Properly link flecs library

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

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+#include <monitor.h>
+#include <stdio.h>
+
+// A monitor observer triggers when an entity starts/stop matching the observer
+// filter. The observer communicates whether an entity is "entering/leaving" the
+// monitor by setting ecs_iter_t::event to EcsOnAdd (for entering) or 
+// EcsOnRemove (for leaving).
+//
+// To specify that an observer is a monitor observer, the EcsMonitor tag must be
+// provided as event. No additional event kinds should be provided for a monitor
+// observer.
+
+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;
+
+    for (int i = 0; i < it->count; i ++) {
+        ecs_entity_t e = it->entities[i];
+        
+        if (event == EcsOnAdd) {
+            printf(" - Enter: %s: %s\n", 
+                ecs_get_name(ecs, event_id),
+                ecs_get_name(ecs, e));
+        } else if (event == EcsOnRemove) {
+            printf(" - Leave: %s: %s\n", 
+                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);
+    ECS_COMPONENT(ecs, Velocity);
+
+    ecs_observer(ecs, {
+        .filter = { .terms = {{ .id = ecs_id(Position) }, { .id = ecs_id(Velocity) }}},
+        .events = { EcsMonitor }, // Monitor entities entering/leaving the query
+        .callback = Observer,
+    });
+
+    // Create entity
+    ecs_entity_t e = ecs_new_entity(ecs, "e");
+
+    // This does not yet trigger the monitor, as the entity does not yet match.
+    ecs_set(ecs, e, Position, {10, 20});
+
+    // This triggers the monitor with EcsOnAdd, as the entity now matches.
+    ecs_set(ecs, e, Velocity, {1, 2});
+
+    // This triggers the monitor with EcsOnRemove, as the entity no longer matches.
+    ecs_remove(ecs, e, Position);
+
+    ecs_fini(ecs);
+
+    // Output
+    //   - Enter: Velocity: e
+    //   - Leave: Position: e
+}