detecting contact end in box2d cocos2d

Question

I've been using ray wenderlich's contact listener and it's been working well for detecting contacts but I don't understand how to modify it to also tell me when a contact ends.

I want to add a sensor to my enemies so they can detect when a floor ends and turn around instead of falling .

can anyone explain how to modify this listener or point me to a different listener than detect contact ends?

//MyContactListener.mm

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);
}

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) {
}

For detecting contacts I use the following..

for(pos = _contactListener->_contacts.begin();

    pos != _contactListener->_contacts.end(); ++pos) {

    MyContact contact = *pos;
    b2Body *bodyA = contact.fixtureA->GetBody();
    b2Body *bodyB = contact.fixtureB->GetBody();
        CCSprite *spriteA = (CCSprite *) bodyA->GetUserData();
        CCSprite *spriteB = (CCSprite *) bodyB->GetUserData();

//...do stuff
 }

Answer 1

You are storing contacts that have begun in _contacts. Instead you can have two vectors std::vector<MyContact>. First for begun contacts (_begin) and second for ended contacts (_end). When your listener receives message that contact has ended - remove it from _begin and insert into _end. In that case you will be able to iterate over both types of contacts. Just don't forget to clear _end contacts when you are done with them.

Also for detecting if floor ended is better to use ray cast. You can google for using ray cast in box2d

Answer 2

When the size of _contactListner->_contact.size() is zero at that time there is no contact. So when there is contact in world there will be some value in the _contact vector and when it become zero , that suggests that there is no contact further more:that is the end of the contact.