Watch for changing properties in Flex

In AS3 there is a handy utility class called ChangeWatcher, which lives inside the mx.binding.utils package. It provides you with a handy way of watching other object’s properties.

Here’s how it’s used:

ChangeWatcher.watch(object, "property", handler);

Object – the object which owns the property
“property” – the property you want to watch
handler – the function name of the handler

Whenever the property of the object changes, the handler will respond. Quite handy, right?

Here’s an example for you:

Let’s say we have a class called WatchMe. Inside that class we have a public variable called ImBeingWatched. We’d like to trigger a function whenever this variable changes. To do this, we simply add one line of code:

package
{
    import mx.binding.utils.*;
    import flash.display.*;
    import flash.events.*;

    public class WatchMe extends MovieClip
    {
        [Bindable] public var ImBeingWatched:Boolean = true;

        public function WatchMe():void
        {
            ChangeWatcher.watch(this, "ImBeingWatched", doSomething);
        }

        private function doSomething(e:Event):void
        {
            // do something ...
        }
    }
}

The property that ChangeWatcher is watching must be [Bindable] and Public for it to work.