Creation of a Flash Stabilize! clone using Box2D – part 4

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Here I am with the 4th part of this prototype. In part 3 we saw the “next” feature, this time we’ll trigger the collision among crates of the same color.

This will allow you to create the classic “match stuff of the same color to remove it from the game” or maybe something more creative.

In this example every crate has a counter, and every time a crate hits another crate of the same color, counters of both crates increase by one.

I already showed you how Box2D manages collisions but I made it directly editing the built in b2ContactListener.as class.

It’s not the best practice as I needed to modify the original library.

The right practice is writing your own class extending b2ContactListener, then overriding the functions you need.

Let’s look at the main class:

package {
	import flash.display.Sprite;
	import flash.events.Event;
	import flash.events.MouseEvent;
	import Box2D.Dynamics.*;
	import Box2D.Collision.*;
	import Box2D.Collision.Shapes.*;
	import Box2D.Common.Math.*;
	public class stabilize extends Sprite {
		public var m_sprite: Sprite = new Sprite();
		public var m_world:b2World;
		// the_crate is the crate sprite
		public var the_crate:crate=new crate();
		// next_crate
		public var next_crate:crate=new crate();
		// droppable_area is the sprite representing the area your mouse
		// must overlap to drop a crate
		public var droppable_area:drop_area = new drop_area();
		// this is the weight of the current crate I am dropping
		public var cur_drop:int=Math.ceil(Math.random()*9);
		// this is the color of the current crate I am dropping
		public var cur_color:int=Math.ceil(Math.random()*5);
		// weight and color of "next" crate
		public var next_drop:int=Math.ceil(Math.random()*9);
		public var next_color:int=Math.ceil(Math.random()*5);
		// this is my custom contact listener class
		public var contact_listener=new custom_contact_listener();
		// the progressive number just keep track of dropped crates and
		// assign an unique number to each of them
		public var progressive:int=1;
		//
		public function stabilize() {
			var gravity:b2Vec2=new b2Vec2(0,9.8);
			var worldAABB:b2AABB = new b2AABB();
			worldAABB.lowerBound.Set(-1000,-1000);
			worldAABB.upperBound.Set(1000,1000);
			m_world=new b2World(worldAABB,gravity,true);
			// adding the contact listener
			m_world.SetContactListener(contact_listener);
			m_sprite = new Sprite();
			addChild(m_sprite);
			addChild(droppable_area);
			addChild(the_crate);
			// assigning the crate current weight and color
			the_crate.weight.text=cur_drop.toString();
			the_crate.gotoAndStop(cur_color);
			addChild(next_crate);
			// assigning "next" crate weight and color
			next_crate.weight.text=next_drop.toString();
			next_crate.gotoAndStop(next_color);
			next_crate.x=250;
			next_crate.y=385;
			debug_draw();
			AddBox(250/30,350/30,10/30,10/30,0,false);
			AddBox(250/30,350/30,200/30,10/30,20,false);
			addEventListener(Event.ENTER_FRAME,Update);
			stage.addEventListener(MouseEvent.MOUSE_DOWN,mousePressed);
			stage.addEventListener(MouseEvent.MOUSE_MOVE,mouseMoved);
		}
		public function mousePressed(e:MouseEvent) {
			// look at hitTestPoint... the final true value says it's going to to check against the actual pixels
			// instead of the bounding box
			if (droppable_area.hitTestPoint(mouseX,mouseY,true)) {
				// notice the cur_drop parameter, I am passing the weight of the current crate to the AddBox function
				AddBox(mouseX/30,mouseY/30,0.5,0.5,cur_drop,true);
			}
		}
		public function mouseMoved(e:MouseEvent) {
			the_crate.x=mouseX;
			the_crate.y=mouseY;
			if (droppable_area.hitTestPoint(mouseX,mouseY,true)) {
				the_crate.alpha=1;
			} else {
				the_crate.alpha=0.5;
			}
		}
		// function AddBox
		// px: x position
		// py: y position
		// _halfwidth: half of the box width
		// _halfheight: half of the box height
		// density: density of the box (0: static)
		// is_crate: if true, it's a crate (and you should render the proper movieclip
		public function AddBox(px:Number,py:Number,_halfwidth:Number,_halfheight:Number,density:Number,is_crate:Boolean) {
			var bodyDef:b2BodyDef = new b2BodyDef();
			bodyDef.position.Set(px,py);
			var boxDef:b2PolygonDef = new b2PolygonDef();
			boxDef.SetAsBox(_halfwidth,_halfheight);
			boxDef.density=density;
			boxDef.friction=0.3;
			boxDef.restitution=0.2;
			if (is_crate) {
				bodyDef.userData = new crate();
			}
			var body:b2Body=m_world.CreateBody(bodyDef);
			body.CreateShape(boxDef);
			body.SetMassFromShapes();
			if (is_crate) {
				addChild(bodyDef.userData);
				// assigning current weight and color to the Box2D sprite bound to the box I created
				bodyDef.userData.weight.text=cur_drop.toString();
				// collz is the text that will count the number of collisions
				bodyDef.userData.collz.text=0;
				// assigning the progressive number to the crate...
				bodyDef.userData.prog=progressive;
				// ... and incrementing it for next one
				progressive++;
				bodyDef.userData.gotoAndStop(cur_color);
				// updating current weight and color with the next ones
				cur_drop=next_drop;
				cur_color=next_color;
				// changing current crate movieclip according to its weight and color
				the_crate.weight.text=cur_drop.toString();
				the_crate.gotoAndStop(cur_color);
				// calculating next crate weight and color
				next_drop=Math.ceil(Math.random()*9);
				next_color=Math.ceil(Math.random()*5);
				// updating next crate movieclip according to its weight and color
				next_crate.weight.text=next_drop.toString();
				next_crate.gotoAndStop(next_color);
			}
		}
		public function Update(e:Event) {
			var crate_num:int;
			m_world.Step(1/30,10);
			for (var bb:b2Body = m_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);
				}
			}
		}
		public function debug_draw() {
			var dbgDraw:b2DebugDraw = new b2DebugDraw();
			var dbgSprite:Sprite = new Sprite();
			m_sprite.addChild(dbgSprite);
			dbgDraw.m_sprite=m_sprite;
			dbgDraw.m_drawScale=30;
			dbgDraw.m_alpha=1;
			dbgDraw.m_fillAlpha=0.5;
			dbgDraw.m_lineThickness=1;
			dbgDraw.m_drawFlags=b2DebugDraw.e_shapeBit;
			m_world.SetDebugDraw(dbgDraw);

		}
	}
}

at line 27 I am creating a new variable as custom_contact_listener class, then I simply set it as the contact listener at line 39

Now I can create a custom_contact_listener class in a custom_contact_listener.as file to extend b2ContactListener class and override existing functions.

package {
	//
	import Box2D.Collision.*;
	import Box2D.Collision.Shapes.*;
	import Box2D.Dynamics.Contacts.*;
	import Box2D.Dynamics.*;
	import Box2D.Common.Math.*;
	import Box2D.Common.*;
	//
	public class custom_contact_listener extends b2ContactListener {
		// this array will hold all collisions among crates
		var collz:Array=new Array;
		//
		public override function Add(point:b2ContactPoint):void {
		}
		//
		public override function Persist(point:b2ContactPoint):void {
			// if the bodies that collided have an userData different from null,
			// then we can say they are crates.
			// So, if collided bodies are crates...
			if (point.shape1.GetBody().GetUserData()!=null&&point.shape2.GetBody().GetUserData()!=null) {
				// if both crates have the same color...
				if (point.shape1.GetBody().GetUserData().currentFrame==point.shape2.GetBody().GetUserData().currentFrame) {
					// getting progressive number of both crates
					var num1:int=point.shape1.GetBody().GetUserData().prog;
					var num2:int=point.shape2.GetBody().GetUserData().prog;
					// if the collision array of the num1-th crate does not exists...
					if (collz[num1]==undefined) {
						// create it
						collz[num1]=new Array;
						// populate it with the num2-th crate
						collz[num1].push(num2);
					} else {
						// if already exists, check if the num2-th crate already exists...
						if (collz[num1].indexOf(num2)==-1) {
							// if not, add it just like before
							collz[num1].push(num2);
						}
					}
					// same thing for the collision array of the num2-th crate
					if (collz[num2]==undefined) {
						collz[num2]=new Array  ;
						collz[num2].push(num1);
					} else {
						if (collz[num2].indexOf(num1)==-1) {
							collz[num2].push(num1);
						}
					}
					// updating the number of collision shown in both crates
					point.shape1.GetBody().GetUserData().collz.text=collz[num1].length;
					point.shape2.GetBody().GetUserData().collz.text=collz[num2].length;
				}
			}
		}
		//
		public override function Remove(point:b2ContactPoint):void {
		}
		//
		public override function Result(point:b2ContactResult):void {
		}
	}
}

This is how you extend a class… at line 10 I am declaring custom_contact_listener class extending b2ContactListener one.

Then at line 17 I override (think as “overwrite”) Persist function.

The remaining interesting code is already commented.

This is the result:

Download the source code.

Next time, more gameplay…

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