Add ownership post example

crates/gpui/examples/ownership_post.rs

@@ -0,0 +1,34 @@
+use gpui::{prelude::*, App, AppContext, EventEmitter, Model, ModelContext};
+
+struct Counter {
+    count: usize,
+}
+
+fn main() {
+    App::new().run(|cx: &mut AppContext| {
+        let counter: Model<Counter> = cx.new_model(|_cx| Counter { count: 0 });
+        let observer = cx.new_model(|cx: &mut ModelContext<Counter>| {
+            cx.observe(&counter, |observer, observed, cx| {
+                observer.count = observed.read(cx).count * 2;
+            })
+            .detach();
+
+            Counter {
+                count: counter.read(cx).count * 2,
+            }
+        });
+
+        counter.update(cx, |counter, cx| {
+            counter.count += 1;
+            cx.notify();
+        });
+
+        assert_eq!(observer.read(cx).count, 2);
+    });
+}
+
+struct Change {
+    delta: isize,
+}
+
+impl EventEmitter<Change> for Counter {}