Dispatching Events

There are a few steps you have to take in order to dispatch an event from your custom element:

Use the function syntax to define your component. This will give you access to this, the instance of your custom element.
When defining your callback that you wish to dispatch from, make sure it is bound to this. You can do this by using the fat arrow syntax.
Finally, you can create a new CustomEvent and pass that to this.dispatchEvent.

Here are a couple of examples of dispatching events from a pion custom element:

function Product({ name, price, productId }) {
  const buyProduct = () => {
    const event = new CustomEvent('buy-product', {
      bubbles: true, // this let's the event bubble up through the DOM
      composed: true, // this let's the event cross the Shadow DOM boundary
      detail: { name, price, productId } // all data you wish to pass must be in `detail`
    });
    this.dispatchEvent(event);
  }

  return html`
    <article>
      <h3>${name}</h3>
      <p>Price: ${price} USD</p>
      <button @click=${buyProduct}>Purchase</button>
    </article>
  `;
}

Product.observedAttributes = ['name', 'price', 'product-id'];

With this, you can now listen for the buy-product event either on an instance of <store-product> itself or higher up in the DOM. Here are examples of both of these instances:

Listening on an element

function Store() {
  const [{ name }, setPurchased] = useState({});

  return html`
    <store-product
      name="T-Shirt"
      price="10.00"
      product-id="0001"
      @buy-product=${event => setPurchased(event.detail)}
    ></store-product>

    <p ?hidden=${!name}><output>${name} Purchased</output></p>
  `;
}

Listening higher up in the DOM

import { component, html, useEffect, useState } from '@pionjs/pion';
import './my-store.js';

function App(element) {
  const [report, setReport] = useState('');
  useEffect(() => {
    // Because the event bubbled all the way up here,
    // we can listen directly on this web component itself!
    const onBuyProduct = ({ detail: { productId }}) =>
      setReport(`product id ${productId} ordered`);

    element.addEventListener('buy-product', onBuyProduct);

    // very important to remove the event listener!
    return () =>
      element.removeEventListener('buy-product', onBuyProduct);
  }, []); // make sure you list all dependencies

  return html`
    <my-store></my-store>
    <p><output>${report}</output></p>
  `;
}

customElements.define('my-app', component(App));

If you want to look more into firing events, here are some links: