I can't catch an Error - apache-flex

Question

I can't catch the thrown error in my simplified code below. Why is that?
According to requirements of the stackoverflow I must insert some more info but this example is very simple. Can you help me with this example?
package com.myserver {
public class ReturnInfo extends Sprite {
public function ReturnInfo(urlParamsArr:Array) {
try {
var client:HttpClient = new HttpClient();
var uri:URI = new URI("http://valid-url.com/aaa.php");
client.listener.onData = function(event:HttpDataEvent):void {
throw new Error();
};
client.listener.onError = function(event:IOErrorEvent):void {
trace("error");
};
client.postFormData(uri, variables);
}
catch (e:Error){
trace("Error was caught.");
}
}
} //class
} //package
I tried also:
try {
new ReturnInfo(urlParamsArr);
}
catch(e:Error){
trace("caught error");
}
It didn't work either.

Answer 1

The code does not work because the code that throws error is executed later, so you need to use try-catch in the client.listener.onData handler. That handler I assume is called sometimes later so there when you parse or handle the data,make sure to catch/handle the errors

Answer 2

Adding on to what Simion said, the problem is method closure. In order for an exception to be caught somewhere in the "food chain" the catch needs to be in the stack - you will know what is in the current stack by getStackTrace(). In this example, there is no stack pointer that sits at the constructor (or any method) like there is one for client.listener.onData - which is why the postFormData will execute. When the event is triggered it's stack pointer goes back to the origination point of what actually started the event trigger in the first place (not the method that declared it). This is also why the 2nd attempt was unsuccessful.
Add on to the fact that the FP executes discrete chunks in frames (think of this like a heap), anything that executes in the scope of the dispatchEvent will generally have a very small or no stack at all (eg the first stack pointer is usually the dispatcher itself - not a method that actually called it).
try-catch is best attempted within the same scope of a method.
A pseudo example:
function getOrCreateWidget():Widget {
var a:Widget;
try {
a = getWidet();
}
catch(e:TypeError) {
a = createNewWidget();
}
//finally can be debatable - most of us leave it off
//bc it executes anyway just as it would in the function scope.
finally {
a.property = 'foo';
}
return a;
}
If this isn't possible - a last ditch effort is to attach a listener to the loaderInfo.uncaughtErrorEvents. Generally associating this with the systemManager is the best option because the SM knows about every branch of the display tree right down to the root stage. It's neither good practice nor practical to assign all deviations in this method because a lot of context to the programmer is usually lost. It's more an "oh S#!) sorry user, our app just verped."