Mapping between a topic (represented by T::Hash) and a vector of indexes
of events in the <Events<T>> list.
All topic vectors have deterministic storage locations depending on the topic. This
allows light-clients to leverage the changes trie storage tracking mechanism and
in case of changes fetch the list of events of interest.
The value has the type (BlockNumberFor<T>, EventIndex) because if we used only just
the EventIndex then in case if the topic has the same contents on the next block
no notification will be triggered thus the event might be lost.
Mapping between a topic (represented by T::Hash) and a vector of indexes of events in the
<Events<T>>list.All topic vectors have deterministic storage locations depending on the topic. This allows light-clients to leverage the changes trie storage tracking mechanism and in case of changes fetch the list of events of interest.
The value has the type
(BlockNumberFor<T>, EventIndex)because if we used only just theEventIndexthen in case if the topic has the same contents on the next block no notification will be triggered thus the event might be lost.