Flex: Event Listeners and Array Collections

While working with some code I ran into a situation where I needed to know when an ArrayCollection was updated. Normally an event is triggered when using addItem, but I needed to know when it was set to another ArrayCollection. It's not very obvious, but it is possible.

ArrayCollection is really a wrapper for an Array. You can access the "real" array in an ArrayCollection by using ".source". When you set the source property an event called "listChanged" is triggered. You can watch for this event and then do whatever you need.

So this

var newAC1:ArrayCollection = new ArrayCollection;
var newAC2:ArrayCollection = new ArrayCollection;
newAC1 = newAC2;

Is the same as this, but with an event trigger.

var newAC1:ArrayCollection = new ArrayCollection;
var newAC2:ArrayCollection = new ArrayCollection;

newAC1.addEventListener('listChanged', function(event:Event){trace('changed');});

newAC1.source = newAC2.source;