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LRUD spatial

Move focus around a HTML document using Left, Right, Up, Down keys.

API

getNextFocus(currentFocus, keyOrKeyCode, [scope])

Parameters

  • currentFocus should be an HTMLElement that you want LRUD spatial to consider as the element you are navigating from. In simple applications, this can just be a reference to document.activeElement.
  • keyOrKeyCode should be a key string or a keyCode decimal representing the directional key pressed.
  • scope is an optional HTMLElement that you only want to look for focusable candidates inside of. Defaults to the whole page if not provided.

Returns

An HTMLElement that LRUD spatial thinks you should navigate to.

Focusables

LRUD spatial defines focusable elements as those which match any of the following CSS selectors:

  • [tabindex] (for tabindex >= 0)
  • a
  • button
  • input

Ignoring Focusables

Any potential candidate with the lrud-ignore class, or inside any parent with the lrud-ignore class, will not be considered focusable and will be skipped over. By default LRUD will not ignore candidates that have opacity: 0 or have a parent with opacity: 0, so this class can be used for that.

Focusables with a tabindex="-1" attribute will be skipped over, however any focusable inside any parent with tabindex="-1" will still be considered focusable.

Focusable Overlap

By default, LRUD will measure to all candidates that are in the direction to move. It will also include candidates that overlap the current focus by up to 30%, allowing for e.g. a 'right' movement to include something that is above the current focus, but has half of it's size expanding to the right.

This threshold can be adjusted on a per-element basis with the data-lrud-overlap-threshold attribute, as a float from 0.0 to 1.0. An overlap of 0.0 will make a candidate only be considered if it is located entirely in the direction of movement.

Please also note that LRUD does not consider the Z Axis, which can cause surprising results with elements that are overlapped in this way, including in the case of full screen overlays on existing UIs. The above attribute can help alleviate this issue.

Containers

Focusables can be wrapped in containers. Containers can keep track of the last active focusable item within them using a data-focus attribute.

Focusables that should be tracked must have a unique ID attribute.

When a container has no previous focus state, it's first focusable element is used instead.

At this time, containers are defined as matching the CSS selectors: nav, section or .lrud-container.

Block exits

In some instances, it is desirable to prevent lrud-spatial from selecting another "best candidate" for example at the bottom of a list. To do this, a container element should add an additional data-block-exit attribute to prevent further selection in that direction. This should be a space separated list.

E.g.

<div class="lrud-container" data-block-exit="up down">...</div>

Focusable Containers

By default, LRUD only measures the distances to focusables and does not consider where their container boundaries are. Adding the attribute data-lrud-consider-container-distance to a container will include it in the distance calculations, as well as its children.

If the container is the closest out of all the possible focusables assessed, LRUD will return one of its children - even if they are not necessarily the spatially closest focusable.

The above container focus logic will still be used, and moving into a focusable container will move to its last focused child if there was one, at any level of container depth inside it.

How does it work?

To determine the next element that should be focused;

  1. Uses the key code to get the direction of movement
  2. From the currently focused DOM element, get the coordinates of the edge corresponding to the direction you are moving
  3. For all other focusable elements, get the coordinates of the opposite edge (the edge you are entering)
  4. Find the line between the exit and entry coordinates that has the shortest length

Developing

Requirements: Node.js 18

To get started, run npm ci.

Building

npm run build will emit a transpiled and minified version of the library. This is run during CI to prepare the artifact published to NPM. It can also be useful for integrating against another local project with npm link.

Testing

The test/layouts directory contains HTML designed to mirror various real-world use cases, and allow for behavioural vertification of library features such as containers and grids.

Significant new features should come with corresponding layouts that test them.

Use npm test to run the tests.

Debugging

The tests use puppeteer to spin up a headless browser. The browser loads the layouts mentioned above and runs scenarios from lrud.test.js against the unminified code from lib/.

To investigate why a test is failing, or just to hack on some changes... run npm run server. Then go to http://localhost:3005 and select a layout.

Contact

TVOpenSource@bbc.co.uk - we aim to respond to emails within a week.