From 62e2fb654d2522073ab51203631d0385a1f445b2 Mon Sep 17 00:00:00 2001
From: Greeble <166992735+greeble-dev@users.noreply.github.com>
Date: Thu, 27 Mar 2025 16:06:57 +0000
Subject: [PATCH] First pass at `test_animation_transitions`.

---
 Cargo.toml                                    |   8 +
 tests/animation/test_animation_transitions.rs | 137 ++++++++++++++++++
 2 files changed, 145 insertions(+)
 create mode 100644 tests/animation/test_animation_transitions.rs

diff --git a/Cargo.toml b/Cargo.toml
index af2d9d356cb59..73aef78b71d13 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -4317,3 +4317,11 @@ name = "`no_std` Compatible Library"
 description = "Example library compatible with `std` and `no_std` targets"
 category = "Embedded"
 wasm = true
+
+[[example]]
+name = "test_animation_transitions"
+path = "tests/animation/test_animation_transitions.rs"
+doc-scrape-examples = true
+
+[package.metadata.example.test_animation_transitions]
+hidden = true
diff --git a/tests/animation/test_animation_transitions.rs b/tests/animation/test_animation_transitions.rs
new file mode 100644
index 0000000000000..a777897c390e2
--- /dev/null
+++ b/tests/animation/test_animation_transitions.rs
@@ -0,0 +1,137 @@
+//! Test animation transitions by spawning several meshes and randomly starting
+//! transitions.
+
+use bevy::{prelude::*, scene::SceneInstanceReady};
+use rand::{seq::SliceRandom, Rng, SeedableRng};
+use rand_chacha::ChaCha8Rng;
+use std::time::Duration;
+
+const GLTF_PATH: &str = "models/animated/Fox.glb";
+
+fn main() {
+    App::new()
+        .insert_resource(AmbientLight {
+            color: Color::WHITE,
+            brightness: 3500.0,
+            ..default()
+        })
+        .add_plugins(DefaultPlugins)
+        .add_systems(Startup, setup)
+        .add_systems(Update, play_transitions)
+        .run();
+}
+
+#[derive(Component)]
+struct Animations {
+    node_indices: Vec<AnimationNodeIndex>,
+    graph_handle: Handle<AnimationGraph>,
+    time_until_transition: f32,
+}
+
+fn setup(
+    mut commands: Commands,
+    asset_server: Res<AssetServer>,
+    mut graphs: ResMut<Assets<AnimationGraph>>,
+) {
+    let scene = SceneRoot(asset_server.load(GltfAssetLabel::Scene(0).from_asset(GLTF_PATH)));
+
+    let (graph, node_indices) = AnimationGraph::from_clips([
+        asset_server.load(GltfAssetLabel::Animation(0).from_asset(GLTF_PATH)),
+        asset_server.load(GltfAssetLabel::Animation(1).from_asset(GLTF_PATH)),
+        asset_server.load(GltfAssetLabel::Animation(2).from_asset(GLTF_PATH)),
+    ]);
+
+    let graph_handle = graphs.add(graph);
+
+    for x in [-160.0, -80.0, 0.0, 80.0, 160.0] {
+        commands
+            .spawn((
+                scene.clone(),
+                Transform::from_xyz(x, 0.0, 0.0),
+                Animations {
+                    node_indices: node_indices.clone(),
+                    graph_handle: graph_handle.clone(),
+                    time_until_transition: 0.0,
+                },
+            ))
+            .observe(setup_animations);
+    }
+
+    commands.spawn((
+        Camera3d::default(),
+        Projection::Perspective(PerspectiveProjection {
+            fov: 10.0_f32.to_radians(),
+            ..Default::default()
+        }),
+        Transform::from_xyz(900.0, 900.0, 900.0).looking_at(Vec3::new(0.0, 30.0, 0.0), Vec3::Y),
+    ));
+}
+
+fn setup_animations(
+    trigger: Trigger<SceneInstanceReady>,
+    mut commands: Commands,
+    query_animations: Query<&Animations>,
+    children: Query<&Children>,
+    mut players: Query<&mut AnimationPlayer>,
+) {
+    if let Ok(animations) = query_animations.get(trigger.target()) {
+        for child in children.iter_descendants(trigger.target()) {
+            if let Ok(mut player) = players.get_mut(child) {
+                let mut transitions = AnimationTransitions::new();
+
+                transitions
+                    .play(&mut player, animations.node_indices[0], Duration::ZERO)
+                    .repeat();
+
+                commands
+                    .entity(child)
+                    .insert(transitions)
+                    .insert(AnimationGraphHandle(animations.graph_handle.clone()));
+            }
+        }
+    }
+}
+
+fn play_transitions(
+    query_animations: Query<(Entity, &mut Animations)>,
+    children: Query<&Children>,
+    mut players: Query<(&mut AnimationPlayer, &mut AnimationTransitions)>,
+    mut rng: Local<Option<ChaCha8Rng>>,
+    time: Res<Time>,
+) {
+    let rng = rng.get_or_insert(ChaCha8Rng::seed_from_u64(729593));
+
+    for (entity, mut animations) in query_animations {
+        animations.time_until_transition =
+            (animations.time_until_transition - time.delta_secs()).max(0.0);
+
+        if animations.time_until_transition > 0.0 {
+            continue;
+        }
+
+        // Choose a random animation.
+        let Some(&node_index) = animations.node_indices.choose(rng) else {
+            continue;
+        };
+
+        // Randomize the blend duration.
+        let duration: f32 = rng.gen_range(0.2..1.0);
+
+        // Play a new transition after this one has finished blending.
+        animations.time_until_transition = duration + 0.5;
+
+        for child in children.iter_descendants(entity) {
+            if let Ok((mut player, mut transitions)) = players.get_mut(child) {
+                // Don't play the animation if it's already playing. This would
+                // look like a bug as it snaps the animation to the start.
+                if transitions.get_main_animation() == Some(node_index) {
+                    continue;
+                }
+
+                transitions
+                    .play(&mut player, node_index, Duration::from_secs_f32(duration))
+                    .repeat();
+            }
+        }
+    }
+}