Is it possible to determine that all Sprite Kit scene collisions have played out?

Question

I've made my way through the pages of SO questions regarding sprite-kit, the docs, available demos, and the vast empty space that is my brain to try and figure this one out but have come up short.

Working on a proof-of-concept that is a turn-based (using Game Center), physics-based game. Players take turns placing a node amongst a collection of other nodes and letting the physics play out. Where I'm flummoxed is in determining that all collisions have played out and the turn has ended.

It would seem that the nodes never stop colliding.

Answer 1

This is a tricky task with physics simulations.

First of all, with gravity enabled you always have a force acting upon bodies that may prevent it from resting. So you need to test every body every couple of steps (or every frame if you must) to see whether its velocity vector length is smaller than a given threshold. Then manually set the body's resting state to YES.

The bodies may wake up again during contacts, so it is crucial to perform this step after contact resolution in didSimulatePhysics.

The trick is in finding a threshold that is large enough to guarantee an end to the "physics entanglements" while not being too high to make objects stop where they clearly shouldn't.

Even then you probably also need to set a timer that starts when every body's velocity is below another (higher) threshold indicating that there will likely be little forces coming from those bodies anyway, and if after a grace period (ie 10 seconds) the bodies are still not all resting but below this threshold, consider the collisions resolved (as far as the game is concerned).

Also the number of bodies still not resting may be an indicator to end the game, ie if there are only 3 or 4 bodies of dozens still moving, there's probably not much going to happen as well.