Properly link flecs library

engine/libs/flecs/examples/c/observers/propagate/src/main.c

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+#include <propagate.h>
+#include <stdio.h>
+
+// Events are propagated along relationship edges. This means that observers can
+// listen for events from a parent or prefab, like triggering when a component
+// inherited from a prefab was set.
+//
+// Event propagation happens automatically when an observer contains a filter
+// with the EcsUp flag set (indicating upwards traversal). Observers use the 
+// same matching logic as queries: if a query with upwards traversal matches an
+// entity, so will an observer.
+//
+// Events are only propagated along traversable relationship edges.
+
+typedef struct {
+    double x, y;
+} Position;
+
+void Observer(ecs_iter_t *it) {
+    ecs_world_t *ecs = it->world;
+    
+    // The event kind
+    ecs_entity_t event = it->event;
+
+    // The (component) id for which the event was emitted
+    ecs_entity_t event_id = it->event_id;
+
+    // Grab Position from self and parent
+    Position *p_self = ecs_field(it, Position, 1);
+    Position *p_parent = ecs_field(it, Position, 2);
+
+    for (int i = 0; i < it->count; i ++) {
+        ecs_entity_t e = it->entities[i];
+
+        printf(" - %s: %s: %s: self: {%f, %f}, parent: {%f, %f}\n", 
+            ecs_get_name(ecs, event), 
+            ecs_get_name(ecs, event_id),
+            ecs_get_name(ecs, e),
+            p_self[i].x, p_self[i].y,
+            p_parent[i].x, p_parent[i].y);
+    }
+}
+
+int main(int argc, char *argv[]) {
+    ecs_world_t *ecs = ecs_init_w_args(argc, argv);
+
+    ECS_COMPONENT(ecs, Position);
+
+    // Create observer that listens for events from both self and parent
+    ecs_observer(ecs, {
+        .filter = { .terms = {
+            // Listen for Position events from self
+            { .id = ecs_id(Position) },
+            // Listen for Position events from parent
+            { .id = ecs_id(Position), .src.flags = EcsUp, .src.trav = EcsChildOf }
+        }},
+        .events = { EcsOnSet },
+        .callback = Observer
+    });
+
+    // Create entity and parent
+    ecs_entity_t p = ecs_new_entity(ecs, "p");
+    ecs_entity_t e = ecs_new_entity(ecs, "p.e"); // Create as child of p
+
+    // Set Position on entity. This doesn't trigger the observer yet, since the
+    // parent doesn't have Position yet.
+    ecs_set(ecs, e, Position, {10, 20});
+
+    // Set Position on parent. This event will be propagated and trigger the 
+    // observer, as the observer query now matches.
+    ecs_set(ecs, p, Position, {1, 2});
+
+    ecs_fini(ecs);
+
+    // Output
+    //   - OnSet: Position: e: self: {10.000000, 20.000000}, parent: {1.000000, 2.000000}
+}