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and provided a simple example for viewing planetary data. The objects representing the planets in the solar system each contained several parameters, and we marked a couple of simple textual properties—the diameter and distance from the sun—as editable. Changes made to any properties in the system were captured by a central event listener function, which we used to write some debug information to the browser status bar. (The ability to write to the status bar is being restricted in recent builds of Mozilla Firefox. In appendix A, we present a pure JavaScript logging console that could be used to provide status messages to the user in the absence of a native status bar.) This event listener mechanism also provides an ideal way of capturing updates in order to send them to the server.

Let’s suppose that we have a script updateDomainModel.jsp running on our server that captures the following information:

■

The unique ID of the planet being updated

■

The name of the property being updated

■

The value being assigned to the property

We can write an event handler to fire all changes to the server like so: function updateServer(propviewer){

var planetObj=propviewer.viewer.object;

var planetId=planetObj.id;

var propName=propviewer.name;

var val=propviewer.value;

net.ContentLoader(

'updateDomainModel.jsp',

someResponseHandler,

null,

'POST',

'planetId='+encodeURI(planetId)

+'&propertyName='+encodeURI(propName)

+'&value='+encodeURI(val)

);

}

Licensed to jonathan zheng

198

CHAPTER 5

The role of the server

And we can attach it to our ObjectViewer:

myObjectViewer.addChangeListener(updateServer);

This is easy to code but can result in a lot of very small bits of traffic to the server, which is inefficient and potentially confusing. If we want to control our traffic, we can capture these updates and queue them locally and then send them to the server in batches at our leisure. A simple update queue implemented in JavaScript is shown in listing 5.13. Listing 5.13 CommandQueue object

net.CommandQueue=function(id,url,freq){ b

Create a queue object

this.id=id;

net.cmdQueues[id]=this;

this.url=url;

this.queued=new Array();

this.sent=new Array();

if (freq){

this.repeat(freq);

}

}

net.CommandQueue.prototype.addCommand=function(command){

if (this.isCommand(command)){

this.queue.append(command,true);

}

}

net.CommandQueue.prototype.fireRequest=function(){ c

Send request to server

if (this.queued.length==0){

return;

}

var data="data=";

for(var i=0;ivar cmd=this.queued[i];

if (this.isCommand(cmd)){

data+=cmd.toRequestString();

this.sent[cmd.id]=cmd;

}

}

this.queued=new Array();

this.loader=new net.ContentLoader(

this.url,

net.CommandQueue.onload,net.CommandQueue.onerror,

"POST",data

);

}

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Writing to the server

199

net.CommandQueue.prototype.isCommand=function(obj){ d

Test object type

return (

obj.implementsProp("id")

&& obj.implementsFunc("toRequestString")

&& obj.implementsFunc("parseResponse")

);

}

net.CommandQueue.onload=function(loader){ e

Parse server response

var xmlDoc=net.req.responseXML;

var elDocRoot=xmlDoc.getElementsByTagName("commands")[0]; if (elDocRoot){

for(i=0;ielChild=elDocRoot.childNodes[i];

if (elChild.nodeName=="command"){

var attrs=elChild.attributes;

var id=attrs.getNamedItem("id").value;

var command=net.commandQueue.sent[id];

if (command){

command.parseResponse(elChild);

}

}

}

}

}

net.CommandQueue.onerror=function(loader){

alert("problem sending the data to the server");

}

net.CommandQueue.prototype.repeat=function(freq){ f

Poll the server

this.unrepeat();

if (freq>0){

this.freq=freq;

var cmd="net.cmdQueues["+this.id+"].fireRequest()"; this.repeater=setInterval(cmd,freq*1000);

}

}

net.CommandQueue.prototype.unrepeat=function(){ g

Switch polling off

if (this.repeater){

clearInterval(this.repeater);

}

this.repeater=null;

}

The CommandQueue object (so called because it queues Command objects