How can I remove objects in Box2d after collision, but with a delay?

Question

I am using cocos2d and box2d in my iPhone game.

When two balls hit each other, I get a notification though the ContactListener and I have a reference to both bodies.

I can destroy both of them, but I would like to do it with a delay.

So, two balls hit each other, they bounce off and then after a second disappear.

Answer 1

Start a timer in the BeginContact or EndContact and remove the Body then.

Answer 2

this is the perfect demo for your problem

http://www.raywenderlich.com/505/how-to-create-a-simple-breakout-game-with-box2d-and-cocos2d-tutorial-part-22

Answer 3

I do my collision processing using a class (below) that filters out duplicate collisions and sends notifications to the "Entity" objects via a messaging system.

NOTE: This is part of a larger code base; feel free to ask any questions if you need clarification because the code is not here.

class EntityContactListener : public ContactListener
{
private:
   GameWorld* _gameWorld;
   EntityContactListener() {}

   typedef struct 
   {
      Entity* entA;
      Entity* entB;
   } CONTACT_PAIR_T;

   vector<CONTACT_PAIR_T> _contactPairs;

public:
   virtual ~EntityContactListener() {}

   EntityContactListener(GameWorld* gameWorld) :
      _gameWorld(gameWorld)
   {
      _contactPairs.reserve(128);
   }

   void NotifyCollisions()
   {
      Message* msg;
      MessageManager& mm = GameManager::Instance().GetMessageMgr();

      for(uint32 idx = 0; idx < _contactPairs.size(); idx++)
      {
         Entity* entA = _contactPairs[idx].entA;
         Entity* entB = _contactPairs[idx].entB;

         //DebugLogCPP("Contact Notification %s<->%s",entA->ToString().c_str(),entB->ToString().c_str());

         msg = mm.CreateMessage();
         msg->Init(entA->GetID(), entB->GetID(), Message::MESSAGE_COLLISION);
         mm.EnqueueMessge(msg, 0);

         msg = mm.CreateMessage();
         msg->Init(entB->GetID(), entA->GetID(), Message::MESSAGE_COLLISION);
         mm.EnqueueMessge(msg, 0);         
      }
      _contactPairs.clear();
   }

   void PreSolve(b2Contact* contact, const b2Manifold* oldManifold)
   {

   }

   // BEWARE:  You may get multiple calls for the same event.
   void BeginContact(b2Contact* contact)
   {
      Entity* entA = (Entity*)contact->GetFixtureA()->GetBody()->GetUserData();
      Entity* entB = (Entity*)contact->GetFixtureB()->GetBody()->GetUserData();
      //DebugLogCPP("Begin Contact %s->%s",entA->ToString().c_str(),entB->ToString().c_str());
      if(entA->GetGroupID() == entB->GetGroupID())
      {  // Can't collide if they are in the same group.
         return;
      }

      assert(entA != NULL);
      assert(entB != NULL);

      for(uint32 idx = 0; idx < _contactPairs.size(); idx++)
      {
         if(_contactPairs[idx].entA == entA && _contactPairs[idx].entB == entB)
            return;
         // Not sure if this is needed...
         if(_contactPairs[idx].entA == entB && _contactPairs[idx].entA == entB)
            return;
      }
      CONTACT_PAIR_T pair;
      pair.entA = entA;
      pair.entB = entB;
      _contactPairs.push_back(pair);
   }

   // BEWARE:  You may get multiple calls for the same event.
   void EndContact(b2Contact* contact)
   {
      /*
      Entity* entA = (Entity*)contact->GetFixtureA()->GetBody()->GetUserData();
      Entity* entB = (Entity*)contact->GetFixtureB()->GetBody()->GetUserData();
      DebugLogCPP("End Contact %s->%s",entA->ToString().c_str(),entB->ToString().c_str());
       */
   }
};

Once your entities receive the message, each can start a timer and destroy itself when needed. Or you could have your messages enqueued with a delay so that they are destroyed during the message processing phase of your game (if you have one). This is good because it gives you explicit control over when the destruction occurs and doesn't make it happen while the physics is going on, which can really cause problems with the engine.