AJAX Design Patterns
Snyke wrote this terribly early in the morning:
We’ll define an API that abstracts from the actual page so that we can then create modules that do more complex tasks.
The first and morst important thing the engine has to do is initialize everything.
Now we move on to the real magic: loadPage(). It will get a URI as input and it will then load the correct module and pass control to the module, which will then take care of the rest:
The URL is interpreted in the following way:
All that is left now to do is implementing some helper functions that will later be used by the modules:
But you can already see that we can create really complex modules too as will be shown in a later part of this tutorial when we’ll create a Forum as a Module.
The source of the Page module can be found here.
By now the entire World has heard about AJAX, even those who don’t care about Web-Development have seen the potential of this new technology. Everybody is tired of endless introductions on how cool AJAX is and those endless lists of good examples like Google Suggest, GMail and alike, so I decided to cut a long story short and jump right into the real tutorial.
Is this tutorial any different from the others? Well yes and no, it is different in being a tutorial on how to design and build a complete site and not just some fancy little details like how to turn caching in AJAX off or how to create a fancy widget. To keep the tutorial readable, and to avoid having to implement low level functionality, I’m using the dojo toolkit, I tried prototype too and really enjoyed working with being a really nice and easy to use Library, but dojo provides much more functionality bundled with it. For both frameworks one thing is true: documentation is scarse, and I spent alot time debugging and reading posts on the newsgroups.
For debugging I suggest using Firefox Venkman and the really nice Firebug extension, which make AJAX a lot easier to understand, especially FireBug’s "Log each Request" Feature.
In this tutorial we will try to design a community portal as it has a wide range of different components that give a good overview of what is archievable with AJAX, also it should provide you with the basic tools that will help you in more complex applications.
What we want
As developers we need to know where we want to go, before starting right away. The requirements analysis is not part of this tutorial so I’ll just write down the basic functionality for our Portal:- Pages: Load and show simple HTML Formatted content.
- User: we want to be a community so the possibility for users to register themselfs, create a Profile and communicate with others is fundamental.
- Messages: some sort of messaging center is nice too, and not too difficult to implement, it allows private communication between the users.
- Forum: talking about communications, what could be better than a Forum to let the Users talk about whatever they want?
The Layout
Although the focus should be on the development of the functionality, the layout is still as important as the application itself. It gets even more important because the application can be good but without an interface that is functional it is useless. We’ll use a layout that is easy yet functional:- The content area.
- A sidebar for context related options.
- The main menu (basically this just selects between modules).
- The title, nothing fancy here
- A list with the online users.
Modules
Basically with AJAx we step away from the classic one page-at-a-time view of doing things, and we have to start talking about a more Event Driven architecture, or MVC-Model if you prefer.We’ll define an API that abstracts from the actual page so that we can then create modules that do more complex tasks.
To get back to this tutorial we will have several small modules that represent parts on the site, along with real modules we will have some virtual modules that take care of some functionality, but more on this later. Modules in this tutorial include:
- Page: this will be the first module we will be implementing, it allows to simply display the contents of a file in the contents area of the site, it does nothing fancy.
- User: this module is used for several different tasks:
- Register: register a new User
- Login
- Profile: show the profile of a User
- Online: once we have users we can easily implement a component that allows us to show which users are online, and with AJAX this is possible almost in realtime. It is a nice effect and very easy to implement.
- Forum: what would a community site be without a Forum?
The implementation
We will encapsulate all of our functionality in JavaScript variables, this is similar to static classes in Java and makes it possible to have some sort of clean namespace division between the modules. First of all we will have to create the engine. The engine is the part that handles loading of all the other modules and gives them abstract ways to interact with the page. The idea behind this is that the page interface (layout, stylesheet, …) and Engine build an abstraction layer so that it is easier to implement the functionality easier, without having to bother about representational issues. In other words we build an API.The first and morst important thing the engine has to do is initialize everything.
var Engine = {
bootstrap: function(){
this.setStatus(“Loading…”);
dojo.require(“dojo.io.*”);
dojo.hostenv.writeIncludes();
if((“”+document.location).indexOf('#’) < 0){
Engine.loadPage(“#Page/Welcome”);
}else{
Engine.loadPage(“” + document.location);
}
},
loadedModules: new Array(),
modules: new Array(),
loadModule: function(module){
if(Engine.loadedModules[module])
return;
dojo.io.bind({
url: “javascript/” + module + “.js”,
sync: true,
error: function(type, evaldObj){Engine.showError(“Error while loading module.”);},
load: function(type, data, evt){
eval(data);
Engine.modules[module].execute(uri);
}
});
},
Now we move on to the real magic: loadPage(). It will get a URI as input and it will then load the correct module and pass control to the module, which will then take care of the rest:
uri: "/",
loadPage: function(url){
Engine.setStatus(“Loading…”);
if(!url)
url = “” + document.location;
var hashIndex = url.indexOf('#’);
if(hashIndex < 0 || hashIndex <= url.length-2)
return Engine.hideStatus;
uri = url.substring(hashIndex + 1);
var moduleLength;
if(uri.indexOf('/’) > 0)
moduleLength = uri.indexOf('/’);
else
moduleLength = uri.length;
var module = uri.substring(0,moduleLength);
uri = uri.substring(uri.indexOf('/’));
if(Engine.loadedModules[module] && ! dojo.lang.isUndefined(Engine.modules[module])){
Engine.modules[module].execute(uri);
}else{
Engine.loadModule(module);
}
},
The URL is interpreted in the following way:
- Everything in front of the '#’ is discarded as it is only the location of the application.
- The part between the '#’ and the first '/’ is the module name which will be loaded if it isn’t yet.
- Everything from the '/’ to the end of the URL is the argument that is passed to the Module’s execute() function (see the Page module below as an example).
All that is left now to do is implementing some helper functions that will later be used by the modules:
setStatus: function(message){
if($(’status’) != null){
$(’status’).parentNode.removeChild($(’status’));
}
var body = document.getElementsByTagName(“body”)[0];
var div = document.createElement(“div”);
div.style.position = “absolute”;
div.style.top = “50%”;
div.style.left = “50%”;
div.style.width = “200px”;
div.style.margin = “-12px 0 0 -100px”;
div.style.border = “0px”;
div.style.padding = “20px”;
div.style.opacity = “0.85″;
div.style.backgroundColor = “#353555″;
div.style.border = “1px solid #CFCFFF”;
div.style.color = “#CFCFFF”;
div.style.fontSize = “25px”;
div.style.textAlign = “center”;
div.id = ’status’;
body.appendChild(div);
div.innerHTML = message;
},
hideStatus: function(){
Engine.opacityDown($(’status’));
},
opacityDown: function(theElement){
if(theElement == null)
return;
var opacity = parseFloat(theElement.style.opacity);
if (opacity < 0.08){
theElement.parentNode.removeChild(theElement);
}else{
opacity -= 0.07;
theElement.style.opacity = opacity;
setTimeout(function(){Engine.opacityDown(theElement);}, 50);
}
return true;
},
setContent: function(content){
$('content’).innerHTML = content;
},
showError: function(message){
Engine.setStatus(message);
setTimeout(“Engine.hideStatus()”,10000);
}
}
The first module
Now we’ll move on to implement our first real module. It’s task is to load an external resource (an HTML page in this specific case) asynchronously and then display it in the content area.var Page = {
init: function(){
Engine.modules[“Page”] = Page;
Engine.loadedModules[“Page”] = true;
},
execute: function(uri){
try{
dojo.io.bind({
url: “resources” + uri + “.php”,
sync: false,
error: function(type, evaldObj){
Engine.showError(“Error while loading Content.”);},
load: function(type, data, evt){
Engine.setContent(data);
Engine.hideStatus();
});
}catch(e){
alert(e);
}
}
}
Page.init();
But you can already see that we can create really complex modules too as will be shown in a later part of this tutorial when we’ll create a Forum as a Module.
The source of the Page module can be found here.
