Add an extension trait to EntityCommands to update hierarchy while preserving GlobalTransform (#7024)

# Objective

It is often necessary  to update an entity's parent
while keeping its GlobalTransform static. Currently
it is cumbersome and error-prone (two questions in
the discord `#help` channel in the past week)

- Part 2, resolves #5475
- Builds on: #7020.

## Solution

- Added the `BuildChildrenTransformExt` trait, it is part
  of `bevy::prelude` and adds the following methods to `EntityCommands`:
  - `set_parent_in_place`: Change the parent of an entity and
    update its `Transform` in order to preserve its `GlobalTransform` after the parent change
  - `remove_parent_in_place`: Remove an entity from a hierarchy,
    while preserving its `GlobalTransform`.

---

## Changelog


- Added the `BuildChildrenTransformExt` trait, it is part
  of `bevy::prelude` and adds the following methods to `EntityCommands`:
  - `set_parent_in_place`: Change the parent of an entity and
    update its `Transform` in order to preserve its `GlobalTransform` after the parent change
  - `remove_parent_in_place`: Remove an entity from a hierarchy,
    while preserving its `GlobalTransform`.

Co-authored-by: Nicola Papale <nicopap@users.noreply.github.com>

crates/bevy_hierarchy/src/child_builder.rs

@@ -198,7 +198,8 @@ impl Command for RemoveChildren {
 
 /// Command that removes the parent of an entity, and removes that entity from the parent's [`Children`].
 pub struct RemoveParent {
-    child: Entity,
+    /// `Entity` whose parent must be removed.
+    pub child: Entity,
 }
 
 impl Command for RemoveParent {

crates/bevy_transform/src/commands.rs

@@ -0,0 +1,101 @@
+//! Extension to [`EntityCommands`] to modify [`bevy_hierarchy`] hierarchies
+//! while preserving [`GlobalTransform`].
+
+use bevy_ecs::{prelude::Entity, system::Command, system::EntityCommands, world::World};
+use bevy_hierarchy::{AddChild, RemoveParent};
+
+#[cfg(doc)]
+use bevy_hierarchy::BuildChildren;
+
+use crate::{GlobalTransform, Transform};
+
+/// Command similar to [`AddChild`], but updating the child transform to keep
+/// it at the same [`GlobalTransform`].
+///
+/// You most likely want to use [`BuildChildrenTransformExt::set_parent_in_place`]
+/// method on [`EntityCommands`] instead.
+pub struct AddChildInPlace {
+    /// Parent entity to add the child to.
+    pub parent: Entity,
+    /// Child entity to add.
+    pub child: Entity,
+}
+impl Command for AddChildInPlace {
+    fn write(self, world: &mut World) {
+        let hierarchy_command = AddChild {
+            child: self.child,
+            parent: self.parent,
+        };
+        hierarchy_command.write(world);
+        // FIXME: Replace this closure with a `try` block. See: https://github.com/rust-lang/rust/issues/31436.
+        let mut update_transform = || {
+            let parent = *world.get_entity(self.parent)?.get::<GlobalTransform>()?;
+            let child_global = *world.get_entity(self.child)?.get::<GlobalTransform>()?;
+            let mut child_entity = world.get_entity_mut(self.child)?;
+            let mut child = child_entity.get_mut::<Transform>()?;
+            *child = child_global.reparented_to(&parent);
+            Some(())
+        };
+        update_transform();
+    }
+}
+/// Command similar to [`RemoveParent`], but updating the child transform to keep
+/// it at the same [`GlobalTransform`].
+///
+/// You most likely want to use [`BuildChildrenTransformExt::remove_parent_in_place`]
+/// method on [`EntityCommands`] instead.
+pub struct RemoveParentInPlace {
+    /// `Entity` whose parent must be removed.
+    pub child: Entity,
+}
+impl Command for RemoveParentInPlace {
+    fn write(self, world: &mut World) {
+        let hierarchy_command = RemoveParent { child: self.child };
+        hierarchy_command.write(world);
+        // FIXME: Replace this closure with a `try` block. See: https://github.com/rust-lang/rust/issues/31436.
+        let mut update_transform = || {
+            let child_global = *world.get_entity(self.child)?.get::<GlobalTransform>()?;
+            let mut child_entity = world.get_entity_mut(self.child)?;
+            let mut child = child_entity.get_mut::<Transform>()?;
+            *child = child_global.compute_transform();
+            Some(())
+        };
+        update_transform();
+    }
+}
+/// Collection of methods similar to [`BuildChildren`], but preserving each
+/// entity's [`GlobalTransform`].
+pub trait BuildChildrenTransformExt {
+    /// Change this entity's parent while preserving this entity's [`GlobalTransform`]
+    /// by updating its [`Transform`].
+    ///
+    /// See [`BuildChildren::set_parent`] for a method that doesn't update the
+    /// [`Transform`].
+    ///
+    /// Note that both the hierarchy and transform updates will only execute
+    /// at the end of the current stage.
+    fn set_parent_in_place(&mut self, parent: Entity) -> &mut Self;
+
+    /// Make this entity parentless while preserving this entity's [`GlobalTransform`]
+    /// by updating its [`Transform`] to be equal to its current [`GlobalTransform`].
+    ///
+    /// See [`BuildChildren::remove_parent`] for a method that doesn't update the
+    /// [`Transform`].
+    ///
+    /// Note that both the hierarchy and transform updates will only execute
+    /// at the end of the current stage.
+    fn remove_parent_in_place(&mut self) -> &mut Self;
+}
+impl<'w, 's, 'a> BuildChildrenTransformExt for EntityCommands<'w, 's, 'a> {
+    fn remove_parent_in_place(&mut self) -> &mut Self {
+        let child = self.id();
+        self.commands().add(RemoveParentInPlace { child });
+        self
+    }
+
+    fn set_parent_in_place(&mut self, parent: Entity) -> &mut Self {
+        let child = self.id();
+        self.commands().add(AddChildInPlace { child, parent });
+        self
+    }
+}

crates/bevy_transform/src/lib.rs

@@ -2,6 +2,7 @@
 #![warn(clippy::undocumented_unsafe_blocks)]
 #![doc = include_str!("../README.md")]
 
+pub mod commands;
 /// The basic components of the transform crate
 pub mod components;
 mod systems;
@@ -9,7 +10,9 @@ mod systems;
 #[doc(hidden)]
 pub mod prelude {
     #[doc(hidden)]
-    pub use crate::{components::*, TransformBundle, TransformPlugin};
+    pub use crate::{
+        commands::BuildChildrenTransformExt, components::*, TransformBundle, TransformPlugin,
+    };
 }
 
 use bevy_app::prelude::*;