How to detect if a player is on a platform - actionscript-3

Question

I am making a basic platform game using as3, and am trying to find a way to detect if my player is on the platform so I can allow him to jump if he is on it, but not be able to jump if he's off the platform.Here is my code:
package {
import flash.display.MovieClip
import flash.events.Event
import flash.events.KeyboardEvent;
import flash.ui.Keyboard;
public class Main extends MovieClip
{
var _vx = 0
var _vy = 0
var _ay = 0.5
var _isOnGround:Boolean
var canJump:Boolean
var _collisionArea:MovieClip
// Constants:
// Public Properties:
// Private Properties:
// Initialization:
public function Main()
{
stage.addEventListener(KeyboardEvent.KEY_UP, onKeyUp)
stage.addEventListener(KeyboardEvent.KEY_DOWN, onKeyDown);
addEventListener(Event.ENTER_FRAME, onEnterFrame);
}
function onKeyDown(event:KeyboardEvent):void
{
if (event.keyCode == Keyboard.LEFT)
{
_vx = -5;
}
else if (event.keyCode == Keyboard.RIGHT)
{
_vx = 5;
}
else if (event.keyCode == Keyboard.UP )
{
if (canJump)
{
_vy = -10
}
}
}
function onKeyUp(event:KeyboardEvent):void
{
if (event.keyCode == Keyboard.LEFT)
{
_vx = 0;
}
else if (event.keyCode == Keyboard.RIGHT)
{
_vx = 0;
}
}
public function onEnterFrame(event:Event):void
{
if(player1.isOnGround)
{
canJump=true
}
else if (!player1.isOnGround)
{
canJump=false
}
if (_vy > 10)
{
_vy = 10
}
player1.y += _vy
player1.x += _vx
vy += _ay
//Player vs wall collision
for (var i:int = 0; i <= 4; i++)
{
CollisionB.platform_BlockRectangles(player1, this["wall" + i]);
//trace("wall" + i);
}
}
public function set vx(vxValue:Number):void
{
_vx = vxValue;
}
public function get vx():Number
{
return _vx;
}
public function set vy(vyValue:Number):void
{
_vy = vyValue;
}
public function get vy():Number
{
return _vy;
}
public function get isOnGround():Boolean
{
return _isOnGround;
}
public function set isOnGround(onGround:Boolean):void
{
_isOnGround = onGround;
}
public function get collisionArea():MovieClip
{
return _collisionArea;
}
}
}
Im also using another class where the code is:
static public function platform_BlockRectangles(objectA:Object,objectB:Object):void
{
//This function requires the following setter properties in objectA:
// _objectIsOnGround:Boolean, vx:Number, vy:Number
var objectA_Halfwidth=objectA.width/2;
var objectA_Halfheight=objectA.height/2;
var objectB_Halfwidth=objectB.width/2;
var objectB_Halfheight=objectB.height/2;
var dx=objectB.x-objectA.x;
var ox=objectB_Halfwidth+objectA_Halfwidth-Math.abs(dx);
if (0<ox)
{
var dy=objectA.y-objectB.y;
var oy=objectB_Halfheight+objectA_Halfheight-Math.abs(dy);
if (0<oy)
{
if (ox<oy)
{
if (dx<0)
{
ox*=-1;
oy=0;
}
else
{
oy=0;
}
//Dampen horizontal velocity
objectA.vx=0;
}
else
{
if (dy<0)
{
ox=0;
oy*=-1;
objectA.isOnGround=true;
}
else
{
ox=0
}
//Dampen vertical velocity
objectA.vy=0;
}
objectA.x-=ox;
objectA.y+=oy;
}
}
}
Someone please help!!!!!!!

Answer 1

I extremely recommend you to use some library like: Flixel, Citrus
Try to follow this tutorial, will explain all necessary logic to help you with your game

Answer 2

You can use AABB collision detection. Here is a tutorial (the Distance function part).
If you google the term you will get a lot of info in the subject.
But i recomend you to use flixel or flashPunk is you are just starting and want to do games, they take care of these stuff for you and you can get to just make the game.