Data-driven triggers #17853
Description
For some applications, it's useful to declare a pane/window to be emitted (or finished) based on its contents. The simplest of these is the AfterCount trigger, but more sophisticated predicates could be constructed.
The requirements for consistent trigger firing are essentially that the state of the trigger form a lattice and that the "should fire?" question is a monotonic predicate on the lattice. Basically it asks "are we high enough up the lattice?"
Because the element types may change between the application of Windowing and the actuation of the trigger, one idea is to extract the relevant data from the element at Windowing and pass it along implicitly where it can be combined and inspected in a type safe way later (similar to how timestamps and windows are implicitly passed with elements).
Imported from Jira BEAM-101. Original Jira may contain additional context.
Reported by: robertwb.