Global Gizmos (#22107)

# Objective

- use gizmos in non-system contexts, such as deep in a math library or
callback without ecs access.
- kind of upstream https://github.com/atlv24/glizmo
- builds off of #22105

## Solution

- mutex + deref

## Testing

-

Author: atlv24

crates/bevy_gizmos/src/global.rs

@@ -0,0 +1,48 @@
+use std::sync::Mutex;
+
+use bevy_app::{App, Last, Plugin};
+use bevy_ecs::schedule::IntoScheduleConfigs;
+
+use crate::{
+    config::DefaultGizmoConfigGroup,
+    gizmos::{GizmoBuffer, Gizmos},
+    GizmoMeshSystems,
+};
+
+static GLOBAL_GIZMO: Mutex<GizmoBuffer<DefaultGizmoConfigGroup, ()>> =
+    Mutex::new(GizmoBuffer::new());
+
+/// Lets you use bevy gizmos outside of systems.
+pub struct GlobalGizmosPlugin;
+
+impl Plugin for GlobalGizmosPlugin {
+    fn build(&self, app: &mut App) {
+        app.add_systems(Last, flush_global_gizmos.before(GizmoMeshSystems));
+    }
+}
+
+fn flush_global_gizmos(mut gizmos: Gizmos) {
+    let mut buffer = GizmoBuffer::new();
+    {
+        core::mem::swap(&mut buffer, &mut GLOBAL_GIZMO.lock().unwrap());
+    }
+    gizmos.strip_positions.extend(buffer.strip_positions);
+    gizmos.strip_colors.extend(buffer.strip_colors);
+    gizmos.list_positions.extend(buffer.list_positions);
+    gizmos.list_colors.extend(buffer.list_colors);
+}
+
+/// A global gizmo context for use outside of bevy systems.
+///
+/// # Example
+/// ```
+/// # use bevy_gizmos::prelude::*;
+/// # use bevy_math::prelude::*;
+/// # use bevy_color::palettes::basic::WHITE;
+/// fn draw() {
+///     gizmo().sphere(Isometry3d::IDENTITY, 0.5, WHITE);
+/// }
+/// ```
+pub fn gizmo() -> impl core::ops::DerefMut<Target = GizmoBuffer<DefaultGizmoConfigGroup, ()>> {
+    GLOBAL_GIZMO.lock().unwrap()
+}

crates/bevy_gizmos/src/lib.rs

@@ -30,6 +30,7 @@ pub mod config;
 pub mod cross;
 pub mod curves;
 pub mod gizmos;
+mod global;
 pub mod grid;
 pub mod primitives;
 pub mod retained;
@@ -52,6 +53,7 @@ pub mod prelude {
             GizmoLineConfig, GizmoLineJoint, GizmoLineStyle,
         },
         gizmos::Gizmos,
+        global::gizmo,
         primitives::{dim2::GizmoPrimitive2d, dim3::GizmoPrimitive3d},
         retained::Gizmo,
         AppGizmoBuilder, GizmoAsset,
@@ -92,7 +94,7 @@ impl Plugin for GizmoPlugin {
             // We insert the Resource GizmoConfigStore into the world implicitly here if it does not exist.
             .init_gizmo_group::<DefaultGizmoConfigGroup>();
 
-        app.add_plugins(aabb::AabbGizmoPlugin);
+        app.add_plugins((aabb::AabbGizmoPlugin, global::GlobalGizmosPlugin));
 
         #[cfg(feature = "bevy_light")]
         app.add_plugins(LightGizmoPlugin);