feat(ecs): add support for optional query components

1 files changed, 86 insertions, 0 deletions

diff --git a/ecs/examples/optional_component.rs b/ecs/examples/optional_component.rs
new file mode 100644
index 0000000..e47bf2e
--- /dev/null
+++ b/ecs/examples/optional_component.rs
@@ -0,0 +1,86 @@
+use ecs::event::Event;
+use ecs::{Component, Query, World};
+
+#[derive(Debug, Component)]
+struct PettingCapacity
+{
+    capacity_left: u32,
+}
+
+#[derive(Debug, Clone, Copy, Component)]
+enum Aggressivity
+{
+    High,
+    Medium,
+    Low,
+}
+
+#[derive(Debug, Component)]
+pub struct CatName
+{
+    name: String,
+}
+
+fn pet_cats(query: Query<(CatName, PettingCapacity, Option<Aggressivity>)>)
+{
+    for (cat_name, mut petting_capacity, aggressivity) in &query {
+        let Some(aggressivity) = aggressivity else {
+            println!("Aggressivity of cat {} is unknown. Skipping", cat_name.name);
+            continue;
+        };
+
+        if let Aggressivity::High = *aggressivity {
+            println!("Cat {} is aggressive. Skipping", cat_name.name);
+            continue;
+        }
+
+        if petting_capacity.capacity_left == 0 {
+            println!(
+                "Cat {} have had enough of being petted. Skipping",
+                cat_name.name
+            );
+            continue;
+        }
+
+        println!("Petting cat {}", cat_name.name);
+
+        petting_capacity.capacity_left -= 1;
+    }
+}
+
+#[derive(Debug)]
+struct PettingTime;
+
+impl Event for PettingTime {}
+
+fn main()
+{
+    let mut world = World::new();
+
+    world.register_system(PettingTime, pet_cats);
+
+    world.create_entity((
+        CatName { name: "Jasper".to_string() },
+        Aggressivity::Medium,
+        PettingCapacity { capacity_left: 5 },
+    ));
+
+    world.create_entity((
+        CatName { name: "Otto".to_string() },
+        PettingCapacity { capacity_left: 9 },
+    ));
+
+    world.create_entity((
+        CatName { name: "Carrie".to_string() },
+        PettingCapacity { capacity_left: 2 },
+        Aggressivity::High,
+    ));
+
+    world.create_entity((
+        CatName { name: "Tommy".to_string() },
+        PettingCapacity { capacity_left: 1 },
+        Aggressivity::Low,
+    ));
+
+    world.emit(PettingTime);
+}