SelectByKey

Transforms each event’s payload using the stream’s key as well; lifetimes are unchanged.

Every Trill stream has a key type, the TKey in IStreamable<TKey, TPayload>, and each event carries a key of that type alongside its payload. SelectByKey passes that key to the selector: (key, payload) => result. On a stream from ToStreamable() the key type is Empty, so the key says nothing. Inside a grouping operator such as GroupApply, the key holds the group: there it is a CompoundGroupKey, and its InnerGroup is the group’s key.

The diagram uses GroupApply to make each event’s Key field the stream key that the selector receives. Each output names its group: a:1 becomes 1 in a.

Two related forms take the start time as well, (start, key, payload). SelectManyByKey is the same for SelectMany, with (key, payload) or (start, key, payload).

See also: GroupApply, Select, SelectMany, SelectManyByKey