问如何在AS3平台游戏中创建平台？

EN

import flash.events.KeyboardEvent;
import flash.events.Event;

var KeyThatIsPressed:uint;
var rightKeyDown:Boolean = false;
var leftKeyDown:Boolean = false;
var upKeyDown:Boolean = false;
var downKeyDown:Boolean = false;

var gravity:Number = 1;
var yVelocity:Number = 0;
var canJump:Boolean = false;
var playerSpeed:Number = 7;
var crouchSpeed:Number = -4;

stage.addEventListener(KeyboardEvent.KEY_DOWN, PressAKey);
stage.addEventListener(KeyboardEvent.KEY_UP, ReleaseAKey);

function PressAKey(event:KeyboardEvent):void{
    if(event.keyCode == 37 || event.keyCode == 65){
        leftKeyDown = true;
    }
    if(event.keyCode == 38 || event.keyCode == 87){
        upKeyDown = true;
    }
    if(event.keyCode == 39 || event.keyCode == 68){
        rightKeyDown = true;
    }
    if(event.keyCode == 40 || event.keyCode == 83){
        downKeyDown = true;
    }
}

function ReleaseAKey(event:KeyboardEvent):void{
    if(event.keyCode == 37 || event.keyCode == 65){
        leftKeyDown = false;
    }
    if(event.keyCode == 38 || event.keyCode == 87){
        upKeyDown = false;
    }
    if(event.keyCode == 39 || event.keyCode == 68){
        rightKeyDown = false;
    }
    if(event.keyCode == 40 || event.keyCode == 83){
        downKeyDown = false;
    }   
}

char_mc.addEventListener(Event.ENTER_FRAME, moveChar);

function moveChar(event:Event):void{
    if(rightKeyDown){
        char_mc.x += playerSpeed;
        char_mc.scaleX = 1;
    }
    if(rightKeyDown && !downKeyDown && canJump == true){
        char_mc.gotoAndStop(2);
    }
    if(leftKeyDown){
        char_mc.x -=playerSpeed;
        char_mc.scaleX = -1;
    }
    if(leftKeyDown && !downKeyDown && canJump == true){
        char_mc.gotoAndStop(2);
    }
    if(upKeyDown && canJump){
        yVelocity = -10;
        canJump = false;
        char_mc.gotoAndStop(3);
    }
    if(downKeyDown && !rightKeyDown && !leftKeyDown){
        char_mc.gotoAndStop(4);
    }
    if(downKeyDown && rightKeyDown && canJump == true){
        char_mc.x +=crouchSpeed;
        char_mc.gotoAndStop(5);
    }
    if(downKeyDown && leftKeyDown && canJump == true){
        char_mc.x -=crouchSpeed;
        char_mc.gotoAndStop(5);
    }
    if(upKeyDown && canJump && rightKeyDown){
        canJump = false;
        char_mc.gotoAndStop(3);
    }
    if(upKeyDown && canJump && leftKeyDown){
        canJump = false;
        char_mc.gotoAndStop(3);
    }

    if(!rightKeyDown && !leftKeyDown && !upKeyDown && !downKeyDown){
        char_mc.gotoAndStop(1);
    }
    if(yVelocity >0){
        canJump = false;
    }
    if (canJump == false){
        char_mc.gotoAndStop(3);
    }

    yVelocity +=gravity;

    if(! floor_mc.hitTestPoint(char_mc.x, char_mc.y,true)){
        char_mc.y+=yVelocity;
    }
    if(yVelocity > 20){
        yVelocity = 20;
    }

    for(var i:int=0;i<10;i++){
        if(floor_mc.hitTestPoint(char_mc.x,char_mc.y,true)){
            char_mc.y--;
            yVelocity=0;
            canJump=true;
        }
    }

    if (char_mc.hitTestObject(fallBox)){
        char_mc.x = 20;
        char_mc.y = 300;
    }
}