Properly link flecs library

engine/libs/flecs/examples/cpp/observers/yield_existing/src/main.cpp

@@ -0,0 +1,36 @@
+#include <yield_existing.h>
+#include <iostream>
+
+// Observers can enable a "yield_existing" feature that upon creation of the
+// observer produces events for all entities that match the observer query. The
+// feature is only implemented for the builtin EcsOnAdd and EcsOnSet events.
+//
+// Custom events can also implement behavior for yield_existing by adding the
+// Iterable component to the event (see EcsIterable for more details).
+
+struct Position {
+    double x, y;
+};
+
+int main(int, char *[]) {
+    flecs::world ecs;
+
+    // Create existing entities with Position component
+    ecs.entity("e1").set<Position>({10, 20});
+    ecs.entity("e2").set<Position>({20, 30});
+
+    // Create an observer for three events
+    ecs.observer<Position>()
+        .event(flecs::OnSet)
+        .yield_existing() // Trigger for existing matching entities
+        .each([](flecs::iter& it, size_t i, Position& p) {
+            std::cout << " - " << it.event().name() << ": " 
+                << it.event_id().str() << ": "
+                << it.entity(i).name() 
+                << ": {" << p.x << ", " << p.y << "}\n";
+        });
+
+    // Output
+    //   - OnSet: Position: e1: {10, 20}
+    //   - OnSet: Position: e2: {20, 30}
+}