Listeners shouldn’t capture references to their index and group?

[mbostock]

This won’t be backwards-compatible, and it would mean that the listener callback signature is different from the standard D3 pattern, but it would simplify things and fix a common memory leak as described in #186. If you need information based on the index and group, you could bake it into the data instead.

[mbostock]

[Recording a Slack discussion for posterity.]

mbostock: … since event listeners are called back asynchronously (unlike normal callback functions in D3, which are invoked immediately before the selection method returns), capturing the index i was confusing (it could be inconsistent with subsequent re-selection) and capturing the sibling nodes could lead to memory leaks.

To quote Emerson, “a foolish consistency is the hobgoblin of little minds”. I was trying to keep the callback pattern consistent, but {event, d} is so much simpler.

vijithassar: sure I get most of this, what I am kind of reaching for here is that “use an argument instead of setting a global” and “the callback function signature is now different” are both sensible points that are distinct from “datum is available but index is not available” and the latter seems to be conceptually important for D3 so I am wondering what the change signals about design and intent and philosophy and so on (edited)

mbostock: The reason that index is not available is this comment above: “event listeners are called back asynchronously…”

The callback pattern {d, i, nodes} isn’t changing for other D3 methods. This signature is only changing for event listeners registered with selection.on, which is a fundamentally different use case than normal D3 callbacks (there’s events!), so it makes sense that there’s a different signature.

See also this comment from the README: “Listeners always see the latest datum for their element, but the index is a property of the selection and is fixed when the listener is assigned; to update the index, re-assign the listener.”

That oddity with the index goes away when listeners don’t capture the index. In other words, the index is a transient property of the selection, whereas the data is a persistent property of the element to which the listener is attached.

Since listeners are asynchronous, it doesn’t make sense for them to capture transient properties of the selection. They should only have access to the element’s bound data and the current event.