Falling object and repeating in a loop ( Action Script 3.0) - actionscript-3

Question

I am trying to make a symbol (made a special class for it) to fall continuously until a timer reaches 0. Also I would like this rock to repeat and show in random places on the stage. I can't figure out how to code that. Pretty much I am still a newbie to action script 3.0.
This is what I have so far:
The Main
package {
import flash.display.Sprite;
import Game_Objects.TinyBird;
import Game_Objects.Rock;
import flash.events.Event;
import flash.utils.Timer;
import flash.events.TimerEvent;
import flash.events.KeyboardEvent;
import flash.ui.Keyboard;
public class Main extends Sprite {
private var userbird:TinyBird;
private var obstacle:Rock;
//containers.
private var obstacleContainer:Sprite = new Sprite();
//timers.
private var enemySpawn:Timer;
public function Main() {
// The main code of the game
// The Bird has to avoid rocks by moving left and right
// Obstacles = rocks
// The Birdie will be controlled by the keyboard
// As long as Birdie alive=1 the loop will continue until alive=0 (where 1=true and 0=false) or timer reaches 0
// if the bird will hit an object it will die (collision detection)
this.userbird = new TinyBird(stage.stageWidth/2, stage.stageHeight-20);
this.obstacle = new Rock;
obstacleContainer.addChild(obstacle);
addChild(userbird);
stage.addEventListener(Event.ENTER_FRAME, startpulse)
stage.addEventListener(KeyboardEvent.KEY_DOWN, keymove);
// event to count the passed rocks in order to set score.
stage.addEventListener(obstacle.KILLED, deadEnemy);
}
private function startpulse(evt:Event):void {
// makes the bird pulsate
this.userbird.pulse_animation();
}
private function keymove(evt:KeyboardEvent):void {
// the keyboard movements for the bird
// if leftArrow = pressed -> tiny bird will move left. Else if rightArrow = pressed -> tiny bird will move right
if (evt.keyCode == Keyboard.LEFT) {
this.userbird.left();
} else if (evt.keyCode == Keyboard.RIGHT) {
this.userbird.right();
}
}
// The obstacle objects are going to fall randomly
private function spawn(e:TimerEvent):void {
// calculate a random starting position
var xPos:Number = Math.floor(Math.random()*stage.stageWidth);
// calculate a random speed between 2 and 6
var speed:Number = Math.floor(Math.random()*4+2) ;
// create the new rock
var enemy:Rock = new Rock(xPos, 42, speed, stage.stageWidth, stage.stageHeight);
// add it to the container
this.obstacleContainer.addChild(enemy);
enemy.name = "Rock " + Rock.createdCount;
}
private function deadEnemy(e:Event) {
var obj:Rock = (e.object as Rock);
this.objectContainer.removeChild(obj)
}
}
}
And this is the rock symbol:
package Game_Objects {
import flash.display.MovieClip;
import flash.display.Sprite;
import flash.events.Event;
import flash.utils.Timer ;
import flash.events.TimerEvent ;
public class Rock extends Sprite {
// The Rock is the obstacle that, if colided with the bird, the game is over.
// public function properties
static public var _createdCount:int = 0;
// private function properties
private var speed:Number;
private var _score:Number = 4;
private var scoreCounter:Number = 0;
// Classes methods
public static function get createdCount():int {
return _createdCount;
}
// instance methods
// Initialization
public function Rock(x:Number, y:Number, s:Number, maxX:Number = 0, maxY:Number = 0) {
// set the speed
this.speed = s;
// If the rock goes off the stage, then
if (x < this.width/2) {
// Put on at left
this.x = this.width/2;
// else if x would put the rocks beyond right side of the stage then
} else if (x > maxX - this.width/2) {
// Position the rock on the stage.
this.x = maxX-this.width/2;
} else {
// Otherwise position at x
this.x = x;
}
// same for Y
if (y < this.height/2) {
this.y = this.height/2;
} else if (y > maxY - this.height/2) {
this.y = maxY-this.height/2;
} else {
this.y = y;
}
// Creating the animation loop in order to repeat the falling motion of the rocks.
configUI();
this.addEventListener(Event.ENTER_FRAME, drop);
// adding a boolean type of loop, taken from
// http://help.adobe.com/en_US/AS2LCR/Flash_10.0/help.html?content=00001283.html#374074
this.cacheAsBitmap = true;
// Add 1 to the public var in order to keep track of the rocks
_createdCount++;
}
// protected function
protected function configUI():void {
}
// private function
private function drop(e:Event) {
// in order to show the dropping effect
// The falling of the rocks
this.y += this.speed;
// if at bottom of stage then
if (this.y-this.height >= stage.stageHeight) {
// Set score to +1 as reward for not hitting the rock.
this._score++;
// Kill the rock that has reached the bottom of the stage
}
}
}
}