Circular Dependencies in an Entity Component System

Question

I'm developing a top-down shooter in C++ using an entity-component system, and have run into a problem when attempting to refactor powerups. Currently I have a PowerupSystem that is essentially a load of if...else statements that directly modifies player entity attributes (for example speed).

I'd like to move towards using different powerup classes that inherit from a base PowerupComponent class that encapsulate powerup behaviour. An example of this would be SpeedPowerup. These derived classes would have a constructor that changes a value in the player entity, and a destructor that sets it back to its original value.

The problem I'm having is that the powerup constructors and destructors need access to the player entity data to make these changes (stored in my World class), giving me a circular dependency issue.

The dependency is:

world -> powerup system -> powerup -> world

Any ideas on how I could overcome this issue are appreciated.

Thanks in advance, Rich.

Answer 1

The way I would design this would be based on physics collisions.

I would register the PowerUpSystem with the PhysicsSystem so that it can be notified about times when game objects collide so that it can handle those collisions.

void PowerUpSystem::OnCollisionEvent(const CollisionEvent& event)
{
  Entity* sender = event->GetSender();
  Entity* receiver = event->GetReceiver();

  //! do something here with the two entities.
}

I would expect the power-up entity to have some component type that your PowerUpSystem would look for and might verify that the player entity is actually the player and not some other entity. You wouldn't want some wandering NPC to be able to greedily acquire a power-up.

At this point, you have a component that knows what attribute to modify in what way and your entity instance. You could simply use delegation here like this:

void SpeedUpComponent::ApplyToEntity(Entity* entity)
{
   //! Apply the speed modification to the provided entity.
}

What I would do is separate the concept of a power-up and an aura/buff that has been applied so that you can easily track them separately and react accordingly.

So SpeedUpComponent applies a SpeedAuraComponent that could either then be tracked by the PowerUpSystem or by a separate aura/buff system.