feat(ecs): add support for singleton components

1 files changed, 98 insertions, 0 deletions

diff --git a/ecs/src/component/single.rs b/ecs/src/component/single.rs
new file mode 100644
index 0000000..a63dbe3
--- /dev/null
+++ b/ecs/src/component/single.rs
@@ -0,0 +1,98 @@
+use std::any::{type_name, Any, TypeId};
+use std::ops::{Deref, DerefMut};
+
+use crate::component::Component;
+use crate::system::{ComponentRefMut, NoInitParamFlag, Param as SystemParam, System};
+use crate::tuple::FilterExclude as TupleFilterExclude;
+use crate::WorldData;
+
+/// Holds a component which has a single instance and is shared globally.
+#[derive(Debug)]
+pub struct Single<'world, SingleComponent: Component>
+{
+    single_component: ComponentRefMut<'world, SingleComponent>,
+}
+
+unsafe impl<'world, SingleComponent> SystemParam<'world>
+    for Single<'world, SingleComponent>
+where
+    SingleComponent: Component,
+{
+    type Flags = NoInitParamFlag;
+    type Input = TupleFilterExclude;
+
+    fn initialize<SystemImpl>(
+        _system: &mut impl System<'world, SystemImpl>,
+        _input: Self::Input,
+    )
+    {
+    }
+
+    fn new<SystemImpl>(
+        _system: &'world impl System<'world, SystemImpl>,
+        world_data: &'world WorldData,
+    ) -> Self
+    {
+        let single_component = world_data
+            .single_component_storage
+            .get::<SingleComponent>()
+            .expect("Single component was not found in world")
+            .write_nonblock()
+            .unwrap_or_else(|_| {
+                panic!(
+                    "Failed to aquire read-write lock to single component {}",
+                    type_name::<SingleComponent>()
+                )
+            });
+
+        Self {
+            single_component: ComponentRefMut::new(single_component),
+        }
+    }
+
+    fn is_compatible<Other: SystemParam<'world>>() -> bool
+    {
+        let other_comparable = Other::get_comparable();
+
+        let Some(comparable) = other_comparable.downcast_ref::<Comparable>() else {
+            // The other system param is not Single
+            return true;
+        };
+
+        TypeId::of::<SingleComponent>() != comparable.single_component_type_id
+    }
+
+    fn get_comparable() -> Box<dyn Any>
+    {
+        Box::new(Comparable {
+            single_component_type_id: TypeId::of::<SingleComponent>(),
+        })
+    }
+}
+
+impl<'world, SingleComponent> Deref for Single<'world, SingleComponent>
+where
+    SingleComponent: Component,
+{
+    type Target = SingleComponent;
+
+    fn deref(&self) -> &Self::Target
+    {
+        &self.single_component
+    }
+}
+
+impl<'world, SingleComponent> DerefMut for Single<'world, SingleComponent>
+where
+    SingleComponent: Component,
+{
+    fn deref_mut(&mut self) -> &mut Self::Target
+    {
+        &mut self.single_component
+    }
+}
+
+struct Comparable
+{
+    single_component_type_id: TypeId,
+}