Component

Components are objects that contain the logic and data pertaining to a particular system in your game. Typically, good Components are largely self-contained.

The data that a Component stores for each Entity is a struct (or sometimes a class) referred to as ComponentData.

Components are registered with an Entity Manager, and the Entity Manager is then responsible for adding Entities to each Component. Once per frame, the Entity Manager will call EntityManager::UpdateComponents(), which calls each Component's ComponentInterface::UpdateAllEntities().

Defining the ComponentData

Before creating a Component, you need to create a struct (or class) that contains all your per-Entity data. In some cases, you may not have any per-Entity data. In that case, just make an empty struct.

struct MyComponentData {
  int my_entity_data = 0;
}

Declaring the Component class

All Components should implement the ComponentInterface (although it is often useful to extend from the Component abstract class).

class MyComponent : public entity::Component<MyComponentData> {
 public:
  // This function is unused in this example, but is required as part
  // of the `ComponentInterface` as a pure-virtual function.
  virtual void AddFromRawData(EntityRef&, const void*) {}

  // This function iterates through every Entity that is registered
  // with this Component.
  virtual void UpdateAllEntities(WorldTime);
}

Inside each header file that declares a Component, you need to use the FPL_ENTITY_REGISTER_COMPONENT() macro. This is required in order to declare the necessary constants for lookups.

Note: This should be declared outside of any namespaces!

FPL_ENTITY_REGISTER_COMPONENT(fpl::entity::example::MyComponent,
                              fpl::entity::example::MyComponentData)

Defining the Component class

Inside each source file that defines a Component, you need to use the FPL_ENTTIY_DEFINE_COMPONENT() macro. This handles defining the storage location for the Component for a given type and ComponentData type.

Note: This should be declared at the top of the file outside of any namespaces!

FPL_ENTITY_DEFINE_COMPONENT(fpl::entity::example::MyComponent,
                            fpl::entity::example::MyComponentData)

After calling the above macro, you can implement any necessary methods for your Component clsas. Namely, you will want to define ComponentInterface::UpdateAllEntities() with the functionality to update each Entity each frame.

void MyComponent::UpdateAllEntities(WorldTime) {
  // Iterate through each of the ComponentData that correspond to each Entity.
  for (auto iter = component_data_.begin(); iter != component_data_.end();
       ++iter) {
    MyComponentData* entity_data = Data<MyComponentData>(iter->entity);

    // Do something with the data for this Entity here.
  }
}