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feat(snackbar): Accessibility: Announce label text to screen readers (#…
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| /** | ||
| * @license | ||
| * Copyright 2018 Google Inc. | ||
| * | ||
| * Permission is hereby granted, free of charge, to any person obtaining a copy | ||
| * of this software and associated documentation files (the "Software"), to deal | ||
| * in the Software without restriction, including without limitation the rights | ||
| * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell | ||
| * copies of the Software, and to permit persons to whom the Software is | ||
| * furnished to do so, subject to the following conditions: | ||
| * | ||
| * The above copyright notice and this permission notice shall be included in | ||
| * all copies or substantial portions of the Software. | ||
| * | ||
| * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR | ||
| * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, | ||
| * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE | ||
| * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER | ||
| * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, | ||
| * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN | ||
| * THE SOFTWARE. | ||
| */ | ||
|
|
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| import {numbers, strings} from './constants'; | ||
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| const {ARIA_LIVE_DELAY_MS} = numbers; | ||
| const {ARIA_LIVE_LABEL_TEXT_ATTR} = strings; | ||
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| /** | ||
| * @param {!HTMLElement} ariaEl | ||
| * @param {!HTMLElement=} labelEl | ||
| */ | ||
| function announce(ariaEl, labelEl = ariaEl) { | ||
| const priority = ariaEl.getAttribute('aria-live'); | ||
| const labelText = labelEl.textContent.trim(); // Ignore ` ` (see below) | ||
| if (!labelText) { | ||
| return; | ||
| } | ||
|
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| // Temporarily disable `aria-live` to prevent JAWS+Firefox from announcing the message twice. | ||
| ariaEl.setAttribute('aria-live', 'off'); | ||
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| // Temporarily clear `textContent` to force a DOM mutation event that will be detected by screen readers. | ||
| // `aria-live` elements are only announced when the element's `textContent` *changes*, so snackbars | ||
| // sent to the browser in the initial HTML response won't be read unless we clear the element's `textContent` first. | ||
| // Similarly, displaying the same snackbar message twice in a row doesn't trigger a DOM mutation event, | ||
| // so screen readers won't announce the second message unless we first clear `textContent`. | ||
| // | ||
| // We have to clear the label text two different ways to make it work in all browsers and screen readers: | ||
| // | ||
| // 1. `textContent = ''` is required for IE11 + JAWS | ||
| // 2. `innerHTML = ' '` is required for Chrome + JAWS and NVDA | ||
| // | ||
| // All other browser/screen reader combinations support both methods. | ||
| // | ||
| // The wrapper `<span>` visually hides the space character so that it doesn't cause jank when added/removed. | ||
| // N.B.: Setting `position: absolute`, `opacity: 0`, or `height: 0` prevents Chrome from detecting the DOM change. | ||
| // | ||
| // This technique has been tested in: | ||
| // | ||
| // * JAWS 2019: | ||
| // - Chrome 70 | ||
| // - Firefox 60 (ESR) | ||
| // - IE 11 | ||
| // * NVDA 2018: | ||
| // - Chrome 70 | ||
| // - Firefox 60 (ESR) | ||
| // - IE 11 | ||
| labelEl.textContent = ''; | ||
| labelEl.innerHTML = '<span style="display: inline-block; width: 0; height: 1px;"> </span>'; | ||
|
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| // Prevent visual jank by temporarily displaying the label text in the ::before pseudo-element. | ||
| // CSS generated content is normally announced by screen readers | ||
| // (except in IE 11; see https://tink.uk/accessibility-support-for-css-generated-content/); | ||
| // however, `aria-live` is turned off, so this DOM update will be ignored by screen readers. | ||
| labelEl.setAttribute(ARIA_LIVE_LABEL_TEXT_ATTR, labelText); | ||
|
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| setTimeout(() => { | ||
| // Allow screen readers to announce changes to the DOM again. | ||
| ariaEl.setAttribute('aria-live', priority); | ||
|
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| // Remove the message from the ::before pseudo-element. | ||
| labelEl.removeAttribute(ARIA_LIVE_LABEL_TEXT_ATTR); | ||
|
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| // Restore the original label text, which will be announced by screen readers. | ||
| labelEl.textContent = labelText; | ||
| }, ARIA_LIVE_DELAY_MS); | ||
| } | ||
|
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||
| export {announce}; |
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