Physics is scary, do I really need it? Please tell me no!

Please don't run away there are no physics monsters under your bed! Your needs might be simple enough to not need to use a physics engine. Perhaps a combination of using a Node objects update() function, Rect objects and a combination of the containsPoint() or intersectsRect() functions might be enough for you? Example:

void update(float dt)
{
  auto p = touch->getLocation();
  auto rect = this->getBoundingBox();

  if(rect.containsPoint(p))
  {
      // do something, intersection
  }
}

This mechanism works for very simple needs, but doesn't scale. What if you had 100 Sprite objects all continuously updating to check for intersections with other objects? It could be done but the the CPU usage and framerate would suffer severely. Your game would be unplayable. A physics engine solves these concerns for us in a scalable and CPU friendly way. Even though this might look foreign, let's take a look at a simple example and then nut and bolt the example, terminology and best practice together.

// create a static PhysicsBody
auto physicsBody = PhysicsBody::createBox(Size(65.0f , 81.0f ), PhysicsMaterial(0.1f, 1.0f, 0.0f));
physicsBody->setDynamic(false);

// create a sprite
auto sprite = Sprite::create("whiteSprite.png");
sprite->setPosition(Vec2(400, 400));

// sprite will use physicsBody
sprite->addComponent(physicsBody);

//add contact event listener
auto contactListener = EventListenerPhysicsContact::create();
contactListener->onContactBegin = CC_CALLBACK_1(onContactBegin, this);
_eventDispatcher->addEventListenerWithSceneGraphPriority(contactListener, this);

Even though this example is simple, it looks complicated and scary. It really isn't if we look closely. Here are the steps that are happening:

• A PhysicsBody object is created.
• A Sprite object is created.
• The Sprite object applies the properties of the PhysicsBody object.
• A listener is created to respond to an onContactBegin() event.

Once we look step by step the concept starts to make sense.