Emanuele Feronato

Talking about Actionscript 3, Box2D, Flash and Game development.

If you followed Box2D: tutorial for the absolute beginners, then you know you can put some object to “sleep” to improve performance by not simulating inactive bodies.

This is CPU saving but can be used for some game concepts too.

Just think about Super Stacker, where you have happy and afraid bricks.

If you spend some time on this game, you will find happy bricks are the inactive ones while the afraid ones are still falling/moving.

This can be achieved by playing on the IsSleeping status.

Let’s take the same script introduced in Box2D: tutorial for the absolute beginners and make some changes:

package {
	import flash.display.Sprite;
	import flash.events.Event;
	import flash.utils.Timer;
	import flash.events.TimerEvent;
	import Box2D.Dynamics.*;
	import Box2D.Collision.*;
	import Box2D.Collision.Shapes.*;
	import Box2D.Common.Math.*;
	public class demo extends Sprite {
		var the_world:b2World;
		var time_count:Timer=new Timer(3000);
		public function demo() {
			var environment:b2AABB=new b2AABB  ;
			environment.lowerBound.Set(-100.0,-100.0);
			environment.upperBound.Set(100.0,100.0);
			var gravity:b2Vec2=new b2Vec2(0.0,10.0);
			the_world=new b2World(environment,gravity,true);
			var final_body:b2Body;
			var the_body:b2BodyDef;
			var the_box:b2PolygonDef;
			the_body=new b2BodyDef  ;
			the_body.position.Set(8.5,14);
			the_box=new b2PolygonDef  ;
			the_box.SetAsBox(8.5,0.5);
			the_box.friction=0.3;
			the_box.density=0;
			final_body=the_world.CreateBody(the_body);
			final_body.CreateShape(the_box);
			final_body.SetMassFromShapes();
			addEventListener(Event.ENTER_FRAME,on_enter_frame);
			time_count.addEventListener(TimerEvent.TIMER,on_time);
			time_count.start();
		}
		public function on_time(e:Event) {
			var final_body:b2Body;
			var the_body:b2BodyDef;
			var the_box:b2PolygonDef;
			var box_width=Math.random() + 0.1;
			var box_height=Math.random() + 0.1;
			the_body=new b2BodyDef;
			the_body.position.Set(Math.random() * 10 + 2,0);
			the_body.userData=new cubeface();
			the_body.userData.width=box_width * 60;
			the_body.userData.height=box_height * 60;
			the_box=new b2PolygonDef;
			the_box.SetAsBox(box_width,box_height);
			the_box.friction=0.3;
			the_box.density=1;
			final_body=the_world.CreateBody(the_body);
			final_body.CreateShape(the_box);
			final_body.SetMassFromShapes();
			addChild(the_body.userData);
		}
		public function on_enter_frame(e:Event) {
			the_world.Step(1 / 30,10);
			for (var bb:b2Body=the_world.m_bodyList; bb; bb=bb.m_next) {
				if (bb.m_userData is Sprite) {
					bb.m_userData.x=bb.GetPosition().x * 30;
					bb.m_userData.y=bb.GetPosition().y * 30;
					bb.m_userData.rotation=bb.GetAngle() * 180 / Math.PI;
					if (bb.IsSleeping()) {
						bb.m_userData.gotoAndStop(2);
					}
					else {
						bb.m_userData.gotoAndStop(1);
					}
				}
			}
		}
	}
}

In the on_time function I added some lines to assign a cubeface object to the rectangle I create.

The cubeface object is just a MovieClip with two frames, one with an happy box and one with a sad one.

Line 43: Assigning the cubeface object to the rectangle

Lines 44-45: Defining movieclip width and height

Line 53: Adding the movieclip on the stage

Later, in the on_enter_frame function, you can find the classic Box2d function used to update sprites (or movieclips) position and rotation.

Lines 62-67: Checking if the body is sleeping with IsSleeping. If the body is sleeping, then stop the movieclip on frame 2 (the happy square), otherwise stop it on frame 1 (the afraid square).

This is the result

And this is the source code to download.