bevy/crates/bevy_ecs/examples/events.rs

use bevy_ecs::prelude::*;

// In this example a system sends a custom event with a 50/50 chance during any frame.
// If an event was send, it will be printed by the console in a receiving system.
fn main() {
    // Create a new empty world and add the event as a resource
    let mut world = World::new();
    world.insert_resource(Events::<MyEvent>::default());

    // Create a schedule to store our systems
    let mut schedule = Schedule::default();

    // Events need to be updated in every frame in order to clear our buffers.
    // This update should happen before we use the events.
    // Here, we use system sets to control the ordering.
    #[derive(SystemSet, Debug, Clone, PartialEq, Eq, Hash)]
    pub struct FlushEvents;

    schedule.add_systems(Events::<MyEvent>::update_system.in_set(FlushEvents));

    // Add systems sending and receiving events after the events are flushed.
    schedule.add_systems((
        sending_system.after(FlushEvents),
        receiving_system.after(sending_system),
    ));

    // Simulate 10 frames of our world
    for iteration in 1..=10 {
        println!("Simulating frame {iteration}/10");
        schedule.run(&mut world);
    }
}

// This is our event that we will send and receive in systems
#[derive(Event)]
struct MyEvent {
    pub message: String,
    pub random_value: f32,
}

// In every frame we will send an event with a 50/50 chance
fn sending_system(mut event_writer: EventWriter<MyEvent>) {
    let random_value: f32 = rand::random();
    if random_value > 0.5 {
        event_writer.send(MyEvent {
            message: "A random event with value > 0.5".to_string(),
            random_value,
        });
    }
}

// This system listens for events of the type MyEvent
// If an event is received it will be printed to the console
fn receiving_system(mut event_reader: EventReader<MyEvent>) {
    for my_event in event_reader.read() {
        println!(
            "    Received message {:?}, with random value of {}",
            my_event.message, my_event.random_value
        );
    }
}