Add entity disabling example (bevyengine#17710)

# Objective

The entity disabling / default query filter work added in bevyengine#17514 and
bevyengine#13120 is neat, but we don't teach users how it works!

We should fix that before 0.16.

## Solution

Write a simple example to teach the basics of entity disabling!

## Testing

`cargo run --example entity_disabling`

## Showcase


![image](https://github.com/user-attachments/assets/9edcc5e1-2bdf-40c5-89b7-5b61c817977a)

---------

Co-authored-by: Zachary Harrold <[email protected]>

Cargo.toml

@@ -2025,6 +2025,17 @@ description = "Demonstrates how to send and receive events of the same type in a
 category = "ECS (Entity Component System)"
 wasm = false
 
+[[example]]
+name = "entity_disabling"
+path = "examples/ecs/entity_disabling.rs"
+doc-scrape-examples = true
+
+[package.metadata.example.entity_disabling]
+name = "Entity disabling"
+description = "Demonstrates how to hide entities from the ECS without deleting them"
+category = "ECS (Entity Component System)"
+wasm = true
+
 [[example]]
 name = "fixed_timestep"
 path = "examples/ecs/fixed_timestep.rs"

examples/README.md

@@ -308,6 +308,7 @@ Example | Description
 [Custom Schedule](../examples/ecs/custom_schedule.rs) | Demonstrates how to add custom schedules
 [Dynamic ECS](../examples/ecs/dynamic.rs) | Dynamically create components, spawn entities with those components and query those components
 [ECS Guide](../examples/ecs/ecs_guide.rs) | Full guide to Bevy's ECS
+[Entity disabling](../examples/ecs/entity_disabling.rs) | Demonstrates how to hide entities from the ECS without deleting them
 [Event](../examples/ecs/event.rs) | Illustrates event creation, activation, and reception
 [Fallible System Parameters](../examples/ecs/fallible_params.rs) | Systems are skipped if their parameters cannot be acquired
 [Fallible Systems](../examples/ecs/fallible_systems.rs) | Systems that return results to handle errors

examples/ecs/entity_disabling.rs

@@ -0,0 +1,153 @@
+//! Disabling entities is a powerful feature that allows you to hide entities from the ECS without deleting them.
+//!
+//! This can be useful for implementing features like "sleeping" objects that are offscreen
+//! or managing networked entities.
+//!
+//! While disabling entities *will* make them invisible,
+//! that's not its primary purpose!
+//! [`Visibility`](bevy::prelude::Visibility) should be used to hide entities;
+//! disabled entities are skipped entirely, which can lead to subtle bugs.
+//!
+//! # Default query filters
+//!
+//! Under the hood, Bevy uses a "default query filter" that skips entities with the
+//! the [`Disabled`] component.
+//! These filters act as a by-default exclusion list for all queries,
+//! and can be bypassed by explicitly including these components in your queries.
+//! For example, `Query<&A, With<Disabled>`, `Query<(Entity, Has<Disabled>>)` or
+//! `Query<&A, Or<(With<Disabled>, With<B>)>>` will include disabled entities.
+
+use bevy::ecs::entity_disabling::Disabled;
+use bevy::prelude::*;
+
+fn main() {
+    App::new()
+        .add_plugins((DefaultPlugins, MeshPickingPlugin))
+        .add_observer(disable_entities_on_click)
+        .add_systems(
+            Update,
+            (list_all_named_entities, reenable_entities_on_space),
+        )
+        .add_systems(Startup, (setup_scene, display_instructions))
+        .run();
+}
+
+#[derive(Component)]
+struct DisableOnClick;
+
+fn disable_entities_on_click(
+    trigger: Trigger<Pointer<Click>>,
+    valid_query: Query<&DisableOnClick>,
+    mut commands: Commands,
+) {
+    let clicked_entity = trigger.target();
+    // Windows and text are entities and can be clicked!
+    // We definitely don't want to disable the window itself,
+    // because that would cause the app to close!
+    if valid_query.contains(clicked_entity) {
+        // Just add the `Disabled` component to the entity to disable it.
+        // Note that the `Disabled` component is *only* added to the entity,
+        // its children are not affected.
+        commands.entity(clicked_entity).insert(Disabled);
+    }
+}
+
+#[derive(Component)]
+struct EntityNameText;
+
+// The query here will not find entities with the `Disabled` component,
+// because it does not explicitly include it.
+fn list_all_named_entities(
+    query: Query<&Name>,
+    mut name_text_query: Query<&mut Text, With<EntityNameText>>,
+    mut commands: Commands,
+) {
+    let mut text_string = String::from("Named entities found:\n");
+    // Query iteration order is not guaranteed, so we sort the names
+    // to ensure the output is consistent.
+    for name in query.iter().sort::<&Name>() {
+        text_string.push_str(&format!("{:?}\n", name));
+    }
+
+    if let Ok(mut text) = name_text_query.get_single_mut() {
+        *text = Text::new(text_string);
+    } else {
+        commands.spawn((
+            EntityNameText,
+            Text::default(),
+            Node {
+                position_type: PositionType::Absolute,
+                top: Val::Px(12.0),
+                right: Val::Px(12.0),
+                ..default()
+            },
+        ));
+    }
+}
+
+fn reenable_entities_on_space(
+    mut commands: Commands,
+    // This query can find disabled entities,
+    // because it explicitly includes the `Disabled` component.
+    disabled_entities: Query<Entity, With<Disabled>>,
+    input: Res<ButtonInput<KeyCode>>,
+) {
+    if input.just_pressed(KeyCode::Space) {
+        for entity in disabled_entities.iter() {
+            // To re-enable an entity, just remove the `Disabled` component.
+            commands.entity(entity).remove::<Disabled>();
+        }
+    }
+}
+
+const X_EXTENT: f32 = 900.;
+
+fn setup_scene(
+    mut commands: Commands,
+    mut meshes: ResMut<Assets<Mesh>>,
+    mut materials: ResMut<Assets<ColorMaterial>>,
+) {
+    commands.spawn(Camera2d);
+
+    let named_shapes = [
+        (Name::new("Annulus"), meshes.add(Annulus::new(25.0, 50.0))),
+        (
+            Name::new("Bestagon"),
+            meshes.add(RegularPolygon::new(50.0, 6)),
+        ),
+        (Name::new("Rhombus"), meshes.add(Rhombus::new(75.0, 100.0))),
+    ];
+    let num_shapes = named_shapes.len();
+
+    for (i, (name, shape)) in named_shapes.into_iter().enumerate() {
+        // Distribute colors evenly across the rainbow.
+        let color = Color::hsl(360. * i as f32 / num_shapes as f32, 0.95, 0.7);
+
+        commands.spawn((
+            name,
+            DisableOnClick,
+            Mesh2d(shape),
+            MeshMaterial2d(materials.add(color)),
+            Transform::from_xyz(
+                // Distribute shapes from -X_EXTENT/2 to +X_EXTENT/2.
+                -X_EXTENT / 2. + i as f32 / (num_shapes - 1) as f32 * X_EXTENT,
+                0.0,
+                0.0,
+            ),
+        ));
+    }
+}
+
+fn display_instructions(mut commands: Commands) {
+    commands.spawn((
+        Text::new(
+            "Click an entity to disable it.\n\nPress Space to re-enable all disabled entities.",
+        ),
+        Node {
+            position_type: PositionType::Absolute,
+            top: Val::Px(12.0),
+            left: Val::Px(12.0),
+            ..default()
+        },
+    ));
+}