Properly link flecs library

engine/libs/flecs/examples/c/queries/singleton/src/main.c

@@ -0,0 +1,71 @@
+#include <singleton.h>
+#include <stdio.h>
+
+// This example shows how to use singleton components in queries.
+
+// Singleton component
+typedef struct {
+    double value; 
+} Gravity;
+
+// Entity component
+typedef struct {
+    double x, y;
+} Velocity;
+
+int main(int argc, char *argv[]) {
+    ecs_world_t *ecs = ecs_init_w_args(argc, argv);
+
+    ECS_COMPONENT(ecs, Gravity);
+    ECS_COMPONENT(ecs, Velocity);
+
+    // Set singleton
+    ecs_singleton_set(ecs, Gravity, { 9.81 });
+
+    ecs_entity_t e1 = ecs_new_entity(ecs, "e1");
+    ecs_entity_t e2 = ecs_new_entity(ecs, "e2");
+    ecs_entity_t e3 = ecs_new_entity(ecs, "e3");
+
+    // Set Velocity
+    ecs_set(ecs, e1, Velocity, {0, 0});
+    ecs_set(ecs, e2, Velocity, {0, 1});
+    ecs_set(ecs, e3, Velocity, {0, 2});
+
+    // Create query that matches Gravity as singleton
+    ecs_query_t *q = ecs_query(ecs, {
+        .filter.terms = {
+            { .id = ecs_id(Velocity) },
+            // A singleton is a component matched on itself
+            { .id = ecs_id(Gravity), .src.id =  ecs_id(Gravity) }
+        },
+
+        // Optional, but lets us iterate entities in bulk even though the query
+        // returns fields with mixed counts (see query manual).
+        .filter.instanced = true
+    });
+
+    // In a query string expression you can use the $ shortcut for singletons:
+    //   Velocity, Gravity($)
+
+    ecs_iter_t it = ecs_query_iter(ecs, q);
+    while (ecs_query_next(&it)) {
+        Velocity *v = ecs_field(&it, Velocity, 1);
+        Gravity *g = ecs_field(&it, Gravity, 2);
+
+        for (int i = 0; i < it.count; i ++) {
+            // Make sure to access g as a pointer, as we only have a single
+            // instance of the Gravity component.
+            v[i].y += g->value;
+
+            printf("%s velocity is {%f, %f}\n",
+                ecs_get_name(ecs, it.entities[i]), v[i].x, v[i].y);
+        }
+    }
+
+    // Output
+    //   e1 velocity is {0.000000, 9.810000}
+    //   e2 velocity is {0.000000, 10.810000}
+    //   e3 velocity is {0.000000, 11.810000}
+
+    ecs_fini(ecs);
+}