move spawn_hand_bones from openxr to xr and add spawn handtracker command

Signed-off-by: Schmarni <marnistromer@gmail.com>

crates/bevy_xr/src/hands.rs

@@ -1,16 +1,50 @@
 use bevy::{
-    ecs::{component::Component, entity::Entity},
+    app::Plugin,
+    ecs::{component::Component, entity::Entity, world::Command},
+    hierarchy::BuildWorldChildren,
+    log::{error, warn},
     math::bool,
-    prelude::{Deref, DerefMut},
+    prelude::{Bundle, Commands, Deref, DerefMut, Resource, SpatialBundle, With, World},
+};
+
+use crate::{
+    session::{XrSessionCreated, XrTrackingRoot},
+    spaces::XrSpaceLocationFlags,
 };
 pub const HAND_JOINT_COUNT: usize = 26;
 
+pub fn spawn_hand_bones<T: Bundle>(
+    cmds: &mut Commands,
+    mut get_bundle: impl FnMut(HandBone) -> T,
+) -> [Entity; HAND_JOINT_COUNT] {
+    let mut bones: [Entity; HAND_JOINT_COUNT] = [Entity::PLACEHOLDER; HAND_JOINT_COUNT];
+    for bone in HandBone::get_all_bones().into_iter() {
+        bones[bone as usize] = cmds
+            .spawn((
+                SpatialBundle::default(),
+                bone,
+                HandBoneRadius(0.0),
+                XrSpaceLocationFlags::default(),
+            ))
+            .insert((get_bundle)(bone))
+            .id();
+    }
+    bones
+}
+
+#[derive(Clone, Copy, Component, Debug)]
+pub enum HandSide {
+    Left,
+    Right,
+}
+
 #[derive(Clone, Copy, Component, Debug)]
 pub struct LeftHand;
 
 #[derive(Clone, Copy, Component, Debug)]
 pub struct RightHand;
 
+/// Hand Joint Entities orderd
 #[derive(Deref, DerefMut, Component, Clone, Copy)]
 pub struct XrHandBoneEntities(pub [Entity; HAND_JOINT_COUNT]);
 
@@ -141,3 +175,43 @@ impl HandBone {
         ]
     }
 }
+
+/// Use by a backend to run custom logic when spawning a hand tracker
+#[derive(Resource)]
+pub struct SpawnHandTrackerCommandExecutor(pub fn(&mut World, Entity, HandSide));
+
+/// `tracker_bundle` is inserted after the backend specific code is run
+pub struct SpawnHandTracker<B: Bundle> {
+    pub joints: XrHandBoneEntities,
+    pub tracker_bundle: B,
+    pub side: HandSide,
+}
+
+impl<B: Bundle> Command for SpawnHandTracker<B> {
+    fn apply(self, world: &mut bevy::prelude::World) {
+        let Some(executor) = world.remove_resource::<SpawnHandTrackerCommandExecutor>() else {
+            warn!("no SpawnHandTracker executor defined, skipping handtracker creation");
+            return;
+        };
+        let Ok(root) = world
+            .query_filtered::<Entity, With<XrTrackingRoot>>()
+            .get_single(world)
+        else {
+            error!("unable to get tracking root, skipping handtracker creation");
+            return;
+        };
+        let mut tracker = world.spawn(self.joints);
+        match &self.side {
+            HandSide::Left => tracker.insert(LeftHand),
+            HandSide::Right => tracker.insert(LeftHand),
+        };
+        let tracker = tracker.id();
+        world.entity_mut(root).push_children(&[tracker]);
+        executor.0(world, tracker, self.side);
+        if let Some(mut tracker) = world.get_entity_mut(tracker) {
+            tracker.insert(self.side);
+            tracker.insert(self.tracker_bundle);
+        }
+        world.insert_resource(executor);
+    }
+}