fix(widgetsFor): return widgets for variable type lists

[domcleal]

Summary

When using a Variable Type list widget and a custom preview component, the widgetsFor helper would only return a data list with each of the items in the list, not a widgets list, e.g.

{"data" =>
    {"markdown" => "# Title"}
    {"type" => "block_body"}
}
{"widgets" => undefined} 🚫

The widgets list should also be supplied, particularly for nested Markdown widgets, so a fully formatted preview can be rendered:

{"data" =>
    {"markdown" => "# Title"}
    {"type" => "block_body"}
}
{"widgets" =>
    {"markdown" => Object} ✅
}

This extends support in widgetsFor to detect variable type list widgets and correctly construct the widgets return value.

Fixes #2307 (specifically #2307 (comment) which explains it neatly).

Test plan

Using the test backend, add the following custom preview helper:

var KitchenSinkPreview = createClass({
  render: function() {
    return h('div', {},
      h('h1', {}, 'Typed list'),
      this.props.widgetsFor('typed_list').map(function(item, index) {
        return h('div', {key: index},
          h('hr', {}),
          h('strong', {}, item.getIn(['data', 'string']) || "Untitled"),

          h('dl', {},
            h('dt', {}, 'Data'),
            h('dd', {}, item.getIn(['data', 'markdown'])),

            h('dt', {}, 'Widgets'),
            h('dd', {}, item.getIn(['widgets', 'markdown'])),
          ),
        );
      })
    );
  }
});

CMS.registerPreviewTemplate("kitchenSink", KitchenSinkPreview);   

Before:

Only the data value is populated, so only the raw Markdown is available. The widgets return value is empty.

After:

In the preview pane, the Markdown widget is now able to be used to render the field.

Checklist

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[demshy]

This is great, thanks!

If maybe someone could figure out if this could work with what @fgnass did here #7374 it would be even better

[domcleal]

Thanks for reviewing!

If maybe someone could figure out if this could work with what @fgnass did here #7374 it would be even better

I can't really think of any way that they'd interact. This only fixes the existing helper for custom previews, but visual editing and the default preview are separate concerns.

It might be that the visual editing could be extended to work in custom previews (if it doesn't already, I'm unclear) or that default previews should extend to variable type lists, but I believe that's separate to this change to widgetsFor.

[demshy]

This only fixes the existing helper for custom previews, but visual editing and the default preview are separate concerns.

Yep, I think this PR is a prerequisite for us to even start thinking about extending visual editing triggers to the variable types. Since that feature will start as Beta anyway I don't have a problem releasing both.