I am currently trying to exend Herald with new Actions. I was able to put the actions up for selection, but they are not listed in transactions.
An example / how-to / documentation on how to extend Herald (programmatically) would be much appreciated.
see @epriestley's exceedingly helpful reply to T5571 for a starting point. I will try to expand on that and make it into something a bit more general for documentation purposes.
This has been in the update notes for a while, but just for completeness: T8726 is changing this substantially. After those change land, you must extend HeraldAction instead of HeraldCustomAction. The API is similar, but doesn't directly map to the old class.
All upstream actions have been converted to be modular subclasses of HeraldAction, so you can look at them for examples. The actual meat of your actions is likely unchanged (or mostly unchanged -- logging/UI is a little more fleshed out now), but how you get data about them is much different.
Since the upstream is now modularized, I'm not specifically going to pursue any documentation here until after we formally support third-party development (see T5447): this will now be on par with everything else and have some level of "documentation" in the form of a bunch of examples, at least.