YUI Library Examples: Focus Manager Node Plugin: Accessible TabView

Focus Manager Node Plugin: Accessible TabView

Setting Up the HTML

The tabs in the tabview widget will be represented by a list of <a> elements whose href attribute is set to the id of an <div> element that contains its content. Therefore, without JavaScript and CSS, the tabs function as in-page links.

For this example the content of each tab panel is created on the server using the YQL API to fetch the title and URL for news stories made available from the various Yahoo! News RSS feeds. Here's the PHP:

Progressive Enhancement

The markup above will be transformed into a tabview widget using both CSS and JavaScript. To account for the scenario where the user has CSS enabled in their browser but JavaScript is disabled, the CSS used to style the tabview will be loaded via JavaScript using the built-in loader of the YUI Global Object. Additionally, JavaScript will be used to temporarily hide the tabview markup while the JavaScript and CSS are in the process of loading to prevent the user from seeing a flash unstyled content.

ARIA Support

Through the use of CSS and JavaScript the HTML for the tabview can be transformed into something that looks and behaves like a desktop tab control, but users of screen readers won't perceive it as an atomic widget, but rather simply as a set of HTML elements. However, through the application of the WAI-ARIA Roles and States, it is possible to improve the semantics of the markup such that users of screen readers perceive it as a tab control.

Keyboard Functionality

The keyboard functionality for the tabview widget will be provided by the Focus Manager Node Plugin. The Focus Manager's descendants attribute is set to a value of ".yui-tab>a", so that only one tab in the tabview is in the browser's default tab flow. This allows users navigating via the keyboard to use the tab key to quickly move into and out of the tabview. Once the tabview has focus, the user can move focus among each tab using the left and right arrows keys, as defined by the value of the keys attribute. Lastly, the focusClass attribute is used to apply a class of yui-tab-focus to the parent <li> of each <a> when it is focused, making it easy to style the tab's focused state in each of the A-Grade browsers. // Add a class to the documentElement to prevent the user from seeing // the unstyled menu while the necessary CSS and JavaScript // dependancies are being fetched. document.documentElement.className = "yui-loading"; YUI({ base: "../../build/", modules: { "tabview-css": { type: "css", fullpath: "assets/tabview.css" } }, timeout: 10000 }).use("tabview-css", "node-focusmanager", "node-event-simulate", function(Y) { var tabView = Y.Node.get("#tabview-1"), tabList = tabView.query("ul"), tabHeading = Y.Node.get("#tabview-heading"), sInstructionalText = tabHeading.get("innerHTML"); selectedTabAnchor = tabView.query(".yui-tab-selected>a"), bGeckoIEWin = ((Y.UA.gecko || Y.UA.ie) && navigator.userAgent.indexOf("Windows") > -1), panelMap = {}; // Remove the "yui-loading" class from the documentElement // now that the necessary YUI dependencies are loaded. tabView.get("ownerDocument").get("documentElement").removeClass("yui-loading"); // Apply the ARIA roles, states and properties. // Add some instructional text to the heading that will be read by // the screen reader when the first tab in the tabview is focused. tabHeading.set("innerHTML", (sInstructionalText + " <em>Press the enter key to load the content of each tab.</em>")); tabList.set("aria-labelledby", "tabview-heading"); tabList.set("role", "tablist"); tabView.query("div").set("role", "presentation"); tabView.plug(Y.Plugin.NodeFocusManager, { descendants: ".yui-tab>a", keys: { next: "down:39", // Right arrow previous: "down:37" }, // Left arrow focusClass: { className: "yui-tab-focus", fn: function (node) { return node.get("parentNode"); } }, circular: true }); // If the list of tabs loses focus, set the activeDescendant // attribute to the currently selected tab. tabView.focusManager.after("focusedChange", function (event) { if (!event.newVal) { // The list of tabs has lost focus this.set("activeDescendant", selectedTabAnchor); } }); tabView.queryAll(".yui-tab>a").each(function (anchor) { var sHref = anchor.getAttribute("href", 2), sPanelID = sHref.substring(1, sHref.length), panel; // Apply the ARIA roles, states and properties to each tab anchor.set("role", "tab"); anchor.get("parentNode").set("role", "presentation"); // Remove the "href" attribute from the anchor element to // prevent JAWS and NVDA from reading the value of the "href" // attribute when the anchor is focused if (bGeckoIEWin) { anchor.removeAttribute("href"); } // Cache a reference to id of the tab's corresponding panel // element so that it can be made visible when the tab // is clicked. panelMap[anchor.get("id")] = sPanelID; // Apply the ARIA roles, states and properties to each panel panel = Y.Node.get(("#" + sPanelID)); panel.setAttrs({ role: "tabpanel", "aria-labelledby": anchor.get("id") }); }); // Use the "delegate" custom event to listen for the "click" event // of each tab's <A> element. Y.on("delegate", function (event) { var selectedPanel, oTarget = event.currentTarget, sID = oTarget.get("id"); // Deselect the currently selected tab and hide its // corresponding panel. if (selectedTabAnchor) { selectedTabAnchor.get("parentNode").removeClass("yui-tab-selected"); Y.Node.get(("#" + panelMap[selectedTabAnchor.get("id")])).removeClass("yui-tabpanel-selected"); } selectedTabAnchor = oTarget; selectedTabAnchor.get("parentNode").addClass("yui-tab-selected"); selectedPanel = Y.Node.get(("#" + panelMap[sID])); selectedPanel.addClass("yui-tabpanel-selected"); creatingPaging(selectedPanel); // Prevent the browser from following the URL specified by the // anchor's "href" attribute when clicked. event.preventDefault(); }, tabView, "click", ".yui-tab>a"); // Since the anchor's "href" attribute has been removed, the // element will not fire the click event in Firefox when the // user presses the enter key. To fix this, dispatch the // "click" event to the anchor when the user presses the // enter key. if (bGeckoIEWin) { Y.on("delegate", function (event) { if (event.charCode === 13) { event.currentTarget.simulate("click"); } }, "#tabview-1", "keydown", ">ul>li>a"); } });

Accessibility Sugar

One of the challenges faced by users of screen readers is knowing when you've left the context of a given control. In the case of this tabview, if it were adjacent to another ARIA-enabled widget, the user would know they've left the tabview when the screen reader announces the role of the adjacent widget. However, if the tabview is sitting alongside standard HTML content, it would be really difficult for the user to know when they've left the context of the active panel.

One solution to this problem is to add some additional navigation as the last child of each tab panel that allows the user to move to the previous and next panel in the tabview. This will not only help alert users of screen readers that they've reached the end of the tab's panel, but allow all keyboard users to move more quickly to the next/previous panel. Without this additionally navigation, keyboard users would typically have to press shift+tab to navigate back up to the list of tabs to move to the next/previous tab.