Adding and removing components during runtime

During runtime, a worker instance can request SpatialOS to:

• add a component to an entity using Worker_Connection_SendAddComponent
• remove a component from an entity using Worker_Connection_SendRemoveComponent

In order to use these functions, you must set Worker_ConnectionParameters::enable_dynamic_components to 1 (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.

Authority change and component state synchronization

When you set Worker_ConnectionParameters::enable_dynamic_components to 1 (true), worker instances will receive an Worker_AddComponentOp or Worker_RemoveComponentOp (instead of a Worker_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:

• a Worker_AddComponentOp (if the component exists)
• a Worker_RemoveComponentOp (if the component is non-existent)

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

• a Worker_AddComponentOp (if the component exists)
• a Worker_RemoveComponentOp (if the component is non-existent)

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

• a Worker_AddComponentOp, you should replace any existing state for the component in the worker instance’s local view with Worker_AddComponentOp.data
  
• a Worker_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. Worker_AddComponentOp 2. Worker_AuthorityChangeOp (kAuthoritative)	1. Worker_RemoveComponentOp 2. Worker_AuthorityChangeOp (kAuthoritative)
Losing write access authority	1. Worker_AuthorityChangeOp (kNotAuthoritative) 2. Worker_AddComponentOp	1. Worker_AuthorityChangeOp (kNotAuthoritative) 2. Worker_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;
}

Example of adding a component

This example assumes that you have followed the serialization reference to create a Worker_ComponentData for SuperStrength. The worker instance which has write access authority over the SuperStrength component can add it to the entity:

void add_super_strength_component_to_entity(Worker_Connection* connection,
                                            Worker_EntityId entity_id,
                                            Worker_ComponentData* super_strength_component_data) {
  Worker_Connection_SendAddComponent(connection, entity_id, super_strength_component_data,
                                     NULL /* short-circuit */);
}

Example of removing a component

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

void remove_super_strength_component_from_entity(Worker_Connection* connection,
                                                 Worker_EntityId entity_id) {
  Worker_Connection_SendRemoveComponent(connection, entity_id, 2345, NULL /* short-circuit */);
}

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