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Change shared state from "Repeat every N seconds"

Suppose we want to do some shared, session-wide work every N seconds, forever. In this multiplayer example, we're going to spawn shared gems in the scene every 5 seconds.

This pattern is suitable for events which always happen on an endless timer and don't depend on any particular action from one of the players. In a golfing game, we might want to ambiently change the wind direction every few minutes. In a fighting game, we might want to raise the level of the lava below the platform, or spawn new powerups in the arena.

Example deck​

Multiplayer example: Change shared state from Repeat

Structure​

  • We have a session manager blueprint which is shared and session-controlled. One session manager is placed in the scene from the belt.
  • The session manager has a rule like this:
When this shared actor is loaded,
  Repeat every 5 seconds,
    If this shared actor is controlled by the current device,
      Create a gem

For comparison, here's the analogous logic from a non-multiplayer game:

When this is created,
  Repeat every 5 seconds,
    Create a gem

Analysis​

Because the session manager is placed in the initial scene from the belt, When this is loaded fires when the session starts. If the session is persisted from a Party, When this is loaded also fires again when the session is resumed. We can't use When this is created because it would only fire in the first case.

The loaded trigger fires on every device in the session, including both the session-controlling device and all player-controlling devices. Therefore, we need to check If this shared actor is controlled by the current device to make sure this rule is running on the session device before we attempt to spawn shared gems.

Lastly, Castle may change which device is the session-controlling device over the course of the session. For example, if the session-controlling player quits, then session control is automatically passed to someone else. The loaded trigger won't fire when session control changes. Therefore, we need to run the Repeat on all devices, and inside the repeat, check for session ownership, assuming that this device may gain or lose session ownership at any time. As ownership is handed off between devices, the correct device always spawns gems.