system_closure/
system_closure.rs

1//! Shows how anonymous functions / closures can be used as systems.
2
3use bevy::{log::LogPlugin, prelude::*};
4
5fn main() {
6    // create a simple closure.
7    let simple_closure = || {
8        // this is a closure that does nothing.
9        info!("Hello from a simple closure!");
10    };
11
12    // create a closure, with an 'input' value.
13    let complex_closure = |mut value: String| {
14        move || {
15            info!("Hello from a complex closure! {}", value);
16
17            // we can modify the value inside the closure. this will be saved between calls.
18            value = format!("{value} - updated");
19
20            // you could also use an outside variable like presented in the inlined closures
21            // info!("outside_variable! {}", outside_variable);
22        }
23    };
24
25    let outside_variable = "bar".to_string();
26
27    App::new()
28        .add_plugins(LogPlugin::default())
29        // we can use a closure as a system
30        .add_systems(Update, simple_closure)
31        // or we can use a more complex closure, and pass an argument to initialize a Local variable.
32        .add_systems(Update, complex_closure("foo".into()))
33        // we can also inline a closure
34        .add_systems(Update, || {
35            info!("Hello from an inlined closure!");
36        })
37        // or use variables outside a closure
38        .add_systems(Update, move || {
39            info!(
40                "Hello from an inlined closure that captured the 'outside_variable'! {}",
41                outside_variable
42            );
43            // you can use outside_variable, or any other variables inside this closure.
44            // their states will be saved.
45        })
46        .run();
47}
system_closure/system_closure.rs

system_closure/
system_closure.rs
