entities/prefab

• C++

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  #include <prefab.h>
  #include <iostream>
   
  // Prefabs are entities that can be used as templates for other entities. They
  // are created with a builtin Prefab tag, which by default excludes them from
  // queries and systems.
   
  struct Attack { double value; };
  struct Defense { double value; };
  struct FreightCapacity { double value; };
  struct ImpulseSpeed { double value; };
  struct HasFTL { };
   
  struct Position { double x = 0; double y = 0; };
   
  int main(int, char *[]) {
      flecs::world ecs;
   
      // Create a prefab hierarchy.
      flecs::entity spaceship = ecs.prefab("Spaceship")
          // Add components to prefab entity as usual
          .set<ImpulseSpeed>({50})
          .set<Defense>({50})
   
          // By default components in an inheritance hierarchy are shared between
          // entities. The override function ensures that instances have a private
          // copy of the component.
          .auto_override<Position>();
   
          flecs::entity freighter = ecs.prefab("Freighter")
              // Short for .add(flecs::IsA, spaceship). This ensures the entity
              // inherits all components from spaceship.
              .is_a(spaceship)
              .add<HasFTL>()
              .set<FreightCapacity>({100})
              .set<Defense>({100});
   
              flecs::entity mammoth_freighter = ecs.prefab("MammothFreighter")
                  .is_a(freighter)
                  .set<FreightCapacity>({500})
                  .set<Defense>({300});
   
          ecs.prefab("Frigate")
              .is_a(spaceship)
              .add<HasFTL>()
              .set<Attack>({100})
              .set<Defense>({75})
              .set<ImpulseSpeed>({125});
   
      // Create a regular entity from a prefab.
      // The instance will have a private copy of the Position component, because
      // of the override in the spaceship entity. All other components are shared.
      flecs::entity inst = ecs.entity("my_mammoth_freighter")
          .is_a(mammoth_freighter);
   
      // Inspect the type of the entity. This outputs:
      //    Position,(Identifier,Name),(IsA,MammothFreighter)
      std::cout << "Instance type: [" << inst.type().str() << "]\n";
   
      // Even though the instance doesn't have a private copy of ImpulseSpeed, we
      // can still get it using the regular API (outputs 50)
      const ImpulseSpeed& ptr = inst.get<ImpulseSpeed>();
      std::cout << "Impulse speed: " << ptr.value << "\n";
   
      // Prefab components can be iterated just like regular components:
      ecs.each([](flecs::entity e, ImpulseSpeed& is, Position& p) {
          p.x += is.value;
          std::cout << e.name() << ": {" << p.x << ", " << p.y << "}\n"; 
      });
  }