1 files changed, 103 insertions, 0 deletions
diff --git a/engine-ecs/src/event/component.rs b/engine-ecs/src/event/component.rs
new file mode 100644
index 0000000..70ea3e5
--- /dev/null
+++ b/engine-ecs/src/event/component.rs
@@ -0,0 +1,103 @@
+//! Component events.
+
+use std::convert::Infallible;
+
+use crate::Component;
+use crate::component::{Handle as ComponentHandle, HandleMut as ComponentHandleMut};
+use crate::entity::Handle as EntityHandle;
+use crate::pair::Pair;
+use crate::system::observer::{EventMatch, Observed};
+
+/// Implemented by the relations of component event pairs
+pub trait EventRelation: Component {}
+
+/// Pair relation for events emitted when:
+/// a) A entity with the target component is spawned.
+/// b) The target component is added to a entity.
+#[derive(Debug, Component)]
+pub struct Added(Infallible);
+
+impl EventRelation for Added {}
+
+/// Pair relation for events emitted **before**:
+/// a) The target component is removed from a entity.
+/// b) A entity with the target component is despawned.
+#[derive(Debug, Component)]
+pub struct Removed(Infallible);
+
+impl EventRelation for Removed {}
+
+#[derive(Debug, Component)]
+pub struct Changed(Infallible);
+
+impl EventRelation for Changed {}
+
+/// [`EventMatch`] extension trait for component event matches.
+pub trait EventMatchExt<Target>: sealed::Sealed
+{
+    #[must_use]
+    fn get_entity(&self) -> EntityHandle<'_>;
+
+    #[must_use]
+    fn get_ent_target_comp(&self) -> ComponentHandle<'_, Target>
+    where
+        Target: Component;
+
+    #[must_use]
+    fn get_ent_target_comp_mut(&self) -> ComponentHandleMut<'_, Target>
+    where
+        Target: Component;
+}
+
+impl<ComponentEventRelation: EventRelation, Target> EventMatchExt<Target>
+    for EventMatch<'_, Pair<ComponentEventRelation, Target>>
+where
+    Pair<ComponentEventRelation, Target>: Observed,
+{
+    fn get_entity(&self) -> EntityHandle<'_>
+    {
+        let Some(ent) = self.try_get_entity() else {
+            unreachable!();
+        };
+
+        ent
+    }
+
+    fn get_ent_target_comp(&self) -> ComponentHandle<'_, Target>
+    where
+        Target: Component,
+    {
+        let ent = self.get_entity();
+
+        let Some(comp) = ent.get::<Target>() else {
+            unreachable!();
+        };
+
+        comp
+    }
+
+    fn get_ent_target_comp_mut(&self) -> ComponentHandleMut<'_, Target>
+    where
+        Target: Component,
+    {
+        let ent = self.get_entity();
+
+        let Some(comp) = ent.get_mut::<Target>() else {
+            unreachable!();
+        };
+
+        comp
+    }
+}
+
+impl<ComponentEventRelation: EventRelation, Target> sealed::Sealed
+    for EventMatch<'_, Pair<ComponentEventRelation, Target>>
+where
+    Pair<ComponentEventRelation, Target>: Observed,
+{
+}
+
+mod sealed
+{
+    pub trait Sealed {}
+}