How to write a box2d contact listener using cocos2d? - objective-c

Question

I've been reading various tutorials on how to write a contact listener, and I can't wrap my head around it.
Here is what I have so far:
In each of the classes that I have representing a physics object I do:
_body->SetUserData(self);
I write a contact listener class containing the following two methods:
void ContactListener::BeginContact(b2Contact* contact)
{
// Box2d objects that collided
b2Fixture* fixtureA = contact->GetFixtureA();
b2Fixture* fixtureB = contact->GetFixtureB();
// Sprites that collided
MyNode* actorA = (MyNode*) fixtureA->GetBody()->GetUserData();
MyNode* actorB = (MyNode*) fixtureB->GetBody()->GetUserData();
}
void ContactListener::EndContact(b2Contact* contact)
{
// Box2d objects that collided
b2Fixture* fixtureA = contact->GetFixtureA();
b2Fixture* fixtureB = contact->GetFixtureB();
// Sprites that collided
MyNode* actorA = (MyNode*) fixtureA->GetBody()->GetUserData();
MyNode* actorB = (MyNode*) fixtureB->GetBody()->GetUserData();
}
I don't know what to do next. I now have the two sprites which are colliding, but I want to do the following:
1) When they collide, I want to remove one of the sprites from the world, based on the type of object. (for example if one a cat object and the other is a mouse object, I want to remove the mouse object.
2) I want to let the cat object know it ate a mouse
3) I want the cat to continue moving as if it didn't contact with the mouse.
4) I still wan't the cat to collide normally with things like the terrain.
What do I do next ? I'm pretty clueless on what to do? How do I get the cat to continue to collide normally with the terrain, but not with the mouse? When do I remove the mouse?

Answer 1

Having an "Entity" class that hold the reference to the Box2D body and does the manipulations on it is definitely a good way to go. If you have a Spaceship class vs. a Meteor class, each of them can supply their own derived methods of controlling the body (AI), but each of them has common logic and code to support operations on "Things that have a body" (e.g. common "Entity" base class). I think you are on the right track.
It gets a little murky when the contacts start happening. This is where you start getting into the architecture of your overall system, not just the structure of the physics world or a single Coco2d Scene.
Here is how I have done this in the past:
First I set up the contact listener, listed below:
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)
{
b2Fixture* fixtureA = contact->GetFixtureA();
b2Body* bodyA = fixtureA->GetBody();
Entity* entityA = bodyA->GetUserData();
b2Fixture* fixtureB = contact->GetFixtureB();
b2Body* bodyB = fixtureB->GetBody();
Entity* entityB = bodyB->GetUserData();
if(test if entityA and entityB should not have collision response)
{
contact->SetEnabled(false);
}
// Do this if you want there to be collision notification, even if
// there is no response.
AddContactPair(entA,entB);
}
void AddContactPair(Entity* entA, Entity* entB)
{
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 BeginContact(b2Contact* contact)
{
Entity* entA = (Entity*)contact->GetFixtureA()->GetBody()->GetUserData();
Entity* entB = (Entity*)contact->GetFixtureB()->GetBody()->GetUserData();
assert(entA != NULL);
assert(entB != NULL);
// Not sure this is still needed if you add it in the pre-solve.
// May not be necessary...
AddContactPair(entA, entB);
}
// BEWARE: You may get multiple calls for the same event.
void EndContact(b2Contact* contact)
{
}
};
Because of the way the engine works, you can get multiple contact hits for the same bodies. This listener filters them so if two Entities collide, you only get one message.
The Listener only stores the collisions that have occurred. It could be modified to further separate them into "begins" and "ends" for other purposes. Here, contact meant "you've been hit by something". I didn't need to know if it stopped contacting.
The call to NotifyCollisions is the "secret sauce". It sends out a message to both the contacted entities (via a message system) that they hit something and the other Entity is what they hit. Bullet hit ship. Bullet destroys self. Ship damages self based on bullet properties (GetDamageInflicted() method). This in turn signals the graphics system to remove the bullet from the display. If the ship was destroyed, it is destroyed as well.
From an overall execution standpoint:
Before you begin, assign the contact listener.
Each Cycle of your game:
Call "Update" on all your Entities. This updates their physics forces, etc.
Update Box2d world.
Call NotifyCollisions on Listener.
Remove dead Entities from system.
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