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Dynamically spawning NetworkObjects in Boss Room to Resolve Zombie NetworkObjects


Required reading: Object Spawning.

This document serves as a walkthrough of Boss Room’s approach to solving the following issue: Late-joining clients entering networked sessions encountering zombie NetworkObjects. Zombie NetworkObjects represent NetworkObjects that are instantiated for a client due to scene loads but aren't despawned or destroyed by Netcode.

This is a particular Netcode limitation of NetworkObject spawning: NetworkObjects that belong to a scene object should not be destroyed until its scene has been unloaded through Netcode’s scene management.

The scenario in question:

  • A host loads a scene and destroys a NetworkObject that belonged to that scene.
  • A client joins the host’s session and loads the additive scene. This operation loads all the GameObjects included in the additive scene. The NetworkObject that was destroyed on the server won't be destroyed on the client’s machine.

This scenario manifested inside Boss Room, whereby late-joining clients joining a game session encountered zombie NetworkObjects that were not destroyed over the network.

Additive scenes now contain Prefab instances of a custom spawner, NetworkObjectSpawner to accommodate this visual inconsistency.

Compositionally, these additive scenes now only contain the following:

  • Prefab instances of level geometry.
  • Prefab instances of NetworkObjects that will not be despawned nor destroyed for the scene’s lifetime. Examples include BreakablePot and BreakableCrystal.
  • Prefab instances of NetworkObjectSpawner, which spawn NetworkObjects that may be destroyed or despawned during the scene’s lifetime. Examples include Imp, VandalImp, and ImpBoss.