Drop down list in actionscript3 using transitions effects

//Import the required classes
import fl.transitions.Tween;  
import fl.transitions.easing.*;
import fl.transitions.TweenEvent;

//Turn on buttonMode for our movieclips
button_mc.buttonMode=true;
dropdown_mc.buttonMode=true;

//Add the dropDown Event Listener
button_mc.addEventListener(MouseEvent.CLICK, dropDown);

//initial dropDown function to animate on our dropdown movieclip into the mask
function dropDown(e:MouseEvent):void
{
var enterTween:Tween;
enterTween = new Tween(dropdown_mc, "y", Regular.easeOut, dropdown_mc.y,  177, 30, false);
enterTween.addEventListener(TweenEvent.MOTION_FINISH, dropUpEventListener);
}

//This function is called on after the dropdown animation is finished
function dropUpEventListener(e:TweenEvent):void
{
button_mc.removeEventListener(MouseEvent.CLICK, dropDown);  //remove event listener to dropDown function
button_mc.addEventListener(MouseEvent.CLICK, dropUp);  // add event listener for dropUp function
}

//Drop up function tweens out our dropdown movieclip from the mask
function dropUp(e:Event):void
{
var enterTween:Tween;
enterTween = new Tween(dropdown_mc, "y", Regular.easeOut, dropdown_mc.y,  -8, 30, false);
enterTween.addEventListener(TweenEvent.MOTION_FINISH, dropDownEventListener);
}

//Function does the exact opposite of the dropUpEventListener
function dropDownEventListener(e:TweenEvent):void
{
button_mc.removeEventListener(MouseEvent.CLICK, dropUp);
button_mc.addEventListener(MouseEvent.CLICK, dropDown);
}