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detect-focus.js
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detect-focus.js
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import cache from './supports-cache';
// options.element:
// {string} element name
// options.mutate: (optional)
// {function} callback(element, wrapper) to manipulate element prior to focus-test.
// Can return DOMElement to define focus target (default: element)
// options.validate: (optional)
// {function} callback(element) to manipulate test-result
function detectFocus(options) {
// wrap tests in an element hidden from screen readers to prevent them
// from announcing focus, which can be quite irritating to the user
const wrapper = document.createElement('div');
wrapper.setAttribute('style', 'position:fixed; position:absolute; top:0; left:-2px; width:1px; height:1px; overflow:hidden;');
wrapper.setAttribute('aria-live', 'off');
wrapper.setAttribute('aria-busy', 'true');
wrapper.setAttribute('aria-hidden', 'true');
document.body.appendChild(wrapper);
// create dummy element to test focusability of
const element = typeof options.element === 'string' ? document.createElement(options.element) : options.element(wrapper);
// allow callback to further specify dummy element
const focus = options.mutate && options.mutate(element, wrapper) || element;
// element needs to be part of the DOM to be focusable
!element.parentNode && wrapper.appendChild(element);
// remember what had focus to restore after test
const previousActiveElement = document.activeElement;
// remember scroll positions to restore after test
const previousScrollTop = window.scrollTop;
const previousScrollLeft = window.scrollLeft;
const previousBodyScrollTop = document.body.scrollTop;
const previousBodyScrollLeft = document.body.scrollLeft;
// test if the element with invalid tabindex can be focused
focus.focus && focus.focus();
// validate test's result
const allowsFocus = options.validate ? options.validate(element) : document.activeElement === focus;
// restore focus to what it was before test and cleanup
// IE10 does not redirect focus to <body> when the activeElement is removed
document.activeElement && document.activeElement.blur();
previousActiveElement && previousActiveElement.focus() || document.body.focus();
document.body.removeChild(wrapper);
// restore scroll position
window.scrollTop !== previousScrollTop && (window.scrollTop = previousScrollTop);
window.scrollLeft !== previousScrollLeft && (window.scrollLeft = previousScrollLeft);
document.body.scrollTop !== previousBodyScrollTop && (document.body.scrollTop = previousBodyScrollTop);
document.body.scrollLeft !== previousBodyScrollLeft && (document.body.scrollLeft = previousBodyScrollLeft);
return allowsFocus;
}
// cache detected support so we don't have to bother screen readers with unstoppable focus changes
export default function(options) {
let value = cache.get(options.name);
if (value === undefined) {
value = detectFocus(options);
cache.set(options.name, value);
}
return value;
}