Feature Request: New Preview Mode which requires user to code a way to update preview and new global variable '_preview_manual'

[Reptorian1125]

There are filters in which this would benefit such as Color Harmonies made by me, and the upcoming filter Color Wheel by cli345 in GIMPchat - http://gimpchat.com/viewtopic.php?f=28&t=20728&p=287157#p287157 . There's Color Harmonies preview there. The reason being is that you don't want to generate the preview picture over, and over again. So, I would like a way to code in a way to manually update preview render. _preview_manual defines whether to update preview render.

[dtschump]

You can already use the $_persistent variable to manage that. For instance, if you don't want to recompute a preview image for a given set of parameters, then once it's computed, store it in $_persistent with a name that corresponds to the list of "important" parameters.
Then, on next call to the preview command, check if the "important" parameters have changed, and if not, use the preview image stored previously instead of recomputing it.