1 files changed, 101 insertions, 0 deletions
diff --git a/engine-ecs/src/component/local.rs b/engine-ecs/src/component/local.rs
new file mode 100644
index 0000000..29afcee
--- /dev/null
+++ b/engine-ecs/src/component/local.rs
@@ -0,0 +1,101 @@
+use std::any::type_name;
+use std::ops::{Deref, DerefMut};
+
+use crate::component::{
+    HandleMut as ComponentHandleMut,
+    IntoParts as _,
+    Parts as ComponentParts,
+};
+use crate::pair::Pair;
+use crate::system::initializable::Param as InitializableParam;
+use crate::system::{Metadata as SystemMetadata, Param as SystemParam};
+use crate::{Component, World};
+
+/// Holds a component which is local to a single system.
+#[derive(Debug)]
+pub struct Local<'world, LocalComponent: Component>
+{
+    local_component: ComponentHandleMut<'world, LocalComponent>,
+}
+
+impl<'world, LocalComponent> SystemParam<'world> for Local<'world, LocalComponent>
+where
+    LocalComponent: Component,
+{
+    type Input = LocalComponent;
+
+    fn new(world: &'world World, system_metadata: &SystemMetadata) -> Self
+    {
+        let Some(system_ent) = world.get_entity(system_metadata.ent_id) else {
+            panic!(
+                "System entity with ID {} does not exist",
+                system_metadata.ent_id
+            );
+        };
+
+        let Some(local_component) = system_ent.get_with_id_mut::<LocalComponent>(
+            Pair::builder()
+                .relation::<IsLocalComponent>()
+                .target::<LocalComponent>()
+                .build()
+                .id(),
+        ) else {
+            panic!(
+                "Local component {} of system with ID {} is uninitialized",
+                type_name::<LocalComponent>(),
+                system_metadata.ent_id
+            );
+        };
+
+        Self { local_component }
+    }
+}
+
+impl<'world, LocalComponent, SystemT> InitializableParam<'world, SystemT>
+    for Local<'world, LocalComponent>
+where
+    LocalComponent: Component,
+    SystemT: SystemWithLocalComponents,
+    Self: SystemParam<'world, Input = LocalComponent>,
+{
+    fn initialize(system: &mut SystemT, input: Self::Input)
+    {
+        system.add_local_component(
+            Pair::builder()
+                .relation::<IsLocalComponent>()
+                .target_as_data(input)
+                .build()
+                .into_parts(),
+        );
+    }
+}
+
+impl<LocalComponent> Deref for Local<'_, LocalComponent>
+where
+    LocalComponent: Component,
+{
+    type Target = LocalComponent;
+
+    fn deref(&self) -> &Self::Target
+    {
+        &self.local_component
+    }
+}
+
+impl<LocalComponent> DerefMut for Local<'_, LocalComponent>
+where
+    LocalComponent: Component,
+{
+    fn deref_mut(&mut self) -> &mut Self::Target
+    {
+        &mut self.local_component
+    }
+}
+
+pub trait SystemWithLocalComponents
+{
+    fn add_local_component(&mut self, component_parts: ComponentParts);
+}
+
+#[derive(Component)]
+struct IsLocalComponent;