react-dnd-accessible-backend

react-dnd-accessible-backend

An add-on backend for react-dnd that provides support for keyboards and screenreaders by default. Keep writing the same drag and drop code while enabling more users to interact with your app.

Why

react-dnd (and the system of packages it manages) does not directly support drag and drop other than using a mouse (or a finger on mobile devices). It encapsulates the HTML5 Drag and Drop API, which could support other input devices, but most browser implementations today only support pointers. react-dnd also does not provide any way of notifying screenreaders that drag and drop operations are happening. Again, the HTML5 API makes this possible, but it is not supported in any meaningful way.

Other drag and drop implementations out there do support these features, such as react-beautiful-dnd and dndkit, and they do it well. However, react-dnd remains by far the most popular drag and drop library for React applications and is likely stay in that position for a while as migrating between them is not easy, and the limitations of other systems keep many complex applications from moving at all.

This package brings support for both alternative input devices like keyboards (or anything that can trigger keyboard events in the browser) as well as announcements for screenreaders to react-dnd natively, without changing any of the public API that developers are used to or limiting of the structural flexibility it is known for.

Installation

This package is available on npm as react-dnd-accessible-backend.

npm install react-dnd-accessible-backend

Basic Usage

react-dnd-accessible-backend is not a replacement backend for react-dnd, but rather an additional one. This means you will most likely need to compose backends together to get all of the functionality you would like (mouse dragging, keyboards, pointer dragging on mobile, etc).

One of the easiest ways to do this is with react-dnd-multi-backend and it's Transition system. Using that library, just add another backend entry and create a Transition for the keyboard trigger, like so:

import KeyboardBackend, { isKeyboardDragTrigger } from "react-dnd-accessible-backend";
import { DndProvider } from "react-dnd";
import { HTML5Backend } from "react-dnd-html5-backend";
import { MultiBackend, createTransition } from "react-dnd-multi-backend";

const KeyboardTransition = createTransition("keydown", (event) => {
  if (!isKeyboardDragTrigger(event as KeyboardEvent)) return false;
  event.preventDefault();
  return true;
});

const DND_OPTIONS = {
  backends: [
    {
      id: "html5",
      backend: HTML5Backend,
      transition: MouseTransition,
    },
    {
      id: "keyboard",
      backend: KeyboardBackend,
      context: { window, document },
      options: { preview: true },
      transition: KeyboardTransition,
    },
  ],
};

function App() {
  return (
    <DndProvider backend={MultiBackend} options={DND_OPTIONS}>
      ...
    </DndProvider>
  );
}

That's all it takes to get started! There are a few considerations you'll want to keep in mind to ensure a really good experience for your users, but everything else should be automatic.

At the moment, the keybinds used for drag and drop are hard-coded as:

• ctrl+d (command+d on macOS) to pick up a draggable item
• up and down arrow keys to move between drop targets
• Enter or Spacebar to drop the dragged item on a drop target
• Escape while dragging to cancel the drag operation

Options

react-dnd-accessible-backend provides a few options for customizing styles and behavior for use in your app. If you're using react-dnd-multi-backend, these can get passed in as an options field on the backend configuration object, or otherwise as the third argment when calling the backend directly as a factory function (like KeyboardBackend(manager, context, options).

These options are:

getAnnouncementMessages?: () => AnnouncementMessages

This function is called any time a drag and drop action is performed by the keyboard backend and is useful for providing translations or more descriptive messages for screenreader users as they interact with draggable items.

If this option is not provided, a default set of messages in English will be used. Providing a separate function requires that you specify a replacement for all messages that can be announced. These (currently) are pickedUpItem, droppedItem, hoveredTarget and canceledDrag.

Each message getter is defined as a function that takes in an itemId and the HTML node that is relevant to the operation.

// A very naive example of how to provide custom announcement messages.
function getCustomAnnouncementMessages() {
  return {
    pickedUpItem: (itemId: string, node: HTMLElement | null) => `Picked up ${itemId}`,
    droppedItem: (itemId: string, node: HTMLElement | null) => `Dropped ${itemId}`,
    hoveredTarget: (itemId: string, node: HTMLElement | null) => `Hoevered over ${itemId}`,
    canceledDrag: (itemId: string, node: HTMLElement | null) => "Drag cancelled"
  };
}

{
  options: {
    getAnnouncementMessages: getCustomAnnouncementMessages,
  },
}

isDragTrigger?: (event: KeyboardEvent, isFirstEvent: boolean) => boolean

This function is used to determine if a keyboard event that occurs on a draggable element should trigger the start of a drag operation. Overriding this option lets you customize the keybind used to start dragging or perform other checks before the drag is allowed to start.

If this option is not provided, it will default to using the isKeyboardDragTrigger that is exported as part of this package, which triggers when ctrl/command+d is pressed.

Ths isFirstEvent parameter indicates whether this is the first event the backend is receiving after being setup.

{
  options: {
    // This will start a drag whenever the users presses
    // `m` while focused on a draggable element.
    isDragTrigger: (event) => event.key === "m"
  },
}

NOTE: In most cases when react-dnd-multi-backend, you'll want to use the same trigger function in this option for the trigger in createTransition. Otherwise the backend may not be set up when you expect to start a drag. This is also where the isFirstEvent property can come in handy, since react-dnd-multi-backend will sometimes fire cloned events that don't have keyboard properties on them. See the comment in isKeyboardDragTrigger for more information.

announcer

By default, react-dnd-accessible-backend will append an aria-live area to the DOM to provide screen-reader announcements, powered by @react-aria/live-announcer.

If you already have a comparable announcer utility in your app, then you can use the announcer option to replace DragAnnouncer's announcer with your own. Pease note that multiple aria-live areas in an app or web page could conflict, which is why we have provided this override option to re-use any existing aria-live announcers.

interface Announcer {
  announce(message: string, assertiveness?: 'assertive' | 'polite', timeout?: number): void;
  clearAnnouncements(assertiveness?: 'assertive' | 'polite'): void;
}

const LiveAnnouncer = require('@react-aria/live-announcer');
const AccessibilityAnnouncer: Announcer = {
  announce: LiveAnnouncer.announce,
  clearAnnouncements: LiveAnnouncer.clearAnnouncer,
};

{
  options: {
    announcer: AccessibilityAnnouncer,
  },
}

previewerClassName

Similar to the announcerClassName this option provides a custom class name to use for the drag previewer, which is a container that gets populated by a clone of the currently-dragged element and positions itself in the appropriate place on screen for the currently-hovered drop target.

{
  options: {
    previewerClassName: styles.dndDragPreview,
  },
}

NOTE: It is important that this div does not have any styles that affect its spatial positioning on screen, as this is controlled internally by the backend. What it can be used for are things like adding a drop shadow or highlight to the drag preview, changing opacities, borders, scaling, and other stylistic options.

preview

Control whether drag previews are rendered for this backend.

react-dnd-multi-backend provides a configuration field, preview, when creating the backend pipeline, which determines whether previews are enabled or hidden when using a given backend. However, this only works by introspecting the MultiBackend manager when rendering a Preview from inside of a React component context (it relies on the Context API to access the backend manager and read the option).

Because this library renders its own previews outside of React, it has no access to the context, and can't get access to the MultiBackend instance to read that configuration.

Instead, if you would like to hide the automatically-created previews when using this backend, pass this configuration under the options object:

{
  options: {
    preview: false;
  }
}

announcerClassName (deprecated)

Deprecated in version 2. To customize the presentation of drag-and-drop announcements, you can now use the announcer option to build your own custom announcer.

react-dnd-accessible-backend uses @react-aria/live-announcer as its default announcer, which visually hides all announcements.

{
  options: {
    announcerClassName: styles.dndAnnouncer,
  },
}

Considerations

Ensuring accessible labels

The default announcement messages will look at a few properties to try and create the most relevant label possible for the user. At the highest priority, if you specify a data-dnd-name attribute on the target element (either the drag source or the drop target, depending on the operation), that will be used directly. Next, it will look for an aria-label on the same element, and finally it will fall back to using the innerText of the element itself.

As an example:

// This would read out "Picked up Example A" when it is picked up for dragging
<div ref={drag} data-dnd-name="Example A" aria-label="some aria label" />
// This would read out "Over Target B" when the user drags an item over it with the keyboard backend
<dif ref={drop} aria-label="Target B" />
// This would read out "Dropped an element with text inside" when it is dropped on a target
<div ref={drag}>an element with text inside</div>

While this should ensure that any item a user picks up or drops has some kind of label on it, falling back to the inner text should be a last resort, and ideally you should add an aria-label or at least a data-dnd-name to provide a more succinct, helpful label for screenreader users.

"Sort on "hover"

A common pattern with react-dnd is to perform sorting operations in the hover callback on drop targets. In fact this is how most of the react-dnd sorting examples operate.

However, this pattern poses a problem for trying to drag and drop with a keyboard, because the user doesn't have the same granularity of control in movement. In the example linked above, sorting happens based on whether the user's mouse is dragging over the upper or lower half of the drop target. A keyboard user wouldn't be able to choose between those positions, so they effectively miss out on some sorting options, or in some cases the sorting won't happen at all since by default the drag operations go over the exact center of the drop target.

Additionally, sorting on hover means that a user "browsing" through drop targets will inadvertently be reorganizing their lists without ever actually dropping an item. If a user picks up an item, drags it up a few places, then decides to cancel the drag, the item will have moved those spaces anyway because the move happened on hover rather than when the item was actually dropped.

The best way to avoid this issue is to just avoid sorting in the hover callback and use drag placeholders and other indicators to show where an element will drop. This may involve some refactoring and rethinking designs, but the end result will be more accessible (and often more performant!) for everyone.

Thanks to

• react-dnd-keyboard-backend as an inspiration and proof-of-concept for keyboard-based backends.
• react-dnd-multi-backend for making it possible to compose backends together.
• react-beautiful-dnd as a guiding example of what a good keyboard-based dnd interface could feel like.
• @react-aria/live-announcer and the React Aria Team who are dedicated to creating accessible components and tools.