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Writing to the server
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This covers the basic mechanics of submitting data to the server, whether based on textual input from a form or other activity such as drag and drop or mouse movements. In the following section, we ll pick up our ObjectViewer example from chapter 4 and learn how to manage updates to the domain model in an orderly fashion.
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5.5.3 Managing user updates effectively
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In chapter 4, we introduced the ObjectViewer, a generic piece of code for browsing complex domain models, 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:
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The unique ID of the planet being updated The name of the property being updated The value being assigned to the property
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We can write an event handler to fire all changes to the server like so:
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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) ); }
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The role of the server
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And we can attach it to our ObjectViewer:
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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){ this.id=id; net.cmdQueues[id]=this; this.url=url; this.queued=new Array(); this.sent=new Array(); if (freq){ this.repeat(freq); } }
Create a queue object
net.CommandQueue.prototype.addCommand=function(command){ if (this.isCommand(command)){ this.queue.append(command,true); } } net.CommandQueue.prototype.fireRequest=function(){ if (this.queued.length==0){ return; } var data="data="; for(var i=0;i<this.queued.length;i++){ var 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 ); }
Send request to server
Writing to the server
net.CommandQueue.prototype.isCommand=function(obj){ return ( obj.implementsProp("id") && obj.implementsFunc("toRequestString") && obj.implementsFunc("parseResponse") ); }
Test object type
net.CommandQueue.onload=function(loader){ Parse server response var xmlDoc=net.req.responseXML; var elDocRoot=xmlDoc.getElementsByTagName("commands")[0]; if (elDocRoot){ for(i=0;i<elDocRoot.childNodes.length;i++){ elChild=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){ 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(){ Switch polling off if (this.repeater){ clearInterval(this.repeater); } this.repeater=null; }
The CommandQueue object (so called because it queues Command objects we ll get to that in a minute) is initialized b with a unique ID, the URL of a serverside script, and, optionally, a flag indicating whether to poll repeatedly. If it
The role of the server
doesn t, then we ll need to fire it manually every so often. Both modes of operation may be useful, so both are included here. When the queue fires a request to the server, it converts all commands in the queue to strings and sends them with the request c. The queue maintains two arrays. queued is a numerically indexed array, to which new updates are appended. sent is an associative array, containing those updates that have been sent to the server but that are awaiting a reply. The objects in both queues are Command objects, obeying an interface enforced by the isCommand() function d. That is:
It can provide a unique ID for itself. It can serialize itself for inclusion in the POST data sent to the server (see c). It can parse a response from the server (see e) in order to determine whether it was successful or not, and what further action, if any, it should take.
We use a function implementsFunc() to check that this contract is being obeyed. Being a method on the base class Object, you might think it is standard JavaScript, but we actually defined it ourselves in a helper library like this:
Object.prototype.implementsFunc=function(funcName){ return this[funcName] && this[funcName] instanceof Function; }
Appendix B explains the JavaScript prototype in greater detail. Now let s get back to our queue object. The onload method of the queue e expects the server to return with an XML document consisting of <command> tags inside a central <commands> tag. Finally, the repeat() f and unrepeat() g methods are used to manage the repeating timer object that will poll the server periodically with updates. The Command object for updating the planet properties is presented in listing 5.14.
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