Relay / Receiver: observe component field writes

[junkdog]

What?

• subscribe to component fields; notified when value is updated
• supplements systems; can replace "reactive" systems
• desired performance characteristics (dev and artemis-odb-plugin):
  • dev/no plugin: artemis internally copies all component data and compares with actual. somewhat wasteful and slow, but convenient during dev.
  • plugin/build-time: intercepts field writes and replaces them with a generated setter; the generated method sets a global dirty state; overhead should be minimal. this won't work in multi-threaded contexts however (ie, when running simultaneous active worlds on multiple threads).
• possible to control frequency of flushing as of yet not notified listeners.

@Relay: component scaffolding

Only primitive types and String are supported.

public class Layer extends Component {
    @Relay // mark primitive fields and strings
    public int order;
}

RelayProcessor: listening in on updated values

// this is the actual contract - the @Relay on Layer::order
// is only validated and serves as documentation; so that
// we don't accidentally forget that we might be running 
// custom code against the component.
@Relay.Receiver(type = Layer.class, field = "order")
public class LayerOrderRelay
        implements RelayProcessor<Layer> {

    // auto-@Wire 
    private ComponentMapper<HudRenderable> hudRenderableMapper;

    @Override // compare to systems' begin/end
    public void begin(IntBag entites) {}

    @Override
    public void onUpdate(int entityId,
                         Layer layer) {

        boolean isHud = hudRenderableMapper.has(entityId);
        if (layer.isUiLayer() != isHud)
            hudRenderableMapper.set(entityId, !isHud);
    }

    @Override // just like systems; these are executed in batch
    public void end(IntBag entites) {}
}

• A @Relayed field can have multiple registered LayerProcessors.
• RelaySystem (?, not sure about the name) is responsible for executing the processors.
• RelaySystem may flush multiple times per processing round; best controlled via custom invocation strategies

[junkdog]

Nvm, can't properly intercept the entity id when writing to the component field.