add example of creating systems via closures

Cargo.toml

@@ -287,6 +287,11 @@ 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/ecs/system_closures.rs

@@ -0,0 +1,29 @@
+use bevy::prelude::*;
+
+fn main() {
+    let arg = "hello";
+    App::build()
+        .add_system(Box::new(move |mut cmd: Commands| {
+            println!("this system uses the moved arg: {:?}", arg);
+            let id = cmd.spawn().id();
+            println!("also it spawned an entity {:?}", id);
+        }).system())
+        .insert_resource(ExampleResource(123))
+        .add_startup_system(create_system(456))
+        .add_system(normal_system.system())
+        .run();
+}
+
+fn normal_system(resource: Res<ExampleResource>) {
+    println!("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>| {
+        println!("system running with arg {}", arg);
+        resource.0 = arg;
+    }).system()
+}