Box2D Flash game creation tutorial – part 1

package {
	import flash.display.Sprite;
	import flash.events.Event;
	import flash.events.KeyboardEvent;
	import Box2D.Dynamics.*;
	import Box2D.Collision.*;
	import Box2D.Collision.Shapes.*;
	import Box2D.Common.Math.*;
	public class ball01 extends Sprite {
		// world creation
		public var world:b2World=new b2World(new b2Vec2(0,10.0),true);
		public var world_scale:int=30;
		// the player
		public var player:b2Body;
		// force to apply to the player
		public var force:b2Vec2;
		// variables to store whether the keys are pressed or not
		// true = pressed;
		// false = unpressed
		public var left,right,up,down:Boolean=false;
		public function ball01():void {
			// calling thebug draw function
			debug_draw();
			// drawing the boundaries
			draw_box(250,400,500,10,false,"ground");
			draw_box(0,200,10,400,false,"left");
			draw_box(500,200,10,400,false,"right");
			draw_box(250,0,500,10,false,"roof");
			// adding the player at 250,200
			// some listeners
			add_player(250,200);
			addEventListener(Event.ENTER_FRAME, update);
			stage.addEventListener(KeyboardEvent.KEY_DOWN,on_key_down);
			stage.addEventListener(KeyboardEvent.KEY_UP,on_key_up);
		}
		// according to the key pressed, set the proper variable to "true"
		public function on_key_down(e:KeyboardEvent):void {
			switch (e.keyCode) {
				case 37 :
					left=true;
					break;
				case 38 :
					up=true;
					break;
				case 39 :
					right=true;
					break;
				case 40 :
					down=true;
					break;
			}
		}
		// according to the key released, set the proper variable to "false"
		public function on_key_up(e:KeyboardEvent):void {
			switch (e.keyCode) {
				case 37 :
					left=false;
					break;
				case 38 :
					up=false;
					break;
				case 39 :
					right=false;
					break;
				case 40 :
					down=false;
					break;
			}
		}
		// simple function to draw a box
		public function draw_box(px,py,w,h,d,ud):void {
			var my_body:b2BodyDef= new b2BodyDef();
			my_body.position.Set(px/world_scale, py/world_scale);
			if (d) {
				my_body.type=b2Body.b2_dynamicBody;
			}
			var my_box:b2PolygonShape = new b2PolygonShape();
			my_box.SetAsBox(w/2/world_scale, h/2/world_scale);
			var my_fixture:b2FixtureDef = new b2FixtureDef();
			my_fixture.shape=my_box;
			var world_body:b2Body=world.CreateBody(my_body);
			world_body.SetUserData(ud);
			world_body.CreateFixture(my_fixture);
		}
		// function to add the player
		public function add_player(px,py):void {
			var my_body:b2BodyDef= new b2BodyDef();
			my_body.position.Set(px/world_scale, py/world_scale);
			my_body.type=b2Body.b2_dynamicBody;
			var my_circle:b2CircleShape=new b2CircleShape(10/world_scale);
			var my_fixture:b2FixtureDef = new b2FixtureDef();
			my_fixture.shape=my_circle;
			player=world.CreateBody(my_body);
			player.CreateFixture(my_fixture);
		}
		// debug draw
		public function debug_draw():void {
			var debug_draw:b2DebugDraw = new b2DebugDraw();
			var debug_sprite:Sprite = new Sprite();
			addChild(debug_sprite);
			debug_draw.SetSprite(debug_sprite);
			debug_draw.SetDrawScale(world_scale);
			debug_draw.SetFlags(b2DebugDraw.e_shapeBit);
			world.SetDebugDraw(debug_draw);
		}
		// functiojn to be executed at every frame
		public function update(e:Event):void {
			// setting the force to null
			force=new b2Vec2(0,0);
			// according to the key(s) pressed, add the proper vector force
			if (left) {
				force.Add(new b2Vec2(-10,0));
			}
			if (right) {
				force.Add(new b2Vec2(10,0));
			}
			if (up) {
				force.Add(new b2Vec2(0,-20));
			}
			if (down) {
				force.Add(new b2Vec2(0,5));
			}
			// if there is any force, then apply it
			if (force.x||force.y) {
				player.ApplyForce(force,player.GetWorldCenter());
			}
			world.Step(1/30,10,10);
			world.ClearForces();
			world.DrawDebugData();
		}
	}
}