From 9dfd4e4b08aba146e4d14e457b7b2ac6560e8ea3 Mon Sep 17 00:00:00 2001
From: Mizu <5565979+l4desu-mizu@users.noreply.github.com>
Date: Tue, 15 Mar 2022 05:49:49 +0000
Subject: [PATCH] Add examples for Transforms (#2441)

# Add Transform Examples

- Adding examples for moving/rotating entities (with its own section) to resolve #2400

I've stumbled upon this project and been fiddling around a little. Saw the issue and thought I might just add some examples for the proposed transformations.
Mind to check if I got the gist correctly and suggest anything I can improve?
---
 Cargo.toml                                    |  21 ++
 examples/README.md                            |  11 +
 examples/transforms/3d_rotation.rs            |  57 ++++++
 .../transforms/global_vs_local_translation.rs | 188 ++++++++++++++++++
 examples/transforms/scale.rs                  |  96 +++++++++
 examples/transforms/transform.rs              | 151 ++++++++++++++
 examples/transforms/translation.rs            |  71 +++++++
 7 files changed, 595 insertions(+)
 create mode 100644 examples/transforms/3d_rotation.rs
 create mode 100644 examples/transforms/global_vs_local_translation.rs
 create mode 100644 examples/transforms/scale.rs
 create mode 100644 examples/transforms/transform.rs
 create mode 100644 examples/transforms/translation.rs

diff --git a/Cargo.toml b/Cargo.toml
index 5efa161158..9f93875617 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -502,6 +502,27 @@ path = "examples/shader/compute_shader_game_of_life.rs"
 name = "bevymark"
 path = "examples/tools/bevymark.rs"
 
+# Transforms
+[[example]]
+name = "global_vs_local_translation"
+path = "examples/transforms/global_vs_local_translation.rs"
+
+[[example]]
+name = "3d_rotation"
+path = "examples/transforms/3d_rotation.rs"
+
+[[example]]
+name = "scale"
+path = "examples/transforms/scale.rs"
+
+[[example]]
+name = "transform"
+path = "examples/transforms/transform.rs"
+
+[[example]]
+name = "translation"
+path = "examples/transforms/translation.rs"
+
 # UI (User Interface)
 [[example]]
 name = "button"
diff --git a/examples/README.md b/examples/README.md
index 53d276d646..9f2e01ae80 100644
--- a/examples/README.md
+++ b/examples/README.md
@@ -52,6 +52,7 @@ git checkout v0.4.0
   - [Shaders](#shaders)
   - [Tests](#tests)
   - [Tools](#tools)
+  - [Transforms](#transforms)
   - [UI (User Interface)](#ui-user-interface)
   - [Window](#window)
 - [Platform-Specific Examples](#platform-specific-examples)
@@ -247,6 +248,16 @@ Example | File | Description
 --- | --- | ---
 `bevymark` | [`tools/bevymark.rs`](./tools/bevymark.rs) | A heavy sprite rendering workload to benchmark your system with Bevy
 
+## Transforms
+
+Example | File | Description
+--- | --- | ---
+`global_vs_local_translation` | [`transforms/global_vs_local_translation.rs`](./transforms/global_vs_local_translation.rs) | Illustrates the difference between direction of a translation in respect to local object or global object Transform
+`3d_rotation` | [`transforms/3d_rotation.rs`](./transforms/3d_rotation.rs) | Illustrates how to (constantly) rotate an object around an axis
+`scale` | [`transforms/scale.rs`](./transforms/scale.rs) | Illustrates how to scale an object in each direction
+`transform` | [`transforms/transfrom.rs`](./transforms/transform.rs) | Shows multiple transformations of objects
+`translation` | [`transforms/translation.rs`](./transforms/translation.rs) | Illustrates how to move an object along an axis
+
 ## UI (User Interface)
 
 Example | File | Description
diff --git a/examples/transforms/3d_rotation.rs b/examples/transforms/3d_rotation.rs
new file mode 100644
index 0000000000..4afa84e417
--- /dev/null
+++ b/examples/transforms/3d_rotation.rs
@@ -0,0 +1,57 @@
+use bevy::prelude::*;
+
+use std::f32::consts::PI;
+
+const FULL_TURN: f32 = 2.0 * PI;
+
+// Define a component to designate a rotation speed to an entity.
+#[derive(Component)]
+struct Rotatable {
+    speed: f32,
+}
+
+fn main() {
+    App::new()
+        .add_plugins(DefaultPlugins)
+        .add_startup_system(setup)
+        .add_system(rotate_cube)
+        .run();
+}
+
+fn setup(
+    mut commands: Commands,
+    mut meshes: ResMut<Assets<Mesh>>,
+    mut materials: ResMut<Assets<StandardMaterial>>,
+) {
+    // Spawn a cube to rotate.
+    commands
+        .spawn_bundle(PbrBundle {
+            mesh: meshes.add(Mesh::from(shape::Cube { size: 1.0 })),
+            material: materials.add(Color::WHITE.into()),
+            transform: Transform::from_translation(Vec3::ZERO),
+            ..Default::default()
+        })
+        .insert(Rotatable { speed: 0.3 });
+
+    // Spawn a camera looking at the entities to show what's happening in this example.
+    commands.spawn_bundle(PerspectiveCameraBundle {
+        transform: Transform::from_xyz(0.0, 10.0, 20.0).looking_at(Vec3::ZERO, Vec3::Y),
+        ..Default::default()
+    });
+
+    // Add a light source for better 3d visibility.
+    commands.spawn_bundle(PointLightBundle {
+        transform: Transform::from_translation(Vec3::ONE * 3.0),
+        ..Default::default()
+    });
+}
+
+// This system will rotate any entity in the scene with an assigned Rotatable around its z-axis.
+fn rotate_cube(mut cubes: Query<(&mut Transform, &Rotatable)>, timer: Res<Time>) {
+    for (mut transform, cube) in cubes.iter_mut() {
+        // The speed is taken as a percentage of a full 360 degree turn.
+        // The timers delta_seconds is used to smooth out the movement.
+        let rotation_change = Quat::from_rotation_y(FULL_TURN * cube.speed * timer.delta_seconds());
+        transform.rotate(rotation_change);
+    }
+}
diff --git a/examples/transforms/global_vs_local_translation.rs b/examples/transforms/global_vs_local_translation.rs
new file mode 100644
index 0000000000..25dde970b4
--- /dev/null
+++ b/examples/transforms/global_vs_local_translation.rs
@@ -0,0 +1,188 @@
+use bevy::prelude::*;
+
+// Define a marker for entities that should be changed via their global transform.
+#[derive(Component)]
+struct ChangeGlobal;
+
+// Define a marker for entities that should be changed via their local transform.
+#[derive(Component)]
+struct ChangeLocal;
+
+// Define a marker for entities that should move.
+#[derive(Component)]
+struct Move;
+
+// Define a resource for the current movement direction;
+#[derive(Default)]
+struct Direction(Vec3);
+
+// Define component to decide when an entity should be ignored by the movement systems.
+#[derive(Component)]
+struct ToggledBy(KeyCode);
+
+fn main() {
+    App::new()
+        .add_plugins(DefaultPlugins)
+        .add_startup_system(setup)
+        .init_resource::<Direction>()
+        .add_system(move_cubes_according_to_global_transform)
+        .add_system(move_cubes_according_to_local_transform)
+        .add_system(update_directional_input)
+        .add_system(toggle_movement)
+        .run();
+}
+
+// Startup system to setup the scene and spawn all relevant entities.
+fn setup(
+    mut commands: Commands,
+    mut meshes: ResMut<Assets<Mesh>>,
+    mut materials: ResMut<Assets<StandardMaterial>>,
+    asset_server: Res<AssetServer>,
+) {
+    // To show the difference between a local transform (rotation, scale and position in respect to a given entity)
+    // and global transform (rotation, scale and position in respect to the base coordinate system of the visible scene)
+    // it's helpful to add multiple entities that are attached to each other.
+    // This way we'll see that the transform in respect to an entity's parent is different to the
+    // global transform within the visible scene.
+    // This example focuses on translation only to clearly demonstrate the differences.
+
+    // Spawn a basic cube to have an entity as reference.
+    let mut main_entity = commands.spawn();
+    main_entity
+        .insert_bundle(PbrBundle {
+            mesh: meshes.add(Mesh::from(shape::Cube { size: 1.0 })),
+            material: materials.add(Color::YELLOW.into()),
+            ..Default::default()
+        })
+        .insert(ChangeGlobal)
+        .insert(Move)
+        .insert(ToggledBy(KeyCode::Key1));
+
+    // Spawn two entities as children above the original main entity.
+    // The red entity spawned here will be changed via its global transform
+    // where the green one will be changed via its local transform.
+    main_entity.with_children(|child_builder| {
+        // also see parenting example
+        child_builder
+            .spawn_bundle(PbrBundle {
+                mesh: meshes.add(Mesh::from(shape::Cube { size: 0.5 })),
+                material: materials.add(Color::RED.into()),
+                transform: Transform::from_translation(Vec3::Y - Vec3::Z),
+                ..Default::default()
+            })
+            .insert(ChangeGlobal)
+            .insert(Move)
+            .insert(ToggledBy(KeyCode::Key2));
+        child_builder
+            .spawn_bundle(PbrBundle {
+                mesh: meshes.add(Mesh::from(shape::Cube { size: 0.5 })),
+                material: materials.add(Color::GREEN.into()),
+                transform: Transform::from_translation(Vec3::Y + Vec3::Z),
+                ..Default::default()
+            })
+            .insert(ChangeLocal)
+            .insert(Move)
+            .insert(ToggledBy(KeyCode::Key3));
+    });
+
+    // Spawn a camera looking at the entities to show what's happening in this example.
+    commands.spawn_bundle(PerspectiveCameraBundle {
+        transform: Transform::from_xyz(0.0, 10.0, 20.0).looking_at(Vec3::ZERO, Vec3::Y),
+        ..Default::default()
+    });
+
+    // Add a light source for better 3d visibility.
+    commands.spawn_bundle(PointLightBundle {
+        transform: Transform::from_translation(Vec3::splat(3.0)),
+        ..Default::default()
+    });
+
+    // Add a UI cam and text to explain inputs and what is happening.
+    commands.spawn_bundle(UiCameraBundle::default());
+    commands.spawn_bundle(TextBundle {
+        text: Text::with_section(
+            "Press the arrow keys to move the cubes. Toggle movement for yellow (1), red (2) and green (3) cubes via number keys.
+
+Notice how the green cube will translate further in respect to the yellow in contrast to the red cube.
+This is due to the use of its LocalTransform that is relative to the yellow cubes transform instead of the GlobalTransform as in the case of the red cube.
+The red cube is moved through its GlobalTransform and thus is unaffected by the yellows transform.",
+            TextStyle {
+                font: asset_server.load("fonts/FiraSans-Bold.ttf"),
+                font_size: 22.0,
+                color: Color::WHITE,
+            },
+            TextAlignment {
+                horizontal: HorizontalAlign::Left,
+                ..Default::default()
+            }
+        ),
+        ..Default::default()});
+}
+
+// This system will move all cubes that are marked as ChangeGlobal according to their global transform.
+fn move_cubes_according_to_global_transform(
+    mut cubes: Query<&mut GlobalTransform, (With<ChangeGlobal>, With<Move>)>,
+    direction: Res<Direction>,
+    timer: Res<Time>,
+) {
+    for mut global_transform in cubes.iter_mut() {
+        global_transform.translation += direction.0 * timer.delta_seconds();
+    }
+}
+
+// This system will move all cubes that are marked as ChangeLocal according to their local transform.
+fn move_cubes_according_to_local_transform(
+    mut cubes: Query<&mut Transform, (With<ChangeLocal>, With<Move>)>,
+    direction: Res<Direction>,
+    timer: Res<Time>,
+) {
+    for mut transform in cubes.iter_mut() {
+        transform.translation += direction.0 * timer.delta_seconds();
+    }
+}
+
+// This system updates a resource that defines in which direction the cubes should move.
+// The direction is defined by the input of arrow keys and is only in left/right and up/down direction.
+fn update_directional_input(mut direction: ResMut<Direction>, keyboard_input: Res<Input<KeyCode>>) {
+    let horizontal_movement = Vec3::X
+        * (keyboard_input.pressed(KeyCode::Right) as i32
+            - keyboard_input.pressed(KeyCode::Left) as i32) as f32;
+    let vertical_movement = Vec3::Y
+        * (keyboard_input.pressed(KeyCode::Up) as i32
+            - keyboard_input.pressed(KeyCode::Down) as i32) as f32;
+    direction.0 = horizontal_movement + vertical_movement;
+}
+
+// This system enables and disables the movement for each entity if their assigned key is pressed.
+fn toggle_movement(
+    mut commands: Commands,
+    movable_entities: Query<(Entity, &Handle<StandardMaterial>, &ToggledBy), With<Move>>,
+    static_entities: Query<(Entity, &Handle<StandardMaterial>, &ToggledBy), Without<Move>>,
+    mut materials: ResMut<Assets<StandardMaterial>>,
+    keyboard_input: Res<Input<KeyCode>>,
+) {
+    // Update the currently movable entities and remove their Move component if the assigned key was pressed to disable their movement.
+    // This will also make them transparent so they can be identified as 'disabled' in the scene.
+    for (entity, material_handle, toggled_by) in movable_entities.iter() {
+        if keyboard_input.just_pressed(toggled_by.0) {
+            materials
+                .get_mut(material_handle)
+                .unwrap()
+                .base_color
+                .set_a(0.5);
+            commands.entity(entity).remove::<Move>();
+        }
+    }
+    // Update the currently non-movable entities and add a Move component if the assigned key was pressed to enable their movement.
+    // This will also make them opaque so they can be identified as 'enabled' in the scene.
+    for (entity, material_handle, toggled_by) in static_entities.iter() {
+        if keyboard_input.just_pressed(toggled_by.0) {
+            materials
+                .get_mut(material_handle)
+                .unwrap()
+                .base_color
+                .set_a(1.0);
+            commands.entity(entity).insert(Move);
+        }
+    }
+}
diff --git a/examples/transforms/scale.rs b/examples/transforms/scale.rs
new file mode 100644
index 0000000000..09e6a265f2
--- /dev/null
+++ b/examples/transforms/scale.rs
@@ -0,0 +1,96 @@
+use bevy::math::Vec3Swizzles;
+use bevy::prelude::*;
+use std::f32::consts::PI;
+
+// Define a component to keep information for the scaled object.
+#[derive(Component)]
+struct Scaling {
+    scale_direction: Vec3,
+    scale_speed: f32,
+    max_element_size: f32,
+    min_element_size: f32,
+}
+
+// Implement a simple initialisation.
+impl Scaling {
+    fn new() -> Self {
+        Scaling {
+            scale_direction: Vec3::X,
+            scale_speed: 2.0,
+            max_element_size: 5.0,
+            min_element_size: 1.0,
+        }
+    }
+}
+
+fn main() {
+    App::new()
+        .add_plugins(DefaultPlugins)
+        .add_startup_system(setup)
+        .add_system(change_scale_direction)
+        .add_system(scale_cube)
+        .run();
+}
+
+// Startup system to setup the scene and spawn all relevant entities.
+fn setup(
+    mut commands: Commands,
+    mut meshes: ResMut<Assets<Mesh>>,
+    mut materials: ResMut<Assets<StandardMaterial>>,
+) {
+    // Spawn a cube to scale.
+    commands
+        .spawn_bundle(PbrBundle {
+            mesh: meshes.add(Mesh::from(shape::Cube { size: 1.0 })),
+            material: materials.add(Color::WHITE.into()),
+            transform: Transform::from_rotation(Quat::from_rotation_y(PI / 4.0)),
+            ..Default::default()
+        })
+        .insert(Scaling::new());
+
+    // Spawn a camera looking at the entities to show what's happening in this example.
+    commands.spawn_bundle(PerspectiveCameraBundle {
+        transform: Transform::from_xyz(0.0, 10.0, 20.0).looking_at(Vec3::ZERO, Vec3::Y),
+        ..Default::default()
+    });
+
+    // Add a light source for better 3d visibility.
+    commands.spawn_bundle(PointLightBundle {
+        transform: Transform::from_translation(Vec3::ONE * 3.0),
+        ..Default::default()
+    });
+}
+
+// This system will check if a scaled entity went above or below the entities scaling bounds
+// and change the direction of the scaling vector.
+fn change_scale_direction(mut cubes: Query<(&mut Transform, &mut Scaling)>) {
+    for (mut transform, mut cube) in cubes.iter_mut() {
+        // If an entity scaled beyond the maximum of its size in any dimension
+        // the scaling vector is flipped so the scaling is gradually reverted.
+        // Additionally, to ensure the condition does not trigger again we floor the elements to
+        // their next full value, which should be max_element_size at max.
+        if transform.scale.max_element() > cube.max_element_size {
+            cube.scale_direction *= -1.0;
+            transform.scale = transform.scale.floor();
+        }
+        // If an entity scaled beyond the minimum of its size in any dimension
+        // the scaling vector is also flipped.
+        // Additionally the Values are ceiled to be min_element_size at least
+        // and the scale direction is flipped.
+        // This way the entity will change the dimension in which it is scaled any time it
+        // reaches its min_element_size.
+        if transform.scale.min_element() < cube.min_element_size {
+            cube.scale_direction *= -1.0;
+            transform.scale = transform.scale.ceil();
+            cube.scale_direction = cube.scale_direction.zxy();
+        }
+    }
+}
+
+// This system will scale any entity with assigned Scaling in each direction
+// by cycling through the directions to scale.
+fn scale_cube(mut cubes: Query<(&mut Transform, &Scaling)>, timer: Res<Time>) {
+    for (mut transform, cube) in cubes.iter_mut() {
+        transform.scale += cube.scale_direction * cube.scale_speed * timer.delta_seconds();
+    }
+}
diff --git a/examples/transforms/transform.rs b/examples/transforms/transform.rs
new file mode 100644
index 0000000000..e35f43445c
--- /dev/null
+++ b/examples/transforms/transform.rs
@@ -0,0 +1,151 @@
+use bevy::prelude::*;
+
+use std::f32::consts::PI;
+
+// A struct for additional data of for a moving cube.
+#[derive(Component)]
+struct CubeState {
+    start_pos: Vec3,
+    move_speed: f32,
+    turn_speed: f32,
+}
+
+// A struct adding information to a scalable entity,
+// that will be stationary at the center of the scene.
+#[derive(Component)]
+struct Center {
+    max_size: f32,
+    min_size: f32,
+    scale_factor: f32,
+}
+
+fn main() {
+    App::new()
+        .add_plugins(DefaultPlugins)
+        .add_startup_system(setup)
+        .add_system(move_cube)
+        .add_system(rotate_cube)
+        .add_system(scale_down_sphere_proportional_to_cube_travel_distance)
+        .run();
+}
+
+// Startup system to setup the scene and spawn all relevant entities.
+fn setup(
+    mut commands: Commands,
+    mut meshes: ResMut<Assets<Mesh>>,
+    mut materials: ResMut<Assets<StandardMaterial>>,
+) {
+    // Add an object (sphere) for visualizing scaling.
+    commands
+        .spawn_bundle(PbrBundle {
+            mesh: meshes.add(Mesh::from(shape::Icosphere {
+                radius: 3.0,
+                subdivisions: 32,
+            })),
+            material: materials.add(Color::YELLOW.into()),
+            transform: Transform::from_translation(Vec3::ZERO),
+            ..Default::default()
+        })
+        .insert(Center {
+            max_size: 1.0,
+            min_size: 0.1,
+            scale_factor: 0.05,
+        });
+
+    // Add the cube to visualize rotation and translation.
+    // This cube will circle around the center_sphere
+    // by changing its rotation each frame and moving forward.
+    // Define a start transform for an orbiting cube, that's away from our central object (sphere)
+    // and rotate it so it will be able to move around the sphere and not towards it.
+    let angle_90 = PI / 2.0;
+    let mut cube_spawn = Transform::from_translation(Vec3::Z * -10.0);
+    cube_spawn.rotation = Quat::from_rotation_y(angle_90);
+    commands
+        .spawn_bundle(PbrBundle {
+            mesh: meshes.add(Mesh::from(shape::Cube { size: 1.0 })),
+            material: materials.add(Color::WHITE.into()),
+            transform: cube_spawn,
+            ..Default::default()
+        })
+        .insert(CubeState {
+            start_pos: cube_spawn.translation,
+            move_speed: 2.0,
+            turn_speed: 0.2,
+        });
+
+    // Spawn a camera looking at the entities to show what's happening in this example.
+    commands.spawn_bundle(PerspectiveCameraBundle {
+        transform: Transform::from_xyz(0.0, 10.0, 20.0).looking_at(Vec3::ZERO, Vec3::Y),
+        ..Default::default()
+    });
+
+    // Add a light source for better 3d visibility.
+    commands.spawn_bundle(PointLightBundle {
+        transform: Transform::from_translation(Vec3::ONE * 3.0),
+        ..Default::default()
+    });
+}
+
+// This system will move the cube forward.
+fn move_cube(mut cubes: Query<(&mut Transform, &mut CubeState)>, timer: Res<Time>) {
+    for (mut transform, cube) in cubes.iter_mut() {
+        // Move the cube forward smoothly at a given move_speed.
+        let forward = transform.forward();
+        transform.translation += forward * cube.move_speed * timer.delta_seconds();
+    }
+}
+
+// This system will rotate the cube slightly towards the center_sphere.
+// Due to the forward movement the resulting movement
+// will be a circular motion around the center_sphere.
+fn rotate_cube(
+    mut cubes: Query<(&mut Transform, &mut CubeState), Without<Center>>,
+    center_spheres: Query<&Transform, With<Center>>,
+    timer: Res<Time>,
+) {
+    // Calculate the point to circle around. (The position of the center_sphere)
+    let mut center: Vec3 = Vec3::ZERO;
+    for sphere in center_spheres.iter() {
+        center += sphere.translation;
+    }
+    // Update the rotation of the cube(s).
+    for (mut transform, cube) in cubes.iter_mut() {
+        // Calculate the rotation of the cube if it would be looking at the sphere in the center.
+        let look_at_sphere = transform.looking_at(center, transform.local_y());
+        // Interpolate between the current rotation and the fully turned rotation
+        // when looking a the sphere,  with a given turn speed to get a smooth motion.
+        // With higher speed the curvature of the orbit would be smaller.
+        let incremental_turn_weight = cube.turn_speed * timer.delta_seconds();
+        let old_rotation = transform.rotation;
+        transform.rotation = old_rotation.lerp(look_at_sphere.rotation, incremental_turn_weight);
+    }
+}
+
+// This system will scale down the sphere in the center of the scene
+// according to the traveling distance of the orbiting cube(s) from their start position(s).
+fn scale_down_sphere_proportional_to_cube_travel_distance(
+    cubes: Query<(&Transform, &CubeState), Without<Center>>,
+    mut centers: Query<(&mut Transform, &Center)>,
+) {
+    // First we need to calculate the length of between
+    // the current position of the orbiting cube and the spawn position.
+    let mut distances = 0.0;
+    for (cube_transform, cube_state) in cubes.iter() {
+        distances += (cube_state.start_pos - cube_transform.translation).length();
+    }
+    // Now we use the calculated value to scale the sphere in the center accordingly.
+    for (mut transform, center) in centers.iter_mut() {
+        // Calculate the new size from the calculated distances and the centers scale_factor.
+        // Since we want to have the sphere at its max_size at the cubes spawn location we start by
+        // using the max_size as start value and subtract the distances scaled by a scaling factor.
+        let mut new_size: f32 = center.max_size - center.scale_factor * distances;
+
+        // The new size should also not be smaller than the centers min_size.
+        // Therefore the max value out of (new_size, center.min_size) is used.
+        new_size = new_size.max(center.min_size);
+
+        // Now scale the sphere uniformly in all directions using new_size.
+        // Here Vec3:splat is used to create a vector with new_size in x, y and z direction.
+        transform.scale = Vec3::splat(new_size);
+    }
+}
diff --git a/examples/transforms/translation.rs b/examples/transforms/translation.rs
new file mode 100644
index 0000000000..e61dc76f25
--- /dev/null
+++ b/examples/transforms/translation.rs
@@ -0,0 +1,71 @@
+use bevy::prelude::*;
+
+// Define a struct to keep some information about our entity.
+// Here it's an arbitrary movement speed, the spawn location, and a maximum distance from it.
+#[derive(Component)]
+struct Movable {
+    spawn: Vec3,
+    max_distance: f32,
+    speed: f32,
+}
+
+// Implement a utility function for easier Movable struct creation.
+impl Movable {
+    fn new(spawn: Vec3) -> Self {
+        Movable {
+            spawn,
+            max_distance: 5.0,
+            speed: 2.0,
+        }
+    }
+}
+
+fn main() {
+    App::new()
+        .add_plugins(DefaultPlugins)
+        .add_startup_system(setup)
+        .add_system(move_cube)
+        .run();
+}
+
+// Startup system to setup the scene and spawn all relevant entities.
+fn setup(
+    mut commands: Commands,
+    mut meshes: ResMut<Assets<Mesh>>,
+    mut materials: ResMut<Assets<StandardMaterial>>,
+) {
+    // Add a cube to visualize translation.
+    let entity_spawn = Vec3::ZERO;
+    commands
+        .spawn_bundle(PbrBundle {
+            mesh: meshes.add(Mesh::from(shape::Cube { size: 1.0 })),
+            material: materials.add(Color::WHITE.into()),
+            transform: Transform::from_translation(entity_spawn),
+            ..Default::default()
+        })
+        .insert(Movable::new(entity_spawn));
+
+    // Spawn a camera looking at the entities to show what's happening in this example.
+    commands.spawn_bundle(PerspectiveCameraBundle {
+        transform: Transform::from_xyz(0.0, 10.0, 20.0).looking_at(entity_spawn, Vec3::Y),
+        ..Default::default()
+    });
+
+    // Add a light source for better 3d visibility.
+    commands.spawn_bundle(PointLightBundle {
+        transform: Transform::from_translation(Vec3::ONE * 3.0),
+        ..Default::default()
+    });
+}
+
+// This system will move all Movable entities with a Transform
+fn move_cube(mut cubes: Query<(&mut Transform, &mut Movable)>, timer: Res<Time>) {
+    for (mut transform, mut cube) in cubes.iter_mut() {
+        // Check if the entity moved too far from its spawn, if so invert the moving direction.
+        if (cube.spawn - transform.translation).length() > cube.max_distance {
+            cube.speed *= -1.0;
+        }
+        let direction = transform.local_x();
+        transform.translation += direction * cube.speed * timer.delta_seconds();
+    }
+}