man1 man1 - 1 month ago 11
iOS Question

SpriteKit: detect complete node overlap

I have two SKShapeNodes – one with an edge-based SKPhysicsBody, one volume-based – and I want to detect their intersection without collision. I've got this working fine, with the SKPhysicsContactDelegate contact methods getting called as one passes over another, but my issue is that

didEndContact
gets called when the edges no longer intersect, even when one body is completely contained within the other. What's the best way to determine true contact or overlap, not just edge intersection? I've tried
usesPreciseCollisionDetection
, to no avail.

Answer

As meisenman suggested, it looks like the best way to do this is using the containsPoint method to determine true node overlap. The docs state that this "returns a Boolean value that indicates whether a point lies inside the node's bounding box", but in my experimentation it looks like this works for concave edge-based shapes as well.

So if I want to detect when two objects no longer overlap, it makes sense to do a containsPoint check within didEndContact – but it turns out didEndContact gets called when each edge is no longer intersected, even if another edge of the same shape still intersects the same object. For example, a ball exiting a rectangle through its corner will yield two didEndContact events. Therefore, it's necessary to keep an absolute count of contact events (the difference between begin and end), and only test for containment when this count is zero.

My solution, in Swift:

var _contactCount = 0

func didBeginContact(contact: SKPhysicsContact!) {
  if ((contact.bodyA.categoryBitMask | contact.bodyB.categoryBitMask) == (CATEGORY_ONE | CATEGORY_TWO)) {
    if (_contactCount == 0) {
      // Contact is actually beginning
    }
    _contactCount += 1
  }
}

func didEndContact(contact: SKPhysicsContact!) {
  if ((contact.bodyA.categoryBitMask | contact.bodyB.categoryBitMask) == (CATEGORY_ONE | CATEGORY_TWO)) {
    _contactCount -= 1
    let overlap = contact.bodyA.node.containsPoint(contact.bodyB.node.position) || contact.bodyB.node.containsPoint(contact.bodyA.node.position)
    if (!overlap && _contactCount == 0) {
      // Contact is actually ending
    }
  }
}
Comments