Add hierarchy example

Cargo.toml

@@ -205,6 +205,10 @@ path = "examples/ecs/ecs_guide.rs"
 name = "parallel_query"
 path = "examples/ecs/parallel_query.rs"
 
+[[example]]
+name = "hierarchy"
+path = "examples/ecs/hierarchy.rs"
+
 [[example]]
 name = "breakout"
 path = "examples/game/breakout.rs"

examples/ecs/hierarchy.rs

@@ -0,0 +1,112 @@
+use bevy::prelude::*;
+
+fn main() {
+    App::build()
+        .add_default_plugins()
+        .add_startup_system(setup.system())
+        .add_system(rotate.system())
+        .run();
+}
+
+fn setup(
+    mut commands: Commands,
+    asset_server: Res<AssetServer>,
+    mut materials: ResMut<Assets<ColorMaterial>>,
+) {
+    commands.spawn(Camera2dComponents::default());
+    let texture = asset_server.load("assets/branding/icon.png").unwrap();
+
+    // Spawn a root entity with no parent
+    let parent = commands
+        .spawn(SpriteComponents {
+            transform: Transform::from_scale(0.75),
+            material: materials.add(ColorMaterial {
+                color: Color::WHITE,
+                texture: Some(texture),
+            }),
+            ..Default::default()
+        })
+        // With that entity as a parent, run a lambda that spawns its children
+        .with_children(|parent| {
+            // parent is a ChildBuilder, which has a similar API to Commands
+            parent.spawn(SpriteComponents {
+                transform: Transform::from_translation(Vec3::new(250.0, 0.0, 0.0)).with_scale(0.75),
+                material: materials.add(ColorMaterial {
+                    color: Color::BLUE,
+                    texture: Some(texture),
+                }),
+                ..Default::default()
+            });
+        })
+        // Store parent entity for next sections
+        .current_entity()
+        .unwrap();
+
+    // Another way to create a hierarchy is to add a Parent component to an entity,
+    // which would be added automatically to parents with other methods.
+    // Similarly, adding a Parent component will automatically add a Children component to the parent.
+    commands
+        .spawn(SpriteComponents {
+            transform: Transform::from_translation(Vec3::new(-250.0, 0.0, 0.0)).with_scale(0.75),
+            material: materials.add(ColorMaterial {
+                color: Color::RED,
+                texture: Some(texture),
+            }),
+            ..Default::default()
+        })
+        // Using the entity from the previous section as the parent:
+        .with(Parent(parent));
+
+    // Another way is to use the push_children function to add children after the parent
+    // entity has already been spawned.
+    let child = commands
+        .spawn(SpriteComponents {
+            transform: Transform::from_translation(Vec3::new(0.0, 250.0, 0.0)).with_scale(0.75),
+            material: materials.add(ColorMaterial {
+                color: Color::GREEN,
+                texture: Some(texture),
+            }),
+            ..Default::default()
+        })
+        .current_entity()
+        .unwrap();
+
+    // Pushing takes a slice of children to add:
+    commands.push_children(parent, &[child]);
+}
+
+// A simple system to rotate the root entity, and rotate all its children separately
+fn rotate(
+    mut commands: Commands,
+    time: Res<Time>,
+    mut parents_query: Query<(Entity, &mut Transform, &mut Children, &Sprite)>,
+    children_query: Query<(&mut Transform, &Sprite)>,
+) {
+    let angle = std::f32::consts::PI / 2.0;
+    for (parent, mut transform, mut children, _) in &mut parents_query.iter() {
+        transform.rotate(Quat::from_rotation_z(-angle * time.delta_seconds));
+
+        // To iterate through the entities children, just treat the Children component as a Vec
+        // Alternatively, you could query entities that have a Parent component
+        for child in children.iter() {
+            let mut transform = children_query.get_mut::<Transform>(*child).unwrap();
+            transform.rotate(Quat::from_rotation_z(angle * 2.0 * time.delta_seconds));
+        }
+
+        // To demonstrate removing children, we'll start to remove the children after a couple of seconds
+        if time.seconds_since_startup >= 2.0 && children.len() == 3 {
+            // Using .despawn() on an entity does not remove it from its parent's list of children!
+            // It must be done manually if using .despawn()
+            // NOTE: This is a bug. Eventually Bevy will update the children list automatically
+            let child = children.pop().unwrap();
+            commands.despawn(child);
+        }
+
+        if time.seconds_since_startup >= 4.0 {
+            // Alternatively, you can use .despawn_recursive()
+            // This will remove the entity from its parent's list of children, as well as despawn
+            // any children the entity has.
+            commands.despawn_recursive(parent);
+        }
+    }
+}