Add system parameter for computing up-to-date GlobalTransforms (#8603)

# Objective

Add a way to easily compute the up-to-date `GlobalTransform` of an
entity.

## Solution

Add the `TransformHelper`(Name pending) system parameter with the
`compute_global_transform` method that takes an `Entity` and returns a
`GlobalTransform` if successful.

## Changelog
- Added the `TransformHelper` system parameter for computing the
up-to-date `GlobalTransform` of an entity.

---------

Co-authored-by: Alice Cecile <alice.i.cecile@gmail.com>
Co-authored-by: Noah <noahshomette@gmail.com>

crates/bevy_transform/Cargo.toml

@@ -19,6 +19,8 @@ serde = { version = "1", features = ["derive"], optional = true }
 
 [dev-dependencies]
 bevy_tasks = { path = "../bevy_tasks", version = "0.12.0-dev" }
+approx = "0.5.1"
+glam = { version = "0.24", features = ["approx"] }
 
 [features]
 serialize = ["dep:serde", "bevy_math/serialize"]

crates/bevy_transform/src/components/transform.rs

@@ -420,6 +420,15 @@ impl Mul<Transform> for Transform {
     }
 }
 
+impl Mul<GlobalTransform> for Transform {
+    type Output = GlobalTransform;
+
+    #[inline]
+    fn mul(self, global_transform: GlobalTransform) -> Self::Output {
+        GlobalTransform::from(self) * global_transform
+    }
+}
+
 impl Mul<Vec3> for Transform {
     type Output = Vec3;
 

crates/bevy_transform/src/helper.rs

@@ -0,0 +1,142 @@
+//! System parameter for computing up-to-date [`GlobalTransform`]s.
+
+use bevy_ecs::{
+    prelude::Entity,
+    query::QueryEntityError,
+    system::{Query, SystemParam},
+};
+use bevy_hierarchy::{HierarchyQueryExt, Parent};
+
+use crate::components::{GlobalTransform, Transform};
+
+/// System parameter for computing up-to-date [`GlobalTransform`]s.
+///
+/// Computing an entity's [`GlobalTransform`] can be expensive so it is recommended
+/// you use the [`GlobalTransform`] component stored on the entity, unless you need
+/// a [`GlobalTransform`] that reflects the changes made to any [`Transform`]s since
+/// the last time the transform propagation systems ran.
+#[derive(SystemParam)]
+pub struct TransformHelper<'w, 's> {
+    parent_query: Query<'w, 's, &'static Parent>,
+    transform_query: Query<'w, 's, &'static Transform>,
+}
+
+impl<'w, 's> TransformHelper<'w, 's> {
+    /// Computes the [`GlobalTransform`] of the given entity from the [`Transform`] component on it and its ancestors.
+    pub fn compute_global_transform(
+        &self,
+        entity: Entity,
+    ) -> Result<GlobalTransform, ComputeGlobalTransformError> {
+        let transform = self
+            .transform_query
+            .get(entity)
+            .map_err(|err| map_error(err, false))?;
+
+        let mut global_transform = GlobalTransform::from(*transform);
+
+        for entity in self.parent_query.iter_ancestors(entity) {
+            let transform = self
+                .transform_query
+                .get(entity)
+                .map_err(|err| map_error(err, true))?;
+
+            global_transform = *transform * global_transform;
+        }
+
+        Ok(global_transform)
+    }
+}
+
+fn map_error(err: QueryEntityError, ancestor: bool) -> ComputeGlobalTransformError {
+    use ComputeGlobalTransformError::*;
+    match err {
+        QueryEntityError::QueryDoesNotMatch(entity) => MissingTransform(entity),
+        QueryEntityError::NoSuchEntity(entity) => {
+            if ancestor {
+                MalformedHierarchy(entity)
+            } else {
+                NoSuchEntity(entity)
+            }
+        }
+        QueryEntityError::AliasedMutability(_) => unreachable!(),
+    }
+}
+
+/// Error returned by [`TransformHelper::compute_global_transform`].
+#[derive(Debug)]
+pub enum ComputeGlobalTransformError {
+    /// The entity or one of its ancestors is missing the [`Transform`] component.
+    MissingTransform(Entity),
+    /// The entity does not exist.
+    NoSuchEntity(Entity),
+    /// An ancestor is missing.
+    /// This probably means that your hierarchy has been improperly maintained.
+    MalformedHierarchy(Entity),
+}
+
+#[cfg(test)]
+mod tests {
+    use std::f32::consts::TAU;
+
+    use bevy_app::App;
+    use bevy_ecs::system::SystemState;
+    use bevy_hierarchy::BuildWorldChildren;
+    use bevy_math::{Quat, Vec3};
+
+    use crate::{
+        components::{GlobalTransform, Transform},
+        helper::TransformHelper,
+        TransformBundle, TransformPlugin,
+    };
+
+    #[test]
+    fn match_transform_propagation_systems() {
+        // Single transform
+        match_transform_propagation_systems_inner(vec![Transform::from_translation(Vec3::X)
+            .with_rotation(Quat::from_rotation_y(TAU / 4.))
+            .with_scale(Vec3::splat(2.))]);
+
+        // Transform hierarchy
+        match_transform_propagation_systems_inner(vec![
+            Transform::from_translation(Vec3::X)
+                .with_rotation(Quat::from_rotation_y(TAU / 4.))
+                .with_scale(Vec3::splat(2.)),
+            Transform::from_translation(Vec3::Y)
+                .with_rotation(Quat::from_rotation_z(TAU / 3.))
+                .with_scale(Vec3::splat(1.5)),
+            Transform::from_translation(Vec3::Z)
+                .with_rotation(Quat::from_rotation_x(TAU / 2.))
+                .with_scale(Vec3::splat(0.3)),
+        ]);
+    }
+
+    fn match_transform_propagation_systems_inner(transforms: Vec<Transform>) {
+        let mut app = App::new();
+        app.add_plugins(TransformPlugin);
+
+        let mut entity = None;
+
+        for transform in transforms {
+            let mut e = app.world.spawn(TransformBundle::from(transform));
+
+            if let Some(entity) = entity {
+                e.set_parent(entity);
+            }
+
+            entity = Some(e.id());
+        }
+
+        let leaf_entity = entity.unwrap();
+
+        app.update();
+
+        let transform = *app.world.get::<GlobalTransform>(leaf_entity).unwrap();
+
+        let mut state = SystemState::<TransformHelper>::new(&mut app.world);
+        let helper = state.get(&app.world);
+
+        let computed_transform = helper.compute_global_transform(leaf_entity).unwrap();
+
+        approx::assert_abs_diff_eq!(transform.affine(), computed_transform.affine());
+    }
+}

crates/bevy_transform/src/lib.rs

@@ -6,15 +6,16 @@
 pub mod commands;
 /// The basic components of the transform crate
 pub mod components;
+pub mod helper;
 /// Systems responsible for transform propagation
 pub mod systems;
 
 #[doc(hidden)]
 pub mod prelude {
     #[doc(hidden)]
     pub use crate::{
-        commands::BuildChildrenTransformExt, components::*, TransformBundle, TransformPlugin,
-        TransformPoint,
+        commands::BuildChildrenTransformExt, components::*, helper::TransformHelper,
+        TransformBundle, TransformPlugin, TransformPoint,
     };
 }