Hi @Nywi6100,
There are two basic approaches which can be used whenever you want to update a field depending on another field.
- Register a custom control for the inputs which correspond to the values, in your case for the multi-selection. Then whenever the value of the multi select changes, you also update the sum property
- Listen to the data changes of the multi select outside of JSON Forms and modify the data to include the correct sum before handing the data back to JSON Forms
In your case the sum value seems to be read only. So in case the sum value is only needed for the UI but does not need to be actually contained in the data, you could go with with an additional renderer which just listens to the multi-select values and then calculates the sum.
See here for a previous discussion on this topic for a similar use case.