Properly link flecs library

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

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+#include <entity_event.h>
+#include <stdio.h>
+
+// Entity events are events that are emitted and observed for a specific entity.
+// They are a thin wrapper around regular observers, which match against queries
+// instead of single entities. While they work similarly under the hood, entity
+// events provide a much simpler API.
+//
+// An entity event only needs two pieces of data:
+// - The entity on which to emit the event
+// - The event to emit
+//
+// Any (_) is provided as component, indicating that we're not emitting an event 
+// for a specific component but rather for the entity itself.
+
+// Event without payload
+ECS_TAG_DECLARE(Click);
+
+// Event with payload
+typedef struct {
+    double width, height;
+} Resize;
+
+ECS_COMPONENT_DECLARE(Resize);
+
+void OnClick(ecs_iter_t *it) {
+    // The event source can be obtained with ecs_field_src(1). This allows the
+    // same event function to be used for different entities.
+    char *path = ecs_get_fullpath(it->world, ecs_field_src(it, 1));
+    printf("clicked on %s!\n", path);
+    ecs_os_free(path);
+}
+
+void OnResize(ecs_iter_t *it) {
+    // Event payload can be obtained from the it->param member
+    Resize *p = it->param;
+    char *path = ecs_get_fullpath(it->world, ecs_field_src(it, 1));
+    printf("resized %s to {%.0f, %.0f}!\n", path, p->width, p->height);
+    ecs_os_free(path);
+}
+
+int main(int argc, char *argv[]) {
+    ecs_world_t *ecs = ecs_init_w_args(argc, argv);
+
+    ECS_TAG_DEFINE(ecs, Click);
+    ECS_COMPONENT_DEFINE(ecs, Resize);
+
+    // Create a widget entity
+    ecs_entity_t widget = ecs_new_entity(ecs, "MyWidget");
+
+    // Create entity observer. Use EcsAny to indicate we're not interested in
+    // matching specific components.
+    ecs_observer(ecs, {
+        .filter.terms = {{ .id = EcsAny, .src.id = widget }},
+        .events = { Click },
+        .callback = OnClick
+    });
+
+    // Create another one for the Resize event
+    ecs_observer(ecs, {
+        .filter.terms = {{ .id = EcsAny, .src.id = widget }},
+        .events = { ecs_id(Resize) },
+        .callback = OnResize
+    });
+
+    // Emit the Click event
+    ecs_emit(ecs, &(ecs_event_desc_t) {
+        .event = Click,
+        .entity = widget
+    });
+
+    // Emit the Resize event
+    ecs_emit(ecs, &(ecs_event_desc_t) {
+        .event = ecs_id(Resize),
+        .entity = widget,
+        .param = &(Resize){100, 200} // pass payload
+    });
+
+    ecs_fini(ecs);
+
+    // Output
+    //  clicked on MyWidget!
+    //  resized MyWidget to {100, 200}!
+}