Add support for JSONSchema constants

[johnmorrell]

Is your feature request related to a problem? Please describe.
Where a JSONSchema contains a constant value this does not get reflected in the output model. If this was a required property the output JSON would never validate against the Schema.

Describe the solution you'd like
Using JSONSchema constant:

{
   "type":"object",
   "properties":{
      "version":{
         "const":"1.0"
      }
   },
   "required":[
      "version"
   ]
}

Constants to be added to the model, via either a hidden or readonly form element.

Describe alternatives you've considered
Currently forcing constants by using an required Enum with a single and default value:

{
   "type":"object",
   "properties":{
      "version":{
         "type":"string",
         "enum":[
            "1.0"
         ],
         "default":"1.0"
      }
   },
   "required":[
      "version"
   ]
}

Describe for which setup you like to have the improvement
Framework: React
RendererSet: Material

Additional context
JSONForms: v2.2.0

[edgarmueller]

Thanks for the report, we'll try to have a look soon.

[johnmorrell]

Just for reference, I have seen good results with populating oneOf data in v2.2.3 since #1273. I did hit a couple of cases where the intended option was not selected, due to partially complete data or where the oneOf structures are similar with optional fields. I think these cases could be rectified with support for constants. Each of the oneOf types could be given a required const identifier that would have to be valid preventing early validation passes.

[edgarmueller]

Alright, thanks for letting us know. I hope we'll be able to tackle this for 2.3.0.

[edgarmueller]

#1393 added support for const. I'm closing this therefore now. Any additional features related to const should be opened in a new issue. Thank you!