Bevy Systems

Systems are where we trigger side effects that change our game's state.

In Bevy, systems are simple rust functions with one rule: They can only have parameters that implement SystemParam.

Bevy uses some type magic to figure out how to provide these system parameters to our systems without us having to manually pass them in. This technique is a form of dependency injection.

A rust function (or lambda) will be automatically converted to a System via the IntoSystem trait which Bevy calls when you register a system to your App.

// Behold, a system!
fn hello_world() {
  println!("Hello, world!");
}

// Bevy converts this into a real `System` when you register them
// Bevy is going to do this automatically, this is just for illustration
let mut system = IntoSystem::into_system(hello_world);

Resources, commands, and queries, all describe ways of fetching data to and from the ECS. So your parameters are a declarative way of interacting with the data from your game's World.

Scheduling systems

When systems are added to our App they are added to a particular Schedule. These schedules contain the rules of when each system should run over the course of each frame.

By default, Bevy is trying to schedule all systems that don't need mutable access to the same data to run in parallel. This is all in an effort to speed up our game.

To schedule a system we call add_systems and specify the ScheduleLabel and the system(s) we want to run:

fn main() {
  App::new()
    .add_plugins(DefaultPlugins)
    .add_systems(Update, hello_world);
}

Each call to add_systems can also take a tuple of systems:

fn main() {
  App::new()
    .add_plugins(DefaultPlugins)
    .add_systems(Update, (defend, attack));
}

Or, if we need fine grain control over ordering, we can use Bevy's built-in methods like before and after:

fn main() {
  App::new()
    .add_plugins(DefaultPlugins)
    .add_systems(Update, (defend, attack.after(defend)));
}

One thing to note is that having parameters and then using them conditionally won't block parallel execution:

fn system_a(mut commands: Commands) {
  if random_bool() {
    commands.spawn_empty();
  }
}

fn system_b(mut commands: Commands) {
  if random_bool() {
    commands.spawn_empty();
  }
}

There is a chance these two systems run in parallel. The overhead of the mutable borrow depends on whether or not we call it.

Fallibility

If your system returns a Result then Bevy considers it to be failable. This will cause Bevy to handle your error with the default behavior being to panic.

fn failable_system() -> Result<()> {
  println!("Running failable system...");
  Ok(())
}

The default error handling behavior can be modified by setting it before you initialize your App:

use bevy::ecs::error::warn;

GLOBAL_ERROR_HANDLER.set(warn)
  .expect("The error handler can only be set once, globally.");

// initialize Bevy App here

There are a bunch of pre-built error-handlers under the bevy::ecs::error module:

• panic: panics with the system error
• error: logs the system error at the error level
• warn: logs the system error at the warn level
• info: logs the system error at the info level
• debug: logs the system error at the debug level
• trace: logs the system error at the trace level
• ignore: ignores the system error

Fallible system parameters

Even if you do not return a Result from your system, there are certain parameters that your system will fail during a SystemParam::validate_param call.

For example if we were to query for a component with Single that does not yet exist:

fn find_friends(friend: Single<&Ally>) {
    // Uh oh, we don't have any friends.
}

The find_friends system would be skipped in the case the system parameter did not validate.

If we wanted the system to always run and decide ourselves what to do in each case then you can wrap Single in an Option:

fn find_friends(friend: Option<Single<&Ally>>) {
  if let Some(ally) = friend {
    info!("Yay we have a friend");
  }
}

In this case the system would always run.

Ordering

By default systems run in parallel with each other and their order is non-deterministic.

fn main() {
  App::new()
    .add_plugins(DefaultPlugins)
    .add_systems(Update, (defend, attack));
}

In this example it is not possible to tell whether defend is going to run after, before or in parallel with attack.

Normal systems cannot safely access the World instance directly because it would block everyone else. Our World contains all of our components, so mutating arbitrary parts of it in parallel is not thread safe.

Ordering can be controlled by:

• Using core sets like Update, PostUpdate, FixedUpdate, etc..
• Calling the .before(this_system) or .after(that_system) methods when adding them to your schedule
• Adding them to a SystemSet, and then using .configure_set(ThisSet.before(ThatSet)) syntax to configure many systems at once
• Through the use of chaining like (system_a, system_b, system_c).chain()

System params and param sets

System params will automatically fetch data from a World without interacting directly with the World struct.

Systems take SystemParam trait parameters. SystemParam structs have two lifetimes:

1. 'w for data stored in the World
2. 's for data stored in a parameter's state

Some common SystemParam are:

Unfortunately, Rust does not yet have variadics, so functions are limited to a maximum number of up to 16 function arguments.

A notable system parameter is the ParamSet which is a collection of potentially conflicting SystemParam's.

It allows systems to safely access and interact with up to 8 mutually exclusive params. For example: two queries that reference the same mutable data or an event reader and writer of the same type.

We can access the params of a ParamSet with p0, p1, etc according to the order they were defined in the type. A ParamSet can take any SystemParam.

ParamSet can be used when mutably accessing the same component twice in one system:

// This will panic at runtime when the system gets initialized.
fn bad_system(
  mut enemies: Query<&mut Health, With<Enemy>>,
  mut allies: Query<&mut Health, With<Ally>>,
) {
  // ...
}

Instead ParamSet leverages the borrow checker to ensure that only one of the contained parameters are accessed at a given time.

fn good_system(
  mut set: ParamSet<(
    Query<&mut Health, With<Enemy>>,
    Query<&mut Health, With<Ally>>,
  )>,
) {
  // This will access the first `SystemParam`.
  for mut health in set.p0().iter_mut() {
    // Do your fancy stuff here...
  }
  // The second `SystemParam`.
  // This would fail to compile if the previous parameter was still borrowed.
  for mut health in set.p1().iter_mut() {
    // Do even fancier stuff here...
  }
}

Custom system parameters

We can create our own system parameters by deriving the trait on a struct. The only thing we have to be careful of is the two lifetimes mentioned earlier.

// The [`SystemParam`] struct can contain any types that can also be included in
// a system function signature.
//
// In this example, it includes a query and a mutable resource.
#[derive(SystemParam)]
struct PlayerCounter<'w, 's> {
  players: Query<'w, 's, &'static Player>,
  count: ResMut<'w, PlayerCount>,
}

impl PlayerCounter<'_, '_> {
  fn count(&mut self) {
    self.count.0 = self.players.iter().len();
  }
}

// The [`SystemParam`] can be used directly in a system argument.
fn count_players(mut counter: PlayerCounter) {
  counter.count();

  println!("{} players in the game", counter.count.0);
}

This can be very useful for reducing the number of parameters you need to pass your systems and to get around the 16 parameter limit due to the way Bevy uses macros to turn functions into system parameters.

System state

Normally your systems are stateless, but they can be locally stateful by using the Local<T> system parameter:

fn print_at_end_round(mut counter: Local<u32>) {
  *counter += 1;
  println!("In set 'Last' for the {}th time", *counter);
  // Print an empty line between rounds
  println!();
}

The local counter variable will keep its state between invocations of the function.

Combining systems

Higher order systems can be composed of many other systems using the pipe method. This function will take the output of the system and pass it as input to the next system.

As long as our next system has its first parameter as a type In<T> it can be composed this way.

This can be used in combination with ParamSet to avoid SystemParam collisions.

fn main() {
  App::new()
    .add_plugins(DefaultPlugins)
    .add_systems(
      Update,
      (
        parse_message_system.pipe(handler_system),
        data_pipe_system.pipe(info),
        parse_message_system.pipe(debug),
        warning_pipe_system.pipe(warn),
        parse_error_message_system.pipe(error),
        parse_message_system.pipe(ignore),
      ),
    );
}

Exclusive systems

Usually we would schedule our commands with the Command system parameter so they can be executed later in a system that has exclusive access to our World.

However, if we use the World system parameter then we can manipulate the state directly. For example we can run commands exactly when the system runs, instead of scheduling them to run later:

fn spawn_immediately(world: &mut World) {
  world.spawn(Player);
}

Systems with this parameter are called exclusive systems.

The downside of exclusive systems is that they cannot be run in parallel if other systems also need to mutate the world. Otherwise they work exactly the same as your normal systems.