Photo Gallery ....disable a previous button once it reaches img zero

Question

I have made a simple photo gallery that reads from external XML file. The gallery allows the user to see the next image by clicking the next button as well as the previous image by clicking the previous button.
I need a bit of code that would disable the previous button when its at the first image, and disable the next image once the user is on the last image.
This is what I have thus far:
var xmlRequest:URLRequest = new URLRequest("wImgData.xml");
var xmlLoader:URLLoader = new URLLoader(xmlRequest);
var imgData:XML;
var imageLoader:Loader;
var rawImage:String;
var rawH:String;
var rawW:String;
var imgNum:Number = 0;
var checkSec:Timer = new Timer(1);
var numberOfChildren:Number;
xmlLoader.addEventListener(Event.COMPLETE, xmlLoadedF);
next_btn.addEventListener(MouseEvent.CLICK, nextImgF);
prev_btn.addEventListener(MouseEvent.CLICK, prevImgF);
master_mc.buttonMode = true;
function xmlLoadedF(event:Event):void{
    checkSec.start();
    checkSec.addEventListener(TimerEvent.TIMER, checkerF);
    imgData = new XML(event.target.data);   
function packagedF():void{
    checkSec.start();
    checkSec.removeEventListener(TimerEvent.TIMER, checkerF);
    rawImage = imgData.image[imgNum].imgURL;
    numberOfChildren = imgData.*.length();
    rawW = imgData.image[imgNum].imgW;
    rawH = imgData.image[imgNum].imgH;
    imageLoader = new Loader;
    imageLoader.load(new URLRequest(rawImage));
    master_mc.addChild(imageLoader);
    imageLoader.x = (stage.stageWidth - Number(rawW)) /2;
    imageLoader.y = (stage.stageHeight - Number(rawH)) /2;
function checkerF(event:TimerEvent):void{
    if(imgNum == 0){
        packagedF();
    }else if(imgNum < numberOfChildren){
        imageLoader.unload();
        packagedF();
    }else{
        imageLoader.unload();
        imgNum = 0;
        packagedF();
function nextImgF(event:MouseEvent):void{
    checkSec.addEventListener(TimerEvent.TIMER, checkerF);
    imgNum++;
function prevImgF (event:MouseEvent):void{
    checkSec.addEventListener(TimerEvent.TIMER, checkerF);
    imgNum--;

Answer 1

All you need to do is remove the listeners in the event handlers for the buttons. I'll use previous as an example.
function prevImgF (event:MouseEvent):void{
    checkSec.addEventListener(TimerEvent.TIMER, checkerF);
    imgNum--;
     if(imgNum == 0){
          //at first
          prev_btn.removeEventListener(MouseEvent.CLICK, prevImgF);
You'd do the same to the next button's listener when the imgNum == the number of images. You'll also need to add the previous listener back to the button when they press next.