Adding and removing components during runtime

During runtime, a worker instance can request SpatialOS to:

• add a component to an entity using Connection::SendAddComponent
• remove a component from an entity using Connection::SendRemoveComponent

In order to use these functions, you must set ConnectionParameters.EnableDynamicComponents to true.

Non-existent components

This feature introduces the concept of non-existent components. A non-existent component is a component which can be added to an entity but is not currently on the entity. In other words, a worker instance has write access authority over the component, but the component is not currently on the entity in the game world.

Write access authority over non-existent components works the same as write access authority over existing components. If you are unsure of how to change write access authority, see the example of changing authority.

Authority change and component state synchronization

When you set ConnectionParameters.EnableDynamicComponents to true, worker instances will receive an AddComponentOp or RemoveComponentOp (instead of a ComponentUpdateOp) when the worker instance gains or loses write access authority over a component. This is to synchronize the worker instance’s local view with that of the Runtime.

Immediately before a worker instance gains write access authority over a component, the worker instance receives either:

• an AddComponentOp (if the component exists)
• a RemoveComponentOp (if the component is non-existent)

Likewise, after losing write access authority over a component, the worker instance receives either:

• an AddComponentOp (if the component exists)
• a RemoveComponentOp (if the component is non-existent)

AddComponentOp and RemoveComponentOp must be treated in an idempotent way. In other words, if the worker instance receives:

• an AddComponentOp, you should replace any existing state for the component in the worker instance’s local view with AddComponentOp.Data
  
• a RemoveComponentOp, you must remove any existing state for the component in the worker instance’s local view

The following table summarizes the ops and the order in which the worker instance receives them:

	Existing component	Non-existent component
Gaining write access authority	1. AddComponentOp 2. AuthorityChangeOp (kAuthoritative)	1. RemoveComponentOp 2. AuthorityChangeOp (kAuthoritative)
Losing write access authority	1. AuthorityChangeOp (kNotAuthoritative) 2. AddComponentOp	1. AuthorityChangeOp (kNotAuthoritative) 2. RemoveComponentOp

Example

Imagine you have a component which is used to temporarily give super strength to a player:

component SuperStrength {
  id = 2345;
  int32 power_level = 1;
}

The worker instance which has write access authority over the write ACL of a given entity can give a worker instance write access authority over the SuperStrength component, as described in the example of changing authority.

Example of adding a component

The worker instance which has write access authority over the SuperStrength component can add it to the entity:

public static void AddSuperStrengthComponentToEntity(Connection connection, Improbable.EntityId entityId) {
  var componentData = new SuperStrength.Data(SuperStrengthData.Create());
  componentData.Value.powerLevel = 100;
  connection.SendAddComponent(entityId, componentData, new UpdateParameters{ Loopback = ComponentUpdateLoopback.ShortCircuited });
}

Example of removing a component

The worker instance which has write access authority over the SuperStrength component can remove it from the entity:

public static void RemoveSuperStrengthComponentFromEntity(Connection connection, Improbable.EntityId entityId) {
  connection.SendRemoveComponent<SuperStrength>(entityId, new UpdateParameters{ Loopback = ComponentUpdateLoopback.ShortCircuited });
}

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