Define Gtk closure expressions in code when the UI file is NOT a template.

Suppose we have a .blp-file that defines a template. Then we can use Gtk closure expression defined in the (say javascript) code like this for example:

UI file main.blp

using Gtk 4.0;
using Adw 1;

template $CustomWidget : Adw.PreferencesGroup {
  Adw.ComboRow cb {
    title: _("Make a selection:");
    
    model:
    StringList {
      strings ["One", "Two", "Three", "Four"]
    };
  }
  
  Adw.SwitchRow sw1 {  // should only be visible when option "One", "Two" or "Three" is selected.
    title: _("Switch one");
    active: true;
    visible: bind $onlyOneTwoThree(cb.selected-item) as <bool>;
  }
  
  Adw.SwitchRow sw2 { // should only be visible when option "Three" or "Four" is selected
    title: _("Switch two");
    visible: bind $onlyThreeFour(cb.selected-item) as <bool>;
  }
  
  Adw.ActionRow {
    title: _("Status:");
    Label { // should display some status message depending on sw1.visible, sw2.visible and cb.selected-item
      label: bind $msg(sw1.visible, sw2.visible, cb.selected-item) as <string>;
    }
  }
 }

Code file main.js

import GObject from "gi://GObject";
import Gtk from "gi://Gtk";
import Adw from "gi://Adw";

const CustomWidget = GObject.registerClass(
  {
    GTypeName: "CustomWidget",
    Template: workbench.template,
  },
  class CustomWidget extends Adw.PreferencesGroup {
    onlyOneTwoThree(_self, item) {
      const str = item.string;
      return str === "One" || str === "Two" || str === "Three";
    }
    onlyThreeFour(_self, item) {
      const str = item.string;
      return str === "Three" || str === "Four";
    }
    msg(_self, active1, active2, item) {
      const str = item.string;
      if (active1 && active2) {
        return `"${str}" was selected, 2 visible`;
      } else if (active1 || active2) {
        return `"${str}" was selected, 1 visible`;
      } else {
        return "0 active. Discard selection";
      }
    }
  },
);

const box = new Gtk.Box({ orientation: "vertical" });
box.append(new CustomWidget());
workbench.preview(box);

Now suppose I want to have the same behavior in case that the UI file does NOT define a template (remove

template $CustomWidget : 

from the fourth line of main.blp)

How can we define the Gtk closure expressions onlyOneTwoThree, onlyThreeFour and msg in the code? There doesn't seem to be an easy way to do so. According to @andyholmes (conversation in the matrix channel) these closures should be added to Workbench's builder scope in some way (compare gjs guide), yet apparently Workbench is not set up to use closures at the moment.

So I'd like to request Workbench to support closures in a way that it is easy to use and documented (maybe in a Library demo).

Start with the main.blp and main.js examples, then read the linked GJS guide section about template-file closures and inspect how Workbench currently creates its builder scope. Done means non-template UI files can use the three demonstrated closures, with an easy usage path documented in a Library demo.