seldom_state

seldom_state is a component-based state machine plugin for Bevy. It's useful for AI, player state, and other entities that occupy various states. It allows for greater reusability of state logic between entities, compared to managing mutually-exclusive components directly in your systems.

A state is a component attached to an entity that defines its current behavior, such as Jumping or Stunned. A trigger is a system that checks information about entities in the world, such as near_position or health_below_threshold. A transition links two states: one to transition from, and one to transition to; once a given trigger has occurred. A state machine is a component attached to an entity that keeps track of that entity's transitions, and automatically changes the entity's state according to those transitions.

State machines are created like so:

commands.spawn((
    // ... (other inserts)
    MyInitialState::new(),
    StateMachine::default()
        .trans::<MyInitialState, _>(my_trigger_1, my_state_2)
        .trans::<AnyState, _>(my_trigger_3, my_state_4)
        .trans_builder(my_trigger_5, |my_state_6: &MyState6, trigger_data| {
            make_state_7(my_state_6, trigger_data)
        })
        .on_enter::<MyState7>(move |entity| entity.insert(my_bundle.clone()))
        .on_exit::<MyState7>(|entity| entity.remove::<MyBundle>())
        // etc.
));

For more complete examples, see the examples directory. The chase.rs example is written like a guide, so it is good for learning. If you need help, feel free to ping me on the Bevy Discord server (@Seldom)! If anything needs improvement, feel free to submit an issue or pr!

Features

• State machine component with user-defined states and triggers
• 30 built-in triggers
  • always: always triggers
  • NotTrigger, AndTrigger, and OrTrigger: combines triggers with boolean logic
  • done: triggers when the Done component is added to the entity
  • 24 more triggers enabled by the leafwing_input feature: action_data, axis_pair, axis_pair_length_bounds, axis_pair_max_length, axis_pair_min_length, axis_pair_rotation_bounds, axis_pair_unbounded, clamped_axis_pair, clamped_axis_pair_length_bounds, clamped_axis_pair_max_length, clamped_axis_pair_min_length, clamped_axis_pair_rotation_bounds, clamped_axis_pair_unbounded, clamped_value, clamped_value_max, clamped_value_min, clamped_value_unbounded, just_pressed, just_released, pressed, value, value_max, value_min, and value_unbounded
  • on_event: triggers when it reads an event of the given type
  • Bevy's built-in run conditions also work as triggers
• AnyState state, that can be used in type parameters to represent any state
• Transition builders that allow dataflow from outgoing states and triggers to incoming states (StateMachine::trans_builder)
• Automatically perform behavior upon entering or exiting states (StateMachine::on_enter, StateMachine::on_exit, StateMachine::command_on_enter and StateMachine::command_on_exit)

Comparison with big-brain

Finite state machine is an old and well-worn pattern in game AI, so its strengths and limitations are known. It is good for entities that:

1. Do not have a huge number of interconnected states, since the number of transitions can grow quadratically. Then it becomes easy to forget to add a transition, causing difficult bugs.
2. Act rigidly, like the enemies in Spelunky, who act according to clear triggers such as got-jumped-on-by-player and waited-for-5-seconds and are predictable to the player, and unlike the dwarves in Dwarf Fortress, who weigh their options of what to do before taking an action and feel lively.

seldom_state is a finite state machine implementation, so it may not be suitable for all types of game AI. If you need a solution that works with more complex states and transitions, then you may want to implement a behavior tree (I had little luck turning existing implementations into a Bevy plugin without forking them). If you need a solution that operates on fuzzy logic, and do not need to define which transitions should be allowed, then I recommend big-brain. If you need fuzzy logic and discrete transitions, you may want to implement a fuzzy state machine. If you need discrete transitions, but not fuzzy logic, consider seldom_state!

seldom_state is not just an AI crate, though. So, you may want to use big-brain for your enemies' AI, and seldom_state to manage state for your player, and control enemies' animation, or something.

Design patterns

seldom_state is rather versatile, so some problems may be solved in multiple ways. If you're lost, here is some advice.

I have an entity whose animations depend on behavior

There are a few solutions to this. The most straightforward is to add the animations to the entity with on_enter. This works for animation systems that rigidly follow behavior, such as the player controller in a 2D fighter or a basic enemy controller. Of course, this is rigid, and anything that the animations must remember between states must be handled manually. In a platformer like Celeste that has multiple animations for a single state (lets assume it has states Grounded, Airborne, Dashing, and Climbing), you might manage some animations, like the dashing animation, through on_enter, and others, like the walk cycle, through systems. Or, you might manage all animations through systems. This is up to preference.

On the other hand, if your animations are less constrained to behavior, consider using multiple state machines, as in the next section:

My entity needs to be in multiple states at once

Consider a 2D platformer, where the player has a sword. The player can run and jump around, and they can swing the sword. So whether you're running, jumping, or dashing, you always swing the sword the same way, independently of movement state. In this case, you might want to have a movement state machine and an attack state machine. Since entities can only have one state machine, spawn another entity (as a child, I would suggest) with its own state machine, and capture the original Entity in closures in command_on_enter and command_on_exit.

However, perhaps your states are not so independent. Maybe attacking while dashing puts the player in a PowerAttack state, or the attack cooldown doesn't count down while moving. Depending on the scale of the dependency, you might want to just have your state machines communicate through commands and observing each other's states, or you might want to combine the state machines, permuting relevant states into states like DashAttack and IdleAttackCooldown.

Also, consider managing one set of states through a state machine and another through systems.

I have some other problem that's difficult to express through the StateMachine API

Remember that StateMachine is component-based, so you can solve some problems normally through Bevy's ECS. Instead of on_enter::<MyState> you can use Added<MyState> in a system, and you can even change state manually through remove and insert commands. If you do change state manually, callbacks like on_enter will not be called, and you will have to make sure that the state machine remains in exactly one state at a time. Else, it will panic.

Usage

Add to your Cargo.toml

# Replace * with your desired version

[dependencies]
seldom_state = "*"

See the chase.rs example for further usage.

Compatibility

License

seldom_state is dual-licensed under MIT and Apache 2.0 at your option.

Contributing

Unless you explicitly state otherwise, any contribution intentionally submitted for inclusion in the work by you, as defined in the Apache-2.0 license, shall be dual licensed as above, without any additional terms or conditions.