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Version: v4.17

Building Form-Associated Components in Stencil

As of v4.5.0, Stencil has support for form-associated custom elements. This allows Stencil components to participate in a rich way in HTML forms, integrating with native browser features for validation and accessibility while maintaining encapsulation and control over their styling and presentation.

caution

Browser support for the APIs that this feature depends on is still not universal1 and the Stencil team has no plans at present to support or incorporate any polyfills for the browser functionality. Before you ship form-associated Stencil components make sure that the browsers you need to support have shipped the necessary APIs.

Creating a Form-Associated Component​

A form-associated Stencil component is one which sets the new formAssociated option in the argument to the @Component decorator to true, like so:

import { Component } from '@stencil/core';

@Component({
  tag: 'my-face',
  formAssociated: true,
})
export class MyFACE {
}

This element will now be marked as a form-associated custom element via the formAssociated static property, but by itself this is not terribly useful.

In order to meaningfully interact with a <form> element that is an ancestor of our custom element we'll need to get access to an ElementInternals object corresponding to our element instance. Stencil provides a decorator, @AttachInternals, which does just this, allowing you to decorate a property on your component and bind an ElementInternals object to that property which you can then use to interact with the surrounding form.

info

Under the hood the AttachInternals decorator makes use of the very similarly named attachInternals method on HTMLElement to associate your Stencil component with an ancestor <form> element. During compilation, Stencil will generate code that calls this method at an appropriate point in the component lifecycle for both lazy and custom elements builds.

A Stencil component using this API to implement a custom text input could look like this:

src/components/custom-text-input.tsx
import { Component, h, AttachInternals, State } from '@stencil/core';

@Component({
  tag: 'custom-text-input',
  shadow: true,
  formAssociated: true
})
export class CustomTextInput {
  @State() value: string;

  @AttachInternals() internals: ElementInternals;

  handleChange(event) {
    this.value = event.target.value;
    this.internals.setFormValue(event.target.value);
  }

  componentWillLoad() {
    this.internals.setFormValue("a default value");
  }

  render() {
    return (
      <input
        type="text"
        value={this.value}
        onInput={(event) => this.handleChange(event)}
      />
    )
  }
}

If this component is rendered within a <form> element like so:

<form>
  <custom-text-input name="my-custom-input"></custom-text-input>
</form>

then it will automatically be linked up to the surrounding form. The ElementInternals object found at this.internals will have a bunch of methods on it for interacting with that form and getting key information out of it.

In our <custom-text-input> example above we use the setFormValue method to set a value in the surrounding form. This will read the name attribute off of the element and use it when setting the value, so the value typed by a user into the input will added to the form under the "my-custom-input" name.

This example just scratches the surface, and a great deal more is possible with the ElementInternals API, including setting the element's validity, reading the validity state of the form, reading other form values, and more.

Lifecycle Callbacks​

Stencil allows developers building form-associated custom elements to define a standard series of lifecycle callbacks which enable their components to react dynamically to events in their lifecycle. These could allow fetching data when a form loads, changing styles when a form's disabled state is toggled, resetting form data cleanly, and more.

formAssociatedCallback​

This callback is called when the browser both associates the element with and disassociates the element from a form element. The function is called with the form element as an argument. This could be used to set an ariaLabel when the form is ready to use, like so:

src/components/form-associated-cb.tsx
import { Component, h, AttachInternals } from '@stencil/core';

@Component({
  tag: 'form-associated',
  formAssociated: true,
})
export class FormAssociatedCmp {
  @AttachInternals()
  internals: ElementInternals;

  formAssociatedCallback(form) {
    form.ariaLabel = 'formAssociated called';
  }

  render() {
    return <input type="text" />;
  }
}

formDisabledCallback​

This is called whenever the disabled state on the element changes. This could be used to keep a CSS class in sync with the disabled state, like so:

src/components/form-disabled-cb.tsx
import { Component, h, State } from '@stencil/core';

@Component({
  tag: 'form-disabled-cb',
  formAssociated: true,
})
export class MyComponent {
  @State() cssClass: string = "";

  formDisabledCallback(disabled: boolean) {
    if (disabled) {
      this.cssClass = "background-mode";
    } else {
      this.cssClass = "";
    }
  }

  render() {
    return <input type="text" class={this.cssClass}></input>
  }
}

formResetCallback​

This is called when the form is reset, and should be used to reset the form-associated component's internal state and validation. For example, you could do something like the following:

src/components/form-reset-cb.tsx
import { Component, h, AttachInternals } from '@stencil/core';

@Component({
  tag: 'form-reset-cb',
  formAssociated: true,
})
export class MyComponent {
  @AttachInternals()
  internals: ElementInternals;

  formResetCallback() {
    this.internals.setValidity({});
    this.internals.setFormValue("");
  }

  render() {
    return <input type="text"></input>
  }
}

formStateRestoreCallback​

This method will be called in the event that the browser automatically fills out your form element, an event that could take place in two different scenarios. The first is that the browser can restore the state of an element after navigating or restarting, and the second is that an input was made using a form auto-filling feature.

In either case, in order to correctly reset itself your form-associated component will need the previously selected value, but other state may also be necessary. For instance, the form value to be submitted for a date picker component would be a specific date, but in order to correctly restore the component's visual state it might also be necessary to know whether the picker should display a week or month view.

The setFormValue method on ElementInternals provides some support for this use-case, allowing you to submit both a value and a state, where the state is not added to the form data sent to the server but could be used for storing some client-specific state. For instance, a pseudocode sketch of a date picker component that correctly restores whether the 'week' or 'month' view is active could look like:

src/components/fa-date-picker.tsx
import { Component, h, State, AttachInternals } from '@stencil/core';

@Component({
  tag: 'fa-date-picker',
  formAssociated: true,
})
export class MyDatePicker {
  @State() value: string = "";
  @State() view: "weeks" | "months" = "weeks";

  @AttachInternals()
  internals: ElementInternals;

  onInputChange(e) {
    e.preventDefault();
    const date = e.target.value;
    this.setValue(date);
  }

  setValue(date: string) {
    // second 'state' parameter is used to store both
    // the input value (`date`) _and_ the current view
    this.internals.setFormValue(date, `${date}#${this.view}`);
  }

  formStateRestoreCallback(state, _mode) {
    const [date, view] = state.split("#");
    this.view = view;
    this.setValue(date);
  }

  render() {
    return <div>
      Mock Date Picker, mode: {this.view}
      <input class="date-picker" onChange={e => this.onInputChange(e)}></input>
    </div>
  }
}

Note that the formStateRestoreCallback also receives a second argument, mode, which can be either "restore" or "autocomplete", indicating the reason for the form restoration.

For more on form restoration, including a complete example, check out this great blog post on the subject.

Resources​

Footnotes​

  1. See https://caniuse.com/?search=attachInternals for up-to-date adoption estimates. ↩

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