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Invokers (Explainer)

Invoker Buttons

Pitch in Code

<button invoketarget="my-dialog">This opens a dialog</button>

<dialog id="my-dialog">This is the dialog</dialog>

Introduction

Adding invoketarget and invokeaction attributes to <button> and <input type="button"> / <input type="reset"> elements would allow authors to assign behaviour to buttons in a more accessible and declarative way, while reducing bugs and simplifying the amount of JavaScript pages are required to ship for interactivity. Buttons with invoketarget will - when clicked, touched, or enacted via keypress - dispatch an InvokeEvent on the element referenced by invoketarget, with some default behaviours.

Background

All elements within the DOM are capable of having interactions added to them. A long while ago this took the form of adding inline JavaScript to an event attribute, such as <button onclick="other.open()"></button>. Inline JavaScript has (rightly so) fallen out of favour due to the security and maintainability concerns. Newer pages may instead introduce more JavaScript to imperatively discover elements and call addEventListener('click', ...) to invoke the same behaviour. These patterns reduce developer experience and introduce more boilerplate and friction, while remediating security and maintainability concerns. Some frameworks attempt to reintroduce the developer experience of inline handlers by introducing new JavaScript or HTML shorthands, such as React’s onClick={...}, Vue’s @click=".." or HTMX’s hx-trigger="click".

There has also been a rise in the desire to customise controls for components. Many sites, for example, introduce custom controls for File Uploads or dropdown menus. These often require a large amount of work to reintroduce the built in functionality of those controls, and often unintentionally sacrifice accessibility in doing so.

With the new popover attribute, we saw a straightforward declarative way to tell the DOM that a button was interested in being a participant of the popover interaction. popovertarget would indicate to a browser that if that button was interacted with, it should open the element the popovertarget value pointed to. This allows for popovers to be created and interacted with - in an accessible and reliable way - without writing any additional JavaScript, which is a very welcome addition. While popovertarget sufficiently captured the use case for popovers, it fell short of providing the same developer & user experience for other interactive elements such as <dialog>, <details>, <video>, <input type="file">, and so on. This proposal attempts to redress the balance.

Terms

  • Invoke/Invoked/Invoking: The action of Invoking refers to a complete press action of a button, using a Human Input Device (HID). If the HID is a mouse, this would be a click event. If the HID is a touch screen, this would be a press using a stylus or finger. If the HID is a keyboard this would be the Space or Enter (Carriage Return) key on the keyboard. For other HIDs such as eye tracking or pedals or game controllers, the equivalent expected “click” action should be used to invoke the element.
  • Invoker: An invoker is a button element (that is a <button>, <input type="button">, or <input type="reset">) that has an invoketarget attribute set.
  • Invokee: An element which is referenced to by an Invoker, via the invoketarget attribute.

Proposed Plan

In the style of popovertarget, this document proposes we add invoketarget, and invokeaction as available attributes to <button>, <input type="button"> and <input type="reset"> elements.

interface mixin InvokerElement {
  [CEReactions] attribute Element? invokeTargetElement;
  [CEReactions] attribute DOMString invokeAction;
};

HTMLButtonElement extends InvokerElement
HTMLInputElement extends InvokerElement

The invoketarget value should be an IDREF pointing to an element within the document. .invokeTargetElement also exists on the element to imperatively assign a node to be the invoker target, allowing for cross-root invokers (in some cases, see the popovertarget attr-asociated element steps for more).

The invokeaction (and the .invokeAction reflected property) is a freeform hint to the Invokee. InvokeAction can be a “built-in” action or a “custom” action. If invokeaction is a falsey value ('', null, etc.) then it will default to 'auto'.

Custom values must contain a -. They will never trigger a browser default behaviour, aside from dispatching an InvokeEvent. This allows web developers to create custom Invoke actions for their components.

Built-in interactive elements have built-in behaviours (detailed below) which are determined by the invokeaction. The built-in names must not contain a -. An invokeaction without a dash that is not a built-in is considered invalid, and will not dispatch an InvokeEvent.

Valid invokeactions (that is: custom invokeactions or a valid built-in) will dispatch InvokeEvents, allowing custom code to take control of invocations (for example calling .preventDefault() or executing custom side-effects).

Elements with invoketarget set will dispatch an InvokeEvent on the Invokee (the element referenced by invoketarget) when the element is Invoked. The InvokeEvent’s type is always invoke. The event also contains an invoker property that will reference the Invoker element. InvokeEvents are always non-bubbling, composed, trusted, cancellable events.

[Exposed=Window]
interface InvokeEvent : Event {
  constructor(InvokeEventInit invokeEventInit);
  readonly attribute Element invoker;
  readonly attribute DOMString type = "invoke";
  readonly attribute DOMString action;
};
dictionary InvokeEventInit : EventInit {
  DOMString action = "auto";
  Element invoker;
};

If an element also has both a popovertarget and invoketarget attribute, then popovertarget must be ignored: invoketarget takes precedence.

If a <button> is a form participant, or has type=submit, then invoketarget must be ignored.

If an <input> is a form participant, or has a type other than reset or button, then invoketarget and interesttarget must be ignored.

Example Code

Popovers

When pointing to a popover, invoketarget acts much like popovertarget, allowing the toggling of popovers.

<button invoketarget="my-popover">Open Popover</button>
<!-- Effectively the same as popovertarget="my-popover" -->

<div id="my-popover" popover="auto">Hello world</div>

Dialogs

When pointing to a <dialog>, invoketarget can toggle a <dialog>’s openness.

<button invoketarget="my-dialog">Open Dialog</button>

<dialog id="my-dialog">
  Hello world!

  <button invoketarget="my-dialog" invokeaction="close">Close</button>
</dialog>

Details

Note: Invokers targeting a <details> element has been deferred from the initial release.

When pointing to a <details>, invoketarget can toggle a <details>’ openness.

<button invoketarget="my-details">Open Details</button>
<!-- Can be used to replicate the `<summary>` interaction -->

<details id="my-details">
  <summary>Summary...</summary>
  Hello world!
</details>

Customizing input type=file

Note: Invokers targeting a <input> element has been deferred from the initial release.

Pointing an invoketarget to an <input type="file"> acts the same as the rendered button within the input; and can be used to declare a customised alternative button to the input’s button.

<button invoketarget="my-file" invokeaction="showPicker">Pick a file...</button>

<input id="my-file" type="file" />

Customizing video/audio controls

Note: Invokers targeting <audio> and <video> elements has been deferred from the initial release.

The <video> and <audio> tags have many interactions, here invokeaction shines, allowing multiple buttons to handle different interactions with the video player.

<button invoketarget="my-video" invokeaction="playpause">Play/Pause</button>
<button invoketarget="my-video" invokeaction="toggleMuted">Mute/Unmute</button>

<video id="my-video"></video>

Custom behaviour

Invokers will dispatch events on the Invokee element. Using a dash in the invokeaction allows for custom JavaScript to be triggered without having to wire up manual event handlers to the Invokers.

<button invoketarget="my-custom">Invoke a div... to do something?</button>
<!-- Custom invokeactions must contain a dash! -->
<button invoketarget="my-custom" invokeaction="my-frobulate">Frobulate</button>

<div id="my-custom"></div>

<script>
  document.getElementById("my-custom").addEventListener("invoke", (e) => {
    if (e.action === "my-frobulate") {
      alert("Successfully frobulated the div");
    }
  });
</script>

Accessibility

Warning: This section is incomplete PRs welcome.

The Invoker implicitly receives aria-controls=IDREF or aria-details=IDREF (tbd) to expose the Invoker controls another element (the Invokee) for instances where the invokee is not a sibling to the invoker element.

If the Invokee has the popover attribute, the Invoker implicitly receives an aria-expanded state, as well as an aria-details association (for instances where the elements are not DOM siblings) which will match the state of the popover’s openness. It will be aria-expanded=true when the popover is :popover-open and aria-expanded=false otherwise.

If the Invokee is a <details> element the Invoker implicitly receives an aria-expanded state which will match the state of the Invokee’s openness. It will be aria-expanded=true when the Invokee is open and aria-expanded=false otherwise.

If the Invokee is a <dialog> element the Invoker implicitly receives an aria-expanded state which will match the state of the Invokee’s openness. It will be aria-expanded=true when the Invokee is open and aria-expanded=false otherwise.

Defaults

Depending on the target set by invoketarget, invoking the button will trigger additional behaviours alongside the event dispatch. In addition the following table represents how built-in invocations on specific element types are handled. Note that this list is ordered and higher rules take precedence:

Invokee Elementaction hintBehaviour
<* popover>'auto'Call .togglePopover() on the invokee
<* popover>'togglePopover'Call .togglePopover() on the invokee
<* popover>'hidePopover'Call .hidePopover() on the invokee
<* popover>'showPopover'Call .showPopover() on the invokee
<dialog>'auto'If the <dialog> is not open, call showModal(), otherwise close the dialog
<dialog>'showModal'If the <dialog> is not open, call showModal()
<dialog>'close'If the <dialog> is open, close and use the button value for returnValue

Futher behaviours have been designed and may ship after the initial release of Invokers, the names and exact semantics may be subject to change:

Invokee Elementaction hintBehaviour
<details>'auto'If the <details> is open, then close it, otherwise open it
<details>'toggle'If the <details> is open, then close it, otherwise open it
<details>'open'If the <details> is not open, then open it
<details>'close'If the <details> is open, then close it
<dialog>'toggle'If the <dialog> is open, cancel the dialog
<dialog>'cancel'If the <dialog> is open, cancel the dialog
<select>'showPicker'Call .showPicker() on the invokee
<input>'showPicker'Call .showPicker() on the invokee
<video>'playpause'Toggle the .playing value
<video>'pause'If .playing is true, set it to false
<video>'play'If .playing is false, set it to true
<video>'toggleMuted'Toggle the .muted value
<audio>'playpause'Toggle the .playing value
<audio>'pause'If .playing is true, set it to false
<audio>'play'If .playing is false, set it to true
<audio>'toggleMuted'Toggle the .muted value
<*>'toggleFullscreen'If the element is fullscreen, then exit, otherwise request to enter
<*>'requestFullscreen'Request the element to enter into ‘fullscreen’ mode
<*>'exitFullscreen'Request the element to exit ‘fullscreen’ mode

Note: The above tables are an attempt at wide coverage, but ideas are welcome. Please submit a PR if you have one!

Invokers and Custom Elements

As the underlying system for invoke/interest elements uses event dispatch, Custom Elements can make use of InvokeEvent/InterestEvents for their own behaviours. Consider the following:

<button invoketarget="my-element" invokeaction="spin-widget">
  Spin the widget
</button>

<spin-widget id="my-element"></spin-widget>
<script>
  customElements.define(
    "spin-widget",
    class extends HTMLElement {
      connectedCallback() {
        this.addEventListener("invoke", (e) => {
          if (e.action === "spin-widget") {
            this.spin();
          }
        });
      }
    },
  );
</script>

PAQ (Potentially Asked Questions)

Why the name invoker? Why not click?

While click is a fairly well established name in the world of the web, it is quite specific to certain types of HID and is not a term which encompasses all viable methods of interaction. In addition a clickaction attribute is deemed to be a little too ambiguous as it conflates existing concepts. Given the opportunity to supply a new name, invoke was settled on.

What about adding Invoker defaults for <form>?

Defaults for <form> are intentionally omitted as this proposal does not aim to replace Reset or Submit buttons. If you want to control forms, use those.

What about adding Invoker defaults for <a>?

Defaults for <a> are intentionally omitted as this proposal does not aim to replace anchors. If you intend to produce a page navigation, use an <a> tag.

Why is invoketarget limited to buttons?

This is by design, to allow for a “pit of success”; invoking actions on non-button elements such as <div>s or <a>s creates many problems, especially for non-interactive elements. While <a>s are interactive, they should only be used for page navigation and not for invoking other behaviours, and so invoketarget should not be allowed.

Why isn’t input[type=submit] included?

This is not added by design. Submit inputs already have a default action: submitting forms. If you want a button to control the submission of a form, use input[type=submit], if you want a button to control invocation of something other than a form then you should use input[type=button].

Why is input[type=reset] included?

It may stand to reason that if input[type=submit] is excluded then so should input[type=reset], however, there are valid use cases to resetting a form at the same time as some other action, for example closing the dialog that contains a form:

<dialog id="my-dialog">
  <form>
    <input type="text" />
    <!-- This button closes the dialog _and_ resets the form -->
    <input type="reset" invoketarget="my-dialog" value="Cancel" />
  </form>
</dialog>

What does this mean for popovertarget?

Whilst invoketarget does replicate popovertarget’s functionality, the two will co-exist side by side for some while to enable a smooth transition.

It is, however, intended that invoketarget will replace popovertarget leading to popovertarget’s eventual deprecation.

InvokeTarget seems limited, what if I wanted to add arguments?

Custom invokeactions can be used in a freeform way, for your own elements. If you feel it necessary you can invent your own DSLs for passing extra data using this hint. For example:

<button invoketarget="my-counter" invokeaction="add-1">Add 1</button>
<button invoketarget="my-counter" invokeaction="add-2">Add 2</button>
<button invoketarget="my-counter" invokeaction="add-10">Add 10</button>

<input readonly id="my-counter" value="0" />

<script>
  const counter = document.getElementById("my-counter");
  counter.addEventListener("invoke", (e) => {
    let addMatch = /^add-(\d+)$/.match(e.action);
    if (addMatch) {
      counter.value = Number(counter.value) + Number(addMatch[1]);
    }
  });
</script>