Collisions

JamJar provides a way to natively detect and respond to collisions detected by Box2D by allowing systems to listen out and parse collisions from messages.

Listening For a Collision

To listen for a collision, a system must first subscribe to the message event.

#include "standard/2d/box2d/box2d_physics_system.hpp"

...

YourCustomSystem::YourCustomSystem(JamJar::MessageBus *messageBus): JamJar::System(messageBus) {
    this->messageBus->Subscribe(this, JamJar::Standard::_2D::Box2DPhysicsSystem::MESSAGE_COLLISION_ENTER);
    this->messageBus->Subscribe(this, JamJar::Standard::_2D::Box2DPhysicsSystem::MESSAGE_COLLISION_EXIT);
}

This subscribes the YourCustomSystem system to the messages sent when a collision is entered and exited.

Parsing a Collision

After subscribing a system to a collision event it will then recieve any collision messages through the OnMessage method. Messages should be filtered and responded to based on the message type, so it's good practice to use a switch statement here to differentiate between message types.

After getting a collision message it will always have a payload attached to the message, so a static cast can be used to retrieve this collision data in the form of a Box2DCollision.

#include "standard/2d/box2d/box2d_body.hpp"
#include "standard/2d/box2d/box2d_physics_system.hpp"
#include <iostream>

...

void YourCustomSystem::OnMessage(JamJar::Message *message) {
    System::OnMessage(message);
    switch (message->type) {
    case JamJar::Standard::_2D::Box2DPhysicsSystem::MESSAGE_COLLISION_ENTER: {
        auto *collisionMessage = static_cast<JamJar::MessagePayload<JamJar::Standard::_2D::Box2DCollision> *>(message);
        auto collision = collisionMessage->payload;
        std::cout << "Entity A in the collision enter is: " << collision.aID << std::endl;
        std::cout << "Entity B in the collision enter is: " << collision.bID << std::endl;
        break;
    }
    switch (message->type) {
    case JamJar::Standard::_2D::Box2DPhysicsSystem::MESSAGE_COLLISION_EXIT: {
        auto *collisionMessage = static_cast<JamJar::MessagePayload<JamJar::Standard::_2D::Box2DCollision> *>(message);
        auto collision = collisionMessage->payload;
        std::cout << "Entity A in the collision exit is: " << collision.aID << std::endl;
        std::cout << "Entity B in the collision exit is: " << collision.bID << std::endl;
        break;
    }
    }
}

This handles the messages when they are published and dispatched, with the YourCustomSystem system parsing out the entity IDs from each collision and printing them.

Collision Data

The data in a collision is represented by the Box2DCollision object, which holds the following pieces of information:

• The IDs of the entities involved in the collision.
• A Box2D b2Contact object which describes the collision in depth, see the Box2D docs for full details.
• The type of the collision, defined as an enum Box2DCollisionType, this describes the collision event, e.g. collision enter or exit.