add example of creating systems via closures

Cargo.toml

@@ -287,6 +287,10 @@ path = "examples/ecs/startup_system.rs"
 name = "state"
 path = "examples/ecs/state.rs"
 
+[[example]]
+name = "system_closures"
+path = "examples/ecs/system_closures.rs"
+
 [[example]]
 name = "system_chaining"
 path = "examples/ecs/system_chaining.rs"

examples/README.md

@@ -159,6 +159,7 @@ 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_closures` | [`ecs/system_closures.rs`](./ecs/system_closures.rs) | Shows how to create and execute systems from closures.
 `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_closures.rs

@@ -0,0 +1,29 @@
+use bevy::prelude::*;
+
+fn main() {
+    App::build()
+        .add_plugin(bevy::log::LogPlugin::default())
+        .add_system(Box::new(|mut cmd: Commands, arg: Local<String>| {
+            info!("this system uses an argument: {:?}", arg);
+            let id = cmd.spawn().id();
+            info!("also it spawned an entity {:?}", id);
+        }).system().config(|config| config.1 = Some("hello".to_string())))
+        .insert_resource(ExampleResource(123))
+        .add_startup_system(create_system(456))
+        .add_system(normal_system.system())
+        .run();
+}
+
+fn normal_system(resource: Res<ExampleResource>) {
+    info!("resource has value {:?}", resource.0);
+}
+
+pub struct ExampleResource(usize);
+
+// Creates a system that modifies a resource.
+fn create_system(arg: usize) -> impl bevy::ecs::system::System<In = (), Out = ()> {
+    Box::new(move |mut resource: ResMut<ExampleResource>| {
+        info!("system running with captured arg {}", arg);
+        resource.0 = arg;
+    }).system()
+}