do I really need to have an event handler for URLLoader? - actionscript-3

Question

I am new to actionscript, and I am following a tutorial by Joseph Devore. I understand that when I want to do a network query I can create a URLRequest, and send this with a URLLoader. I expected to find the response data in the URLLoader.data attribute, but after making the query this was still null in my case. Then it seemed that I must create an event handling to actually access the response. Is this true?
I wrote this small example code, and wonder how I (the easiest way) can access the response in this get request.
package
{
import flash.events.Event;
import flash.net.URLLoader;
import flash.net.URLRequest;
public class stackQuery
{
public function stackQuery()
{
}
public function makeQuery(): String {
var request: URLRequest = URLRequest("http://www.google.com");
var loader: URLLoader = new URLLoader();
loader.addEventListener(Event.COMPLETE, handleResponse);
try{
loader.load(request);
}
if(loader.data == null){
return "Empty"
}
return "data not empty"
}
private function handleResponse(event:Event):void
{
//something with event.target.data here??
}
}
}

Answer 1

There isn't any nice way to do a synchronous load (like you can in some other languages) with AS3. Your data is null because in AS3 the load command is asynchronous - meaning it doesn't halt the rest of the application (locking it up) until the load completes, it moves on to the next line of code and dispatches an event once the load is complete.
Here is a code example of how you would want to handle this based off of the code you supplied:
package
{
import flash.events.Event;
import flash.events.IOErrorEvent;
import flash.events.SecurityErrorEvent;
import flash.net.URLLoader;
import flash.net.URLRequest;
public class stackQuery
{
private var loader: URLLoader = new URLLoader(); //let's move it out to the class level so you can easily reference it from other methods (though this is purely optional)
public function stackQuery()
{
}
public function makeQuery(): String {
var request:URLRequest = URLRequest("http://www.google.com");
//add you listeners before loading
loader.addEventListener(Event.COMPLETE, handleResponse);
loader.addEventListener(IOErrorEvent.IO_ERROR, handleResponse); //it's good practice to also listen for errors
loader.addEventListener(SecurityErrorEvent.SECURITY_ERROR, handleResponse); //this kind of error will happen if you're trying to load a local resource but have published as network only (or vice-versa)
//load the request, any code after this will run before the load completes.
loader.load(request);
}
//this function will run once the load completes
private function handleResponse(event:Event):void
{
if (event is IOErrorEvent) {
//you had an IO error, better do something
return;
}
if (event is SecurityErrorEvent) {
//url wasn't allowed to load
return;
}
trace(loader.data); //this is what was returned from the page
//you could also get it by referencing the currentTarget of the event
trace(URLLoader(event.currentTarget).data);
}
}
}

Answer 2

Yes, of course it is true. Imagine you load one megabyte of data. You need time in order to load it, right? So there's no way that you will have this data on the very next line of code.
The listener will get executed when the target dispatches the event - in your case when that megabyte is loaded.
This is a normal behavior.

Answer 3

While the answers given are correct, the real explanation is that Actionscript 3 is an asynchronous language as opposed to other language like Python for example that are synchronous. In python after loading something like an url the next line of code will not be executed until the url is loaded (which can create lag or idling execution), in Actionscript 3 the code execution doesn't wait and move on to the next line. That is the reason you need to use events (Actionscript 3 is an event based language) so you can find out when the data you are waiting for is available all that while your code is still executing.