Receive a batch of mutation events.
Each entry maps a mutated resource id to an existence flag: true if the resource was
created, updated, or inserted;
false if it was deleted or removed. When the same
resource is mutated multiple times within a batch, last-write-wins semantics apply.
Observers may be sync or async. Both synchronous throws and asynchronous rejections are caught and silently ignored so that one faulty observer cannot break delivery to others.
Record mapping resource identifiers to existence flags
(true for upserted, false for removed)
Store mutation observer.
Receives batched mutation events. Registered via Store.observe.