1 files changed, 88 insertions, 0 deletions
diff --git a/engine-ecs/src/sole.rs b/engine-ecs/src/sole.rs
new file mode 100644
index 0000000..cf5283b
--- /dev/null
+++ b/engine-ecs/src/sole.rs
@@ -0,0 +1,88 @@
+use std::any::{type_name, Any};
+use std::fmt::Debug;
+
+use crate::component::{
+    HandleMut as ComponentHandleMut,
+    IntoParts as IntoComponentParts,
+    WithAssociatedId as ComponentWithAssociatedId,
+};
+use crate::system::{Metadata as SystemMetadata, Param as SystemParam};
+use crate::{Component, World};
+
+/// A component which has a single instance in the [`World`].
+pub trait Sole: Any + IntoComponentParts + ComponentWithAssociatedId
+{
+    fn type_reflection() -> Option<&'static crate::reflection::Type>
+    where
+        Self: Sized;
+
+    /// Returns the name of this component.
+    fn name(&self) -> &'static str;
+}
+
+/// Marker component for the entities of soles.
+#[derive(Debug, Component)]
+#[non_exhaustive]
+pub struct Is;
+
+/// Holds a reference to a singleton component.
+#[derive(Debug)]
+pub struct Single<'world, SoleT: Sole>
+{
+    sole: Option<ComponentHandleMut<'world, SoleT>>,
+}
+
+impl<'world, SoleT> Single<'world, SoleT>
+where
+    SoleT: Sole,
+{
+    /// Returns a reference to `SoleT`.
+    ///
+    /// # Errors
+    /// Returns `Err` if the sole does not exist.
+    pub fn get(&self) -> Result<&SoleT, DoesNotExistError>
+    {
+        self.sole
+            .as_deref()
+            .ok_or_else(|| DoesNotExistError { name: type_name::<SoleT>() })
+    }
+
+    /// Returns a mutable reference to `SoleT`.
+    ///
+    /// # Errors
+    /// Returns `Err` if the sole does not exist.
+    pub fn get_mut(&mut self) -> Result<&mut SoleT, DoesNotExistError>
+    {
+        self.sole
+            .as_deref_mut()
+            .ok_or_else(|| DoesNotExistError { name: type_name::<SoleT>() })
+    }
+
+    pub(crate) fn new(world: &'world World) -> Self
+    {
+        let sole = world
+            .get_entity(SoleT::id())
+            .and_then(|ent| ent.get_with_id_mut::<SoleT>(SoleT::id()));
+
+        Self { sole }
+    }
+}
+
+impl<'world, SoleT> SystemParam<'world> for Single<'world, SoleT>
+where
+    SoleT: Sole,
+{
+    type Input = ();
+
+    fn new(world: &'world World, _system_metadata: &SystemMetadata) -> Self
+    {
+        Self::new(world)
+    }
+}
+
+#[derive(Debug, thiserror::Error)]
+#[error("Sole {name} does not exist")]
+pub struct DoesNotExistError
+{
+    pub name: &'static str,
+}