Optional collision with SKPhysicsContactDelegate

Question

Is it possible to cancel a collision before it happens in the space?

Example: A and B is about to collide but A has a property so that the collision with B should not take place for the specific SKPhysicsContact. All other collisions in the space should be unaffected.

When reading Apple´s documentation for the class it says:

The physics contact delegate methods are called during the physics simulation step. During that time, the physics world can't be modified and the behavior of any changes to the physics bodies in the simulation is undefined. If you need to make such changes, set a flag inside didBegin(:) or didEnd(:) and make changes in response to that flag in the update(_:for:) method in a SKSceneDelegate.

Does this mean I can detect the state in didBegin() and change the collision mask in update() before the actual collision is a fact?

There is an older thread with a similar topic: SpriteKit SKPhysicsBody collision in one direction like a door you can only go through but not back

Answer 1

Sure

You could set the contactBitMask to call the didBegin method when the contact is detected. Once didBegin has been called and you identify the node you want from the information in the (_ contact: SKPhysicsContact) argument.

Set a flag indicating that the contact has occurred. From there run a flow control (if) statement in the override func didFinishUpdate() to check the value of the flag. If true then change the collision masks of the objects.

Then on the next game loop iteration the physics will not detect the collision. You will still get that initial collision though on the game loop that detects the contact. There are ways around that though. Like making a second larger physics body to detect the contact before the collision occurs.

Answer 2

Based on what I am understanding, You want an object to go through an object some of the time.

The following code should work:

enum CategoryBitMasks{
  case player1Collidable = 1 << 0
  case player2Collidable = 1 << 1
  case player1 = 1 << 2
  case player2 = 1 << 3
  case oneTimePass = 1 << 4
  case unstoppable = 1 << 5
  case player = CategoryBitMasks.player1 | CategoryBitMasks.player2
  case playerCollidable = CategoryBitMasks.player1Collidable | CategoryBitMasks.player2Collidable
}


//Create an obstacle with category (.player1Collidable | .player2Collidable)
//                   with contact 0
//                   with collision 0
//Create a player with category (.player1 | .unstoppable)
//                with contact .player1Collidable
//                with collision .player1Collidable 

func didBeginContact(contact: SKPhysicsContact) {
    var obstacle = contact.bodyA.categoryBitMask >= contact.bodyB.categoryBitMask ?contact.bodyA, contact.bodyB
    var player = contact.bodyB == obstacle ? contact.bodyA, contact.bodyB

    //Only allows the player to pass through 1 object
    if player.categoryBitMask || CategoryBitMasks.oneTimePass && obstacle.categoryBitMask || CategoryBitMasks.playerCollidable  {
       obstacle.categoryBitMask &= ~(player.collisionBitMask & CategoryBitMasks.playerCollidable) //turns player1Collidable off
       player.categoryBitMask &= ~(player.categoryBitMask & CategoryBitMasks.oneTimePass) //turns oneTimePass off

    }

    //Allows player to pass through all objects
    if player.categoryBitMask || CategoryBitMasks.unstoppable && obstacle.categoryBitMask || CategoryBitMasks.playerCollidable  {

       //This will bypass all objects providing their collision is not set to the player
       player.collisionBitMask &= ~(player.collisionBitMask & CategoryBitMasks.playerCollidable) //turns player1Collidable off

       //This will bypass all objects, but is dangerous because the category is now changed, meaning if a contact does happen, .player can't be used
       //player.categoryBitMask &= ~(player.categoryBitMask & CategoryBitMasks.player ) //turns player1Collidable off

    }
}

Remember to reset the masks when needed

This will allow player 1 to go through objects depending on category you gave it, while forcing player 2 to still be obstructed by the same object.

Hopefully these examples will help you tweak the needs of your game.