RewindableStateMachine

Rollback-aware state machine implementation.

State machines are often used in games to implement different behaviors. However, most implementations are not prepared for rollbacks. This class provides an extensible implementation that can be used alongside a RollbackSynchronizer.

For a full example, see multiplayer-state-machine.

Creating a state machine

The first step is to add the RewindableStateMachine to your scene. It also requires a RollbackSynchronizer that manages its state property. Unless these conditions are satisfied, an editor warning will be displayed.

Note: Editor warnings are only updated when the node tree changes, configuration changes don't trigger an update. You may need to reload the scene after fixing a warning.

Notice the RollbackSynchronizer added as a sibling to the RewindableStateMachine, and having its state property configured.

Implementing states

States are where the custom gameplay logic can be implemented. Each state must be an extension of the RewindableState class, and added as a child to the RewindableStateMachine.

States react to the game world using the following callbacks:

• tick(delta, tick, is_fresh) is called for every rollback tick.
• enter(previous_state, tick) is called when entering the state.
• exit(next_state, tick) is called when exiting the state.
• can_enter(previous_state) is called before entering the state. The state is only entered if this method returns true.

You can override any of these callbacks to implement your custom behaviors.

For example, the snippet below implements an idle state, that transitions to other states based on movement inputs:

extends RewindableState

@export var input: PlayerInputStateMachine

func tick(delta, tick, is_fresh):
    if input.movement != Vector3.ZERO:
        state_machine.transition(&"Move")
    elif input.jump:
        state_machine.transition(&"Jump")

Transitions are based on node names, i.e. calling transition(&"Move") will transition to a state node called Move.

States must be added as children under a RewindableStateMachine to work.