pub type ChangeObserver<'a> = &'a mut dyn FnMut(Link, Link);Expand description
Callback invoked once per (before, after) change a write produced.
The upstream decorators turn a single write into a cascade of changes, so
the layers above the storage — names, transactions, the query processor —
only stay in sync if they can see all of them. This is the equivalent of the
WriteHandler the C# implementation threads through every decorator. A
change whose after is null is a deletion.