[TYPO3-mvc] How to calculate a repository value from other inputs?

[Sy Moen]

Is there a standard way to calculate inputs in a BE FCE... or possibly after
the FCE is submitted and before the Repository object is persisted?

For instance, extracting geo data from an image if the image has geolocation
data in the EXIF. If it does not, presenting a map to click.

Or, pre-filing a form field with a category if the FCE is for an element
being created in a Sysfolder or Page that is dedicated to that category.

Like, pre / post manipulation methods for a standard FCE?

I guess this is a general TYPO3 question, or maybe an ExtJS question... but
if there is an extbase / FLOW3 specific answer, that is what I am looking
for.