Dojo and ArcGIS Server 9.3

What is Dojo and why is it important to ArcGIS users?

Recently Alex Russell and Neil Roberts from SitePen were here in Redlands showing  some of the capabilities of the Dojo toolkit and how it can greatly simplify your life if you’re a JavaScript developer. We anticipate that a lot of you will be encountering Dojo soon because the ArcGIS JavaScript API is built on top of it. This post gives an overview of what Dojo is and what you need to know about it to be successful when building ArcGIS JavaScript applications.

What is Dojo?

Dojo is a toolkit that helps you write more robust and performant JavaScript code. JavaScript is a language that runs within the Web browser, and there are various flavors of Web browser that interpret the JavaScript in slightly different ways. Toolkits such as Dojo, YUI, Prototype, and many others are designed to abstract away the browser idiosyncrasies so that you don’t have to learn them all and handle them in your code.

Additionally, there are often several ways to code the same thing using JavaScript. Toolkits like Dojo provide functions you can use to do things in easier or more efficient ways. Using libraries from a toolkit can reduce the lines of code you write and make your JavaScript applications quicker and more stable.

What does it mean that the ArcGIS JavaScript API is built on top of Dojo?

The ESRI developers who created the ArcGIS JavaScript API used Dojo to simplify their development process (Why reinvent things that work?) and to ensure that the applications you build behave the same in different browsers. For example, the map zoom and panning animations use Dojo, as does the graphics layer.

Furthermore, the zoom level slider and info windows that you see in your JavaScript API maps are Dojo widgets (dijits). The slider dijit is provided with Dojo, and the info window is a custom dijit created by ESRI for the ArcGIS JavaScript API.

How much Dojo do I need to know in order to use the ArcGIS JavaScript API?

The amount of Dojo you use when you work with the ArcGIS JavaScript API is up to you, but at a minimum you’ll need to use several common functions:

  • dojo.require() – Similar to the <script> include tag on an HTML page. It imports resources into your JavaScript page.
  • dojo.addOnLoad() – Similar to <body onload=””>. It registers an initializing block called after the page has finished loading and the dijits have been initialized.
  • dojo.connect() – Similar to the Element.addEventListener and Element.attachEvent JavaScript functions. It registers a listener to listen to specific events on an Object or element on the page and returns results from a function.
  • dojo.byId() – Similar to the document.getElementById(id) JavaScript function. The function searches and returns the first HTML element with the argument ID.

When writing your ArcGIS JavaScript applications, you can take advantage of the full Dojo toolkit, which includes buttons, grids, tree views, charts, and other widgets. The toolkit is divided into three parts:

Core – Essential functions like those listed above
Dijit – Themeable widgets such as trees, menus, and buttons
DojoX- Extension projects in various stages of development, such as graphics, grids, and charts

How do I get Dojo? Do I have to install it?

Dojo is included with the ArcGIS JavaScript API. When you include this script tag referencing the ArcGIS JavaScript API, you get access to the full Dojo tookit version 1.1.0:

<script type="text/javascript" src="http://serverapi.arcgisonline.com/jsapi/arcgis/?v=1"></script>

Many Dojo users download Dojo and host it themselves, or they reference the publicly available Dojo hosted on the AOL Content Delivery Network (CDN). When building ArcGIS JavaScript applications, there is no need to download or host Dojo, and you should use the Dojo included with the ArcGIS JavaScript API.

How do I get help with Dojo?

The Dojo Web site, www.dojotoolkit.org contains the official Dojo documentation. Documentation may be sparse, especially for projects in DojoX. Various books have recently become available that describe working with Dojo in more detail than you’ll find on the web site. The Dojo forums and the #dojo chat room on the IRC server irc.freenode.net are other good ways to get help.

Do the ESRI Web ADFs use Dojo?

At 9.3, ESRI’s .NET Web Application Developer Framework (ADF) contains a JavaScript library which should not be confused with the ArcGIS JavaScript API. The .NET Web ADF JavaScript library is dependent on the ASP.NET AJAX JavaScript Library and does not use Dojo.

The Java Web ADF also provides a JavaScript library, which was not written with a toolkit and consequently does not use Dojo.

Further Reading: http://blogs.esri.com/Dev/blogs/arcgisserver/archive/2008/07/09/What-is-Dojo-and-why-is-it-important-to-ArcGIS-users_3F00_.aspx#2369

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