Ixa v4.0 To-Do List

This is a meta issue to keep track of breaking changes slated for Ixa v4.0 that might not have their own dedicated issue yet. For things that already have their own issue, attach the issue to the v4.0 milestone. The point is, for some things it might be too early to do a concrete implementation or even file a concrete issue, but we don't want to forget about anything.

Proposed / Committed

These are items we already committed to or are low-risk or unlikely to be controversial.

• Remove the deprecated plan module. (Deprecate the plan module #977)
• For derived properties, pass the global parameters as Option<&T>, which is the return type of get_global_property, to the derived properties compute function instead of T. (Document global properties failure cases #691)

In-progress / research

These items are expected to generate new issues as they are researched.

• Stats and reporting v4: Stats and reporting #971

Under consideration / not yet committed

These might need more discussion or consensus or research before we commit to them.

• Changes to IxaEvent and event subscribers:
  • Change trait bounds on IxaEvent from Copy to Clone. This allows event types to hold an Rc, for example.
  • Change subscribers to take &IxaEvent instead of IxaEvent. This allows a single IxaEvent to be owned in an Rc, and a clone of the Rc is owned by each callback. The data is released when the last callback is popped from the callback queue and executed, releasing the (shared) IxaEvent data.
  • Remove on_subscribe / on_unsubscribe if we have no clear use case for it...
  • ...or change the signature of on_subscribe / on_unsubscribe to also accept the EventListenerId<E> value of the added/removed subscriber. That way an IxaEvent can actually change the subscription status based on its own logic if it wants to.