Added example of creating a system from a closure (#4327)

Fixes #4262

Cargo.toml

@@ -391,6 +391,10 @@ path = "examples/ecs/state.rs"
 name = "system_chaining"
 path = "examples/ecs/system_chaining.rs"
 
+[[example]]
+name = "system_closure"
+path = "examples/ecs/system_closure.rs"
+
 [[example]]
 name = "system_param"
 path = "examples/ecs/system_param.rs"

examples/README.md

@@ -190,6 +190,7 @@ Example | File | Description
 `startup_system` | [`ecs/startup_system.rs`](./ecs/startup_system.rs) | Demonstrates a startup system (one that runs once when the app starts up)
 `state` | [`ecs/state.rs`](./ecs/state.rs) | Illustrates how to use States to control transitioning from a Menu state to an InGame state
 `system_chaining` | [`ecs/system_chaining.rs`](./ecs/system_chaining.rs) | Chain two systems together, specifying a return type in a system (such as `Result`)
+`system_closure` | [`ecs/system_closure.rs`](./ecs/system_closure.rs) | Show how to use closures as systems, and how to configure `Local` variables by capturing external state
 `system_param` | [`ecs/system_param.rs`](./ecs/system_param.rs) | Illustrates creating custom system parameters with `SystemParam`
 `system_sets` | [`ecs/system_sets.rs`](./ecs/system_sets.rs) | Shows `SystemSet` use along with run criterion
 `timers` | [`ecs/timers.rs`](./ecs/timers.rs) | Illustrates ticking `Timer` resources inside systems and handling their state

examples/ecs/system_closure.rs

@@ -0,0 +1,45 @@
+use bevy::{log::LogPlugin, prelude::*};
+
+fn main() {
+    // create a simple closure.
+    let simple_closure = || {
+        // this is a closure that does nothing.
+        info!("Hello from a simple closure!");
+    };
+
+    // create a closure, with an 'input' value.
+    let complex_closure = |mut value: String| {
+        move || {
+            info!("Hello from a complex closure! {:?}", value);
+
+            // we can modify the value inside the closure. this will be saved between calls.
+            value = format!("{} - updated", value);
+
+            // you could also use an outside variable like presented in the inlined closures
+            // info!("outside_variable! {:?}", outside_variable);
+        }
+    };
+
+    let outside_variable = "bar".to_string();
+
+    App::new()
+        .add_plugin(LogPlugin)
+        // we can use a closure as a system
+        .add_system(simple_closure)
+        // or we can use a more complex closure, and pass an argument to initialize a Local variable.
+        .add_system(complex_closure("foo".into()))
+        // we can also inline a closure
+        .add_system(|| {
+            info!("Hello from an inlined closure!");
+        })
+        // or use variables outside a closure
+        .add_system(move || {
+            info!(
+                "Hello from an inlined closure that captured the 'outside_variable'! {:?}",
+                outside_variable
+            );
+            // you can use outside_variable, or any other variables inside this closure.
+            // their states will be saved.
+        })
+        .run();
+}