OptionalonCallback for each batch of meshes
OptionalonCallback for deferred color updates
OptionalonCallback when processing is complete
OptionalonCallback for errors
OptionalonDeclared but never read. This is the only occurrence of the name in the repository — no bridge implementation ever invokes it, so a callback passed here is never called and no "early metadata bootstrap" is delivered. Unlike its siblings (onBatch, onColorUpdate, onComplete, onError), which are all dispatched, this one is an unfinished stub. There is no substitute callback; metadata must be read from the completed result. Slated for removal; see issue #2731.
Options for streaming geometry processing