Properly link flecs library

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

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+#include <custom_event.h>
+#include <stdio.h>
+
+// Observers can be used to match custom events. Custom events can be emitted
+// using the ecs_emit function. This function is also used by builtin events,
+// so builtin and custom events use the same rules for matching with observers.
+//
+// An event consists out of three pieces of data used to match with observers:
+//  - An single event kind (EcsOnAdd, EcsOnRemove, ...)
+//  - One or more event ids (Position, Velocity, ...)
+//  - A source (either an entity or a table)
+
+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;
+
+    for (int i = 0; i < it->count; i ++) {
+        ecs_entity_t e = it->entities[i];
+
+        printf(" - %s: %s: %s\n", 
+            ecs_get_name(ecs, event), 
+            ecs_get_name(ecs, event_id),
+            ecs_get_name(ecs, e));
+    }
+}
+
+int main(int argc, char *argv[]) {
+    ecs_world_t *ecs = ecs_init_w_args(argc, argv);
+
+    ECS_COMPONENT(ecs, Position);
+
+    // Create custom event
+    ecs_entity_t MyEvent = ecs_new_entity(ecs, "MyEvent");
+
+    // Create observer for custom event
+    ECS_OBSERVER(ecs, Observer, MyEvent, Position);
+
+    // Create entity
+    ecs_entity_t e = ecs_new_entity(ecs, "e");
+    
+    // The observer filter can be matched against the entity, so make sure it
+    // has the Position component before emitting the event. This does not 
+    // trigger the observer yet.
+    ecs_set(ecs, e, Position, {10, 20});
+
+    // Emit the custom event
+    ecs_emit(ecs, &(ecs_event_desc_t) {
+        .event = MyEvent,
+        .ids = &(ecs_type_t){ (ecs_id_t[]){ ecs_id(Position) }, 1 }, // 1 id
+        .entity = e
+    });
+
+    ecs_fini(ecs);
+
+    // Output
+    //    - MyEvent: Position: e
+}