Pausing a Box2D simulation

There is a Box2D feature easy to implement and capable to add interesting gameplay that I haven’t found explained anywhere: the pause.

I want to stop a Box2D simulation when I click the mouse, and to play it again with another click.

The core function of this feature is Step.

Normally is called this way:

b2World.Step(timestep, iterations)

where timestep is the amount of time to simulate (normally 1/30 or 1/60 seconds) and iterations the number of iterations to be used by the constraint solver.

So, playing with timestep is the key to success. In this script, copied/pasted from A smart way to manage sleeping objects with Box2D I am going to stop the simulation with a mouse click, and let it play again with another mouse click.

package {
	import flash.display.Sprite;
	import flash.events.Event;
	import flash.utils.Timer;
	import flash.events.TimerEvent;
	import flash.events.MouseEvent;
	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(1500);
		var timestep:Number=1/30;
		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);
			stage.addEventListener(MouseEvent.CLICK,on_click);
			time_count.addEventListener(TimerEvent.TIMER, on_time);
			time_count.start();
		}
		public function on_click(e:MouseEvent) {
			if (timestep) {
				timestep=0;
			} else {
				timestep=1/30;
			}
		}
		public function on_time(e:Event) {
			if (timestep) {
				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(timestep, 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);
					}
				}
			}
		}
	}
}

Let’s see the interesting lines:

Line 69: Step function is performed with a timestep variable declared at line 15 and set at 1/30

Line 35: Adding a mouse listener waiting for clicks and calling on_click function

Line 39-45: on_click function switches timestep variable between 1/30 and 0. This way we can stop and resume the simulation. The other check for timestep value at line 47 is just to avoid a lot of new boxes to be generated while the simulation is paused

And this is the result:

Click mouse button to pause/play the simulation, and notice how unsleeping objects do not sleep while paused.

No need to download anything, just copy/paste this code in the example you can find at A smart way to manage sleeping objects with Box2D.

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