Erase Box: the tutorial

核心提示：That can be done by checking the x and y positions of it, but I also wanted to explore Box2D features.That was actually good, because I found another way to do

That can be done by checking the x and y positions of it, but I also wanted to explore Box2D features.

That was actually good, because I found another way to do that, and it can be of great help on other projects.

Sensors are bodies that won’t directly interact with others.

They won’t make a moving box stop when they hit, i.e.

That being said, they are still useful.

Box2D will keep track of it’s “contacts” just as if it were a normal body, so they can be used as exactly what their name means, a sensor.

Also, Box2D allows you to “attach” some information to a body, so that it can be distinguished from others.

Let’s see an example:

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.*;
　import　Box2D.Dynamics.Contacts.*;
　public　class　demo　extends　Sprite　{
　　public　var　the_world:b2World;
　　var　time_count:Timer=new　Timer(1000);
　　var　i:int　=　0;
　　public　function　demo()　{
　　　var　environment:b2AABB　=　new　b2AABB();
　　　environment.lowerBound.Set(-5.0,　-5.0);
　　　environment.upperBound.Set(30.0,　30.0);
　　　var　gravity:b2Vec2=new　b2Vec2(0.0,10.0);
　　　the_world=new　b2World(environment,gravity,true);
　　　var　debug_draw:b2DebugDraw　=　new　b2DebugDraw();
　　　var　debug_sprite:Sprite　=　new　Sprite();
　　　addChild(debug_sprite);
　　　debug_draw.m_sprite=debug_sprite;
　　　debug_draw.m_drawScale=30;
　　　debug_draw.m_fillAlpha=0.5;
　　　debug_draw.m_lineThickness=1;
　　　debug_draw.m_drawFlags=b2DebugDraw.e_shapeBit;
　　　the_world.SetDebugDraw(debug_draw);
　　　var　final_body:b2Body;
　　　var　final_body2:b2Body;
　　　var　the_body:b2BodyDef;
　　　var　the_box:b2PolygonDef;
　　　the_body　=　new　b2BodyDef();
　　　the_body.position.Set(8.5,　13);
　　　the_box　=　new　b2PolygonDef();
　　　the_box.SetAsBox(8.5,　0.5);
　　　the_box.friction=0.3;
　　　the_box.density=0;
　　　the_box.isSensor　=　true;
　　　the_body.userData　=　"eraseSensor";
　　　final_body=the_world.CreateBody(the_body);
　　　final_body.CreateShape(the_box);
　　　the_body.position.Set(8.5,　8);
　　　the_body.userData　=　"detSensor";
　　　final_body2=the_world.CreateBody(the_body);
　　　final_body2.CreateShape(the_box);
　　　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;
　　　the_body　=　new　b2BodyDef();
　　　the_body.position.Set(Math.random()*10+2,　0);
　　　the_box　=　new　b2PolygonDef();
　　　the_box.SetAsBox(Math.random()+0.1,Math.random()+0.1);
　　　the_box.friction=0.3;
　　　the_box.density=1;
　　　the_body.userData　=　i;
　　　final_body=the_world.CreateBody(the_body);
　　　final_body.CreateShape(the_box);
　　　final_body.SetMassFromShapes();
　　　i++;
　　}
　　public　function　on_enter_frame(e:Event)　{
　　　the_world.Step(1/30,　10);
　
　　　//　Setting　up　a　sensor-contact　listener
　　　if　(the_world.m_contactList)　{
　
　　　var　contact:b2Contact　=　the_world.m_contactList;　　　
　
　　　if　(　contact.GetShape1().GetBody().GetUserData()　==　"detSensor"　)　{
　　　　　　trace(contact.GetShape2().GetBody().GetUserData());
　　　　　}
　
　
　　　if　(　contact.GetShape1().GetBody().GetUserData()　==　"eraseSensor"　)　{
　　　　　　var　body2　=　contact.GetShape2().GetBody();
　　　　　　the_world.DestroyBody(body2);
　　　　　}
　　　}
　
　　　//if　(　contact.GetShape2().GetBody().GetUserData()　==　"eraseSensor"　)
　　　　　//{
　　　　　　//var　body3　=　contact.GetShape1().GetBody();
　　　　　　//the_world.DestroyBody(body3);
　　　　　//}
　　}
　}
}