Detect initial collision of two box2d bodies without continuous collision - cocos2d-iphone

Question

I have some simple box2d bodies setup with a contact listener like so:
#import "MyContactListener.h"
MyContactListener::MyContactListener() : _contacts() {
}
MyContactListener::~MyContactListener() {
}
void MyContactListener::BeginContact(b2Contact* contact) {
// We need to copy out the data because the b2Contact passed in
// is reused.
MyContact myContact = { contact->GetFixtureA(), contact->GetFixtureB() };
_contacts.push_back(myContact);
b2Body *A = contact->GetFixtureA()->GetBody();
b2Body *B = contact->GetFixtureA()->GetBody();
NSLog(#"Collision detected!");
PLAYSOUND(COLLISION);
}
void MyContactListener::EndContact(b2Contact* contact) {
MyContact myContact = { contact->GetFixtureA(), contact->GetFixtureB() };
std::vector<MyContact>::iterator pos;
pos = std::find(_contacts.begin(), _contacts.end(), myContact);
if (pos != _contacts.end()) {
_contacts.erase(pos);
}
}
void MyContactListener::PreSolve(b2Contact* contact, const b2Manifold* oldManifold) {
}
void MyContactListener::PostSolve(b2Contact* contact, const b2ContactImpulse* impulse) {
}
And I need to play a sound when two bodies have collided. However this implementation detects continuous collisions so the sound is played when the bodies are touching. My knowledge of box2d and C++ is already very limited, is there a simple way to detect a new collision without detecting continuous collisions?

Answer 1

You have the right basic idea, but it needs some refinement.
In your BeginContact(...) call, you have:
PLAYSOUND(COLLISION);
Instead of playing the sound here, what you should do is enqueue some other system to play the sound for this particular pair. Set the userdata tag of your bodies to the pointer to the class (or some other ID to keep track of entities). Like so:
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());
*/
}
};
The last part of this is to NOT play the sound again for a short time even if a collision occurs. You can do this by creating a stopwatch or counting down from a fixed time on the entity update cycles.
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Answer 2

First set a timer like this..
[self schedule:#selector(check collision:)];
and in this method
- (void)tick:(ccTime) dt
{
for(b2Body *b = _world->GetBodyList(); b; b=b->GetNext())
{
//--------------My contact Listener Start-------------------------
std::vector<MyContact>::iterator pos;
for(pos = _contactListener->_contacts.begin(); pos != _contactListener->_contacts.end(); ++pos)
{
MyContact contact = *pos;
// Here get your sprite and make their fixture and check...
if ((contact.fixtureA == tempballFixture && contact.fixtureB == _mainballFixture) ||
(contact.fixtureA == _mainballFixture && contact.fixtureB == tempballFixture))
{
if(mainDelegate.music_playing == TRUE)
{
[[SimpleAudioEngine sharedEngine] playEffect:#"Rock impact.mp3"];
}
//-------------collision count for update score value--------
}
}