The index and inverseRelation shadow variables need to be updated when the list variable is changed by a move, including change, swap, assign and unassign moves.

For example, in order to avoid score corruption during unassign move, the removed element's shadow variables need to be unset.

Currently, the before/afterVariableChanged methods only receive the entity instance, whose list variable is being changed. The information about which element is being moved is missing. Therefore we unset index and inverse relation shadow variables of all elements in the list before variable changes and then reset them after the change is done. This is highly inefficient.

Performance discussions:

• https://github.com/kiegroup/optaplanner/pull/1494#discussion_r695186996
• https://github.com/kiegroup/optaplanner/pull/1645#discussion_r742860038

If the index at which the change occurs was part of the before/afterVariableChanged notification method, then we'd be able to eliminate a lot of unnecessary shadow variable updates on elements that are not affected by the change.

The new ListVariableListener API should also work for custom shadow variables that have a genuine list variable as a source. Example: StartTimeUpdatingVariableListener implements ListVariableListener.