Access of undefined property Keyboard (AS3) - actionscript-3

Question

I'm new to Actionscript 3 and I'm wanting to allow a circle to move down using the down arrow on the keyboard. Here's my code:
package {
import flash.display.MovieClip;
import flash.events.KeyboardEvent;
import flash.ui.Keyboard;
public class Circle extends MovieClip {
public function Circle() {
// constructor code
var speed:int = 3;
addEventListener(KeyboardEvent.KEY_DOWN,keyIsDown);
function keyIsDown(event:KeyboardEvent) {
if(event.keyCode == Keyboard.DOWN) {
y = y+=speed;
}
}
}
}
}
When I test it, nothing happens when I press the down key. Anyone know what's wrong with the code?

Answer 1

Try adding KeyBoard events to the stage instead of to the class. Additionally, I would not nest functions like that, bad practice in general. Also the line y = y+=speed; is confusing, shouldn't it just be y += speed; ?
EDIT: Sorry, I guess stage will be null in the constructor, I've added a ADDED event listener.
Try this:
package {
import flash.display.MovieClip;
import flash.events.Event;
import flash.events.KeyboardEvent;
import flash.ui.Keyboard;
public class Circle extends MovieClip {
public function Circle() {
// constructor code
var speed:int = 3;
addEventListener(Event.ADDED, onAdded);
}
private function onAdded(event:Event) {
stage.addEventListener(KeyboardEvent.KEY_DOWN,keyIsDown);
}
private function keyIsDown(event:KeyboardEvent) {
if(event.keyCode == Keyboard.DOWN) {
y += speed;
}
}
}
}