How to detect when a disconnection has happened?

Unity Engine
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  1. Detecting disconnection from Client in the Server

Is there a callback I can listen to?

I know that when a client disconnects, its “NetworkConnection” entity (the one that has a NetworkIdComponent) is destroyed in the Server World. Clearly, Netcode for Entities does detect when a client disconnects. How can my server’s logic detect that too?

I am aware that there is a “proper” way to disconnect with NetworkStreamRequestDisconnect. I don’t want to rely on that, if the client crashes or the player kills the game’s process I want to know that they disconnected.

  1. Detecting disconnection from Server in the Client

What is the appropriate way?

Thank you for your help!

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Hey Emma,

Yes, you can track both connections and disconnections via the optional ConnectionState ICleanupComponentData.

“PlayerList” is a new sample showing this feature off, found here: EntityComponentSystemSamples/NetcodeSamples/Assets/Samples/PlayerList at master · Unity-Technologies/EntityComponentSystemSamples · GitHub
This sample uses RPCs to create a “feed” of players who have connected/disconnected.
One other approach (which isn’t demonstrated here) is to implement this behaviour via a static-optimized ghost (with an IBufferElementData containing all connected players).

For this implementation, take a look at the server logic here: EntityComponentSystemSamples/NetcodeSamples/Assets/Samples/PlayerList/ServerPlayerListSystem.cs at master · Unity-Technologies/EntityComponentSystemSamples · GitHub

ConnectionState is added on like 67, where we have a query of all new NetworkStreamConnection entities, where we add the optional component.

New joiners are handled in the HandleNewJoinersJob on line 101. We wait for a client to send an RPC containing their Username. Thus, each client must opt-into being seen in the player list. Obviously, for your own game, you can source usernames however you like. E.g. The GameServer could communicate with a backend instead.

For disconnecting players (regardless of disconnect reason): The NotifyPlayersOfDisconnectsJob job on line 184 will show you the query, and how it’s used to detect disconnects.

The ClientPlayerListSystem:

  • Registers a username with the “PlayerList” sub-system.
  • Receives these “PlayerList” change events.
  • Uses a cache of this data to draw the Player List, and all NOTIFICATIONS (via IMGUI).

You can also use new controls in the “Multiplayer > Window: PlayMode Tools” to test various forms of disconnect. The next few pre-releases will contain improvements to this window.

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Nice, this was really helpful.

To continue talk about disconnections, is there a particular reason why ALL ghost entities are automatically destroyed on clients if the server disconnects? Is there a way to keep them, so I can kill the game gracefully? At the moment, because a lot of the visuals are made of ghosted entities, the world disppearing is very jarring, and I don’t particularly want to hard-cut to a fullscreen “connection lost” popup to hide it.

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Supporting ‘reconnecting with existing state’ is a non-trivial feature that we simply haven’t had time to build yet. It’s on our radar, but feature-work likely won’t start for a long time. If you need this, you’ll need to roll it yourself via a package fork.

Unfortunately not easily, but again it’s on our radar.

If you only want to keep them (but not reconnect with this cached state), you can probably hack in a thing that prevents those ghosts from succeeding the delete query. My guess (not at PC) is to remove the GhostInstance component when you detect a disconnect.

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I’m still not sure what the reason is for deleting ghost entities when disconnected, but removing GhostInstance does the trick. Many thanks!

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It’s only in case you reconnect. Doing so - with ‘stale’ ghosts already spawned on the client - would confuse netcode and lead to unknown/undefined behaviour.

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