如何在 Box2D 中收到有关对象碰撞和对象触发的通知？

Question

Unity3D has the OnCollisionEnter2D , OnCollisionExit2D , OnTriggerEnter2D etc. However, I am using Box2D and am looking for a way to implement something similar or the same in my C++ project, but I do not know how. I saw something about implementing a listener but I believe it was for the whole world. I am looking to report these events only to the code that physics body is attached to.

Answer 1

Enabling an event listener is necessary for collision events in Box2D, as explained in this video . Alternatively , you could implement your own collision logic. For rectangular hitboxes, that would look something like this: ( Please note: this only works for rectangles with no rotation relative to the screen. )

if (rect1.x < rect2.x + rect2.w &&
    rect1.x + rect1.w > rect2.x &&
    rect1.y < rect2.y + rect2.h &&
    rect1.h + rect1.y > rect2.y) 
{
    // collision detected
} 
else 
{
    // no collision
}

For circles, that would look something like this:

auto dx = (circle1.x + circle1.radius) - (circle2.x + circle2.radius);
auto dy = (circle1.y + circle1.radius) - (circle2.y + circle2.radius);
auto distance = sqrt(dx * dx + dy * dy);

if (distance < circle1.radius + circle2.radius) 
{
    // collision detected
}
else 
{
    // no collision
}

You can see these detection algorithms in more detail here .

Answer 2

Accoring to the api doc , OnCollisionEnter2D is bind to the object itself, so it meet your needs.

Simply, you can add collision detection code in the ego object and do something when a collision is detected.

class Car {

  void process() {
    if (checkCollision(box2d)) {
      // do something
    }
  }
};