Asynchronous loading with MultiplayerSpawner

[masterthdev]

As far as I understand, there is no way to show a loading screen when using high level multiplayer. I tried to find a workaround but none of these worked:

• Spawning a loading screen instead of level. This leads to “node not found” errors when host loads scene faster than clients.
• Preloading all levels to memory. This wastes memory and takes too long on my project.
• Using rpcs to make sure all peers have scene loaded. This is only suitable for lobby games. It makes everyone wait for no reason, and doesn’t work when a player joins the game later.
• Running MultiplayerSpawner in a thread group. This doesn’t make any difference.
• Disabling public visibility, and manually enabling it when a peer loads the scene. This causes "cache not found" errors and breaks nested spawners, they cannot despawn anything.

MultiplayerSpawner uses ResourceLoader.load() which hangs the main thread. I think MultiplayerSpawner needs some mechanism to load scenes asynchronously, and nested MultiplayerSpawner and MultiplayerSynchronizers should wait until client loads the parent node.