Expressions: Support options from expression result

Currently

Options/code lists currently have a few variants we support:

A simple/static list of options
An optionsId that indicates a remote list of options that needs to be fetched from the server (optionally with mapping)
A complex setup with source that maps to a repeating group in the data model (which, weirdly, has to be configured as a component somewhere to work)

I think we could replace the last (and possibly also the first) using expressions instead. That is, we can already look up keys in the data model using expressions, and after #1179 there should also be support for lists and objects.

The current configuration for source means you have to indicate where the group/array is in the data model, which text resource (?!) should indicate the label (as text resources can look up from repeating groups?) and which field in the data model the value should be derived from.

Note that:

You cannot mix static options and options from repeating groups (without prefilling and hiding the static options from the group)
As mentioned, a Group component needs to be properly configured for this to work (that is not a requirement on the backend).
Description and help text is not supported
Unintuitive trickery is used to make the label translatable

Also, for the very simple case of a static list of options, you always have to provide both a value and a label.

In the future

We should only support two ways of configuring options, either via an expression or via an optionsId as before. For the absolutely simplest cases (i.e. yes/no) it should be sufficient to provide a basic list (using an expression):

["list", "yes", "no"]

The values we save in the data model will then be yes or no, but the labels are displayed via text resources (so if you have text resources for yes and no we might instead show Ja and Nei as labels).

If you want to provide your own labels, you can expand the expression:

["list",
  {"label": ["text", "simple.yes"], "value": "yes"},
  {"label": ["text", "simple.no"], "value": "no"}
]

And if you want to look up from the data model, from a repeating group structure, it can be done similarly:

["map",
  {
    "label": ["pluck", "label", ["argv", 0]],
    "value": ["pluck", "value", ["argv", 0]],
  },
  ["dataModel", "path.to.repeating.group"]
]

This would assume the suggested map function in #1179 is implemented, and supports mapping directly to an object propotype in addition to functions/closures.

To make this example as clear as possible, say we have this repeating group in dot-notation:

path.to.repeating.group[0].label = "Foo"
path.to.repeating.group[0].value = 1234
path.to.repeating.group[1].label = "Bar"
path.to.repeating.group[1].value = 4321

The resulting options would map to:

[
  {"label": "Foo", "value": 1234},
  {"label": "Bar", "value": 4321},
]

Combining `optionsId` with expressions

We should also support fetching a remote list of options, and using expressions to filter/change it before presenting it as options. This could be done by adding a function to fetch options based on an id, and return the options as a list of label/value objects which could then be filtered.

Related issue(s)

1179
936

Altinn / app-frontend-react