example with terrible caching

components.my-component\.js

import Component from '@glimmer/component';
import { tracked } from '@glimmer/tracking';

function getAndClearFieldNamed(name, elements) {
  const field = elements.namedItem(name);
  if (!field) {
    throw 'wat';
  }

  const value = field.value;
  field.value = "";
  return value;
}

export default class MyComponent extends Component {
  _prev;
  @tracked _draft;
  
  get draft() {
    if (this.args.foo !== this._prev) {
      this._draft = this.args.foo;
      this._prev = this.args.foo;
    }

    return this._draft;
  }
  
  add = (event) => {
    event.preventDefault();
    const { elements } = event.target;
    const key = getAndClearFieldNamed('key', elements);
    const value = getAndClearFieldNamed('value', elements);

    this._draft = {...this._draft, [key]: value };
  };

  remove = (key) => {
    delete this._draft[key];
    this._draft = this._draft;
  };
}

controllers.application\.js

import Controller from '@ember/controller';
import { tracked } from '@glimmer/tracking';

export default class ApplicationController extends Controller {
  @tracked values = { a: 1 };

  save = (newValues) => {
    this.values = newValues;
  };
}

templates.application\.hbs

<MyComponent
  @foo={{this.values}}
  @onSave={{this.save}}
/>

templates.components.my-component\.hbs

<p>Original:</p>
<ul>
{{#each-in @foo as |key value|}}
  <li>{{key}}: {{value}}</li>
{{/each-in}}
</ul>

<p>Current:</p>
<ul>
{{#each-in this.draft as |key value|}}
  <li>{{key}}: {{value}}<button {{on "click" (fn this.remove key)}}>X</button></li>
{{/each-in}}
</ul>

<form {{on "submit" this.add}}>
	<label>key<input name='key' type='text'></label>
  <label>value<input name='value' type='text'></label>
  <button type='submit'>add</button>
</form>

<div>
<button {{on "click" (fn @onSave this._draft)}}>save them ALL</button>
</div>

twiddle\.json

{
  "version": "0.17.1",
  "EmberENV": {
    "FEATURES": {},
    "_TEMPLATE_ONLY_GLIMMER_COMPONENTS": false,
    "_APPLICATION_TEMPLATE_WRAPPER": true,
    "_JQUERY_INTEGRATION": true
  },
  "options": {
    "use_pods": false,
    "enable-testing": false
  },
  "dependencies": {
    "jquery": "https://cdnjs.cloudflare.com/ajax/libs/jquery/3.5.1/jquery.js",
    "ember": "3.18.1",
    "ember-template-compiler": "3.18.1",
    "ember-testing": "3.18.1"
  },
  "addons": {
    "@glimmer/component": "1.0.0"
  }
}