Properly link flecs library

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

@@ -0,0 +1,53 @@
+#include <two_components.h>
+#include <stdio.h>
+
+// An observer can match multiple components/tags. Only entities that match the
+// entire observer filter will be forwarded to the callback. For example, an
+// observer for Position,Velocity won't match an entity that only has Position.
+
+typedef struct {
+    double x, y;
+} Position, Velocity;
+
+void Observer(ecs_iter_t *it) {
+    ecs_world_t *ecs = it->world;
+
+    ecs_entity_t event = it->event;
+    ecs_entity_t event_id = it->event_id;
+
+    Position *p = ecs_field(it, Position, 1);
+    Velocity *v = ecs_field(it, Velocity, 2);
+
+    for (int i = 0; i < it->count; i ++) {
+        ecs_entity_t e = it->entities[i];
+        printf("%s: %s: %s: p: {%f, %f} v: {%f, %f}\n",
+            ecs_get_name(ecs, event),
+            ecs_get_name(ecs, event_id),
+            ecs_get_name(ecs, e),
+            p[i].x, p[i].y,
+            v[i].x, v[i].y);
+    }
+}
+
+int main(int argc, char *argv[]) {
+    ecs_world_t *ecs = ecs_init_w_args(argc, argv);
+
+    ECS_COMPONENT(ecs, Position);
+    ECS_COMPONENT(ecs, Velocity);
+
+    ECS_OBSERVER(ecs, Observer, EcsOnSet, Position, Velocity);
+
+    // Create entity
+    ecs_entity_t e = ecs_new_entity(ecs, "e");
+    
+    // Set Position (emits EcsOnSet, does not yet match observer)
+    ecs_set(ecs, e, Position, {10, 20});
+
+    // Set Velocity (emits EcsOnSet, matches observer)
+    ecs_set(ecs, e, Velocity, {1, 2});
+
+    return ecs_fini(ecs);
+
+    // Output:
+    //   OnSet: Velocity: e: p: {10.000000, 20.000000} v: {1.000000, 2.000000}
+}