Adds onAttach triggerMethod.

Adds `triggerAttach` and `beforeTriggerAttach` triggerMethods.

This is the 3rd out of 3 PRs to get #1886 in.
This variable name suggests that it should reference the "attached region" which is unclear. 

I think a better name would be `isRegionAttached` and i think that could be moved to a function (`this.isAttached()`).