1 files changed, 80 insertions, 0 deletions
diff --git a/ecs/examples/component_changed_event.rs b/ecs/examples/component_changed_event.rs
new file mode 100644
index 0000000..643f338
--- /dev/null
+++ b/ecs/examples/component_changed_event.rs
@@ -0,0 +1,80 @@
+use ecs::event::component::OnChanged;
+use ecs::pair::Pair;
+use ecs::phase::UPDATE as UPDATE_PHASE;
+use ecs::system::observer::Observe;
+use ecs::{Component, Query, World};
+
+#[derive(Component)]
+struct SomeData
+{
+    num: u64,
+}
+
+#[derive(Component)]
+struct Greeting
+{
+    greeting: String,
+}
+
+fn say_hello(query: Query<(&SomeData, &mut Greeting)>)
+{
+    for (data, mut greeting) in &query {
+        println!("{}: {}", greeting.greeting, data.num);
+
+        if greeting.greeting == "Good evening" {
+            greeting.greeting = "Good morning".to_string();
+            greeting.set_changed();
+        }
+    }
+}
+
+fn print_changed_greetings(observe: Observe<'_, Pair<OnChanged, Greeting>>)
+{
+    println!("\nChanged greetings:");
+
+    for ent in &observe {
+        let Some(greeting) = ent.get::<Greeting>() else {
+            unreachable!();
+        };
+
+        println!("A greeting changed to {}", greeting.greeting);
+    }
+
+    println!("");
+}
+
+fn main()
+{
+    let mut world = World::new();
+
+    world.register_system(*UPDATE_PHASE, say_hello);
+
+    world.register_observer(print_changed_greetings);
+
+    world.create_entity((
+        SomeData { num: 987_654 },
+        Greeting {
+            greeting: "Good afternoon".to_string(),
+        },
+    ));
+
+    world.create_entity((
+        SomeData { num: 345 },
+        Greeting { greeting: "Good evening".to_string() },
+    ));
+
+    world.step();
+
+    world.step();
+
+    for (mut greeting,) in &world.query::<(&mut Greeting,), ()>() {
+        if greeting.greeting == "Good afternoon" {
+            greeting.greeting = "Yo yo".to_string();
+            greeting.set_changed();
+        }
+    }
+
+    world.step();
+
+    world.step();
+}