observers/monitor

• C

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  #include <monitor.h>
  #include <stdio.h>
   
  // A monitor observer triggers when an entity starts/stop matching the observer
  // query. 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->stage;
   
      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, {
          .query  = { .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_entity(ecs, { .name = "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
  }

• C++

  View on GitHub

  #include <monitor.h>
  #include <iostream>
   
  // A monitor observer triggers when an entity starts/stop matching the observer
  // query. 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.
   
  struct Position {
      double x, y;
  };
   
  struct Velocity {
      double x, y;
  };
   
  int main(int, char *[]) {
      flecs::world ecs;
   
      // Create observer for custom event
      ecs.observer<Position, Velocity>()
          .event(flecs::Monitor) // Monitor entities entering/leaving the query
          .each([](flecs::iter& it, size_t i, Position&, Velocity&) {
              if (it.event() == flecs::OnAdd) {
                  std::cout << " - Enter: "
                      << it.event_id().str() << ": "
                      << it.entity(i).name() << "\n";
              } else if (it.event() == flecs::OnRemove) {
                  std::cout << " - Leave: "
                      << it.event_id().str() << ": "
                      << it.entity(i).name() << "\n";
              }
          });
   
      // Create entity
      flecs::entity e = ecs.entity("e");
   
      // This does not yet trigger the monitor, as the entity does not yet match.
      e.set<Position>({10, 20});
   
      // This triggers the monitor with EcsOnAdd, as the entity now matches.
      e.set<Velocity>({1, 2});
   
      // This triggers the monitor with EcsOnRemove, as the entity no longer matches.
      e.remove<Position>();
   
      // Output
      //   - Enter: Velocity: e
      //   - Leave: Position: e
  }