Vlad 2</p>
   <p>Vlad 3</p>
   <p>Vlad 4</p>
   <p>Vlad 5</p>

Here is a screen shot of the HTML above.

Figure 4: Visual representation of HTML element properties (source MSDN)

Notice that the table height is set to 50, but it still does not scroll.

The two CSS properties overflow:auto; and overflow-x:hidden;* actually make the scroll bar appear on the right within the div element and make the table scroll within the div. The browser's CSS engine must support these two properties. Otherwise, the table would only grow in place without the scroll bar.

* These are CSS2 properties. If content in an element falls outside the element's rendering box for a number of reasons (negative margins, absolute positioning, content exceeding the width/height set for an element, etc.), the 'overflow' and 'overflow-x' properties describe what to do with the content that exceeds the element's width.

These CSS properties only define the appearance of the div and table, but JavaScript APIs interface with the scroll properties. To understand the implementation of a dynamic table, you need to know properties that measure HTML elements' dimension and location on the page. These properties can be programmatically accessed and changed in JavaScript and, thankfully, APIs are the same across most major browsers.

HTML element properties

Here is the cheat sheet of available HTML element properties for HTML BODY and a single DIV element in the center of the page. As you can see, there are a lot of them, but the three of the most interest are scrollTop, scrollHeight, and clientHeight.

Figure 5: Visual representation of Div element properties (source MSDN)

The scrollTop specifies the position of the content upper bound relative to the visible portion of the viewport (defined by the div's height and width), as the user scrolls. scrollTop would indicate the distance in pixels the content moved. The scrollHeight specifies the actual height in pixels of the inner element and clientHeight specifies height of the visible content (viewport height). In my case, the viewport is the outer div element and the content is the inner TABLE. The Height and Top properties do not account for element's borders, margins, or padding, and if your elements have any of these attributes, look into the offset group of properties, among others. Similarity to the height, there are width properties to deal with horizontal scrolling calculations.

Every element placed on the page has these properties and the way elements look (and behave) depends on the values of these properties.

Here is simplified depiction of the div element's properties in the page.

Figure 6: Code to generate scroll behavior within 500px Div

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