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How To: Creating a custom field

Custom fields can be created in just a few steps. All you need is an interaction element (like for instance a slider, a listbox or a combobox) and connect it to the input components.

Prerequisite: an interaction element

An interaction element provides the means for the end user to enter a certain value, just like native elements provide in this (think of <input>, <textarea> and <select>). An example of a non native element is the slider design pattern described here.

For this tutorial, we assume we have a component <my-slider> that exposes its value via property mySliderValue and sends an event my-slider-changed on every value change. To make it focusable, it has a tabindex=“0” applied.

Connecting the interaction element to the field

Now we want to integrate the slider in our form framework to enrich the user interface, get validation support and get all the other benefits of LionField. We start by creating a component <lion-slider> that extends from LionField. Then we follow the steps below:

1. Add your interaction element as ‘input slot'

Here you return the element the user interacts with. By configuring it as a slot, it will end up in light DOM, ensuring the best accessibility for the end user.

2. Proxy event my-slider-changed to user-input-changed event

The user-input-changed event is listened to by the FormatMixin: it should be regarded as the equivalent of the input event of the platform, but for custom built interaction elements.

3. Proxy property <my-slider>.mySliderValue to <lion-slider>.value

Every time the user-input-changed fires, the value of <my-slider> is synchronized with the modelValue of <my-slider>. Now the cycle is complete: the modelValue connects your interaction element to all logic inside the LionField.

Steps as described can be implemented with the following javascript:

import { LionField } from '@lion/form-core';
import './my-slider.js';

export class LionSlider extends LionField {
  // 1. Add your interaction element as ‘input slot'
  get slots() {
    return {
      input: () => document.createElement(‘my-slider’),

  // 2. Proxy event `my-slider-changed` to `user-input-changed` event
  connectedCallback() {
    this.addEventListener('my-slider-changed',  this._proxyChangeEvent);

  _proxyChangeEvent() {
      new CustomEvent('user-input-changed', { bubbles: true, composed: true }),

  // 3. Proxy property `<my-slider>.mySliderValue` to `<lion-slider>.value`
  get value() {
    return Array.from(this.children).find(child => child.slot === 'input').mySliderValue;

  set value(newV) {
    Array.from(this.children).find(child => child.slot === 'input').mySliderValue = newV;

That was all. Now you can enhance your slider by writing custom validators for it or by writing a parser to get a custom modelValue type.