Implemented in 3.6.1
Mechanism allowing users to define custom save hanlers for individual controls introduced.
Official Documentation Available
This topic is now covered in Dialogs reference > Properties.
Filed based saveHandler
This page contains notes and usage of field based savehandler.
Since Magnolia 3.6.1 there is a mechanism to allow users to define custom save hanlers for individual controls, before http://jira.magnolia-cms.com/browse/MAGNOLIA-2272, this functionality was available per dialog.
Since Magnolia 4.0 the interface used for this functionality was renamed from CustomSaveHandler to FieldSaveHandler http://jira.magnolia-cms.com/browse/MAGNOLIA-2403.
- In my opinion it is not completely right as FieldSaveHandler is also used for dialogs.
Create a control for your dialog and in the control definition add property "saveHandler" and set the value to your implementation of FieldSaveHandler.
As an example Data module is using the saveHandler for some on the dialogs like 'Contacts' and Categorization module is using a saveHandler for the field 'categories'.