How to detect `focusin` support?

问题:

Thanks to Perfection kills, we can use the following JavaScript to detect event support:

function hasEvent(ev) {
    var elem = document.createElement('a'),
        type = 'on' + ev,
        supported = elem[type] !== undefined;
    if (!supported) {
        elem.setAttribute(type, 'return;');
        supported = typeof elem[type] === 'function';
    }
    elem = null;
    return supported;
}

This works for about the only time I need it: detecting mouseenter support; hasEvent('mouseenter') will return false in Chrome, Firefox, etc., as it should.

But now I'm trying to "fix" browsers that don't support the focusin and focusout events. According to PPK, that's basically just Firefox. Unfortunately, Chrome and Safari are listed as having "incomplete" support, for the following reason:

Safari and Chrome fire these events only with addEventListener; not with traditional registration.

In general, that's fine; I'd only be using addEventListener anyway. It does mean, however, that detecting support via elem.onfocusin !== undefined won't work. I tested it out, and it's true:

<p>Do I support <a href="#">focusin</a>?</p>

<script>
var elem = document.getElementsByTagName('p')[0];

// hasEvent method defined here
function listener() {
    var response = hasEvent('focusin') ? 'Yes!' : 'No...';
    alert(response);
}

elem.addEventListener('focusin', listener, false);
</script>

The above alerts No... in Chrome!! Is there any way to detect whether the browser supports focusin, without using browser sniffing?

回答1:

This uses the fact that calling focus() triggers focusin: http://jsfiddle.net/pimvdb/YXeD3/.

The element must be visible and inserted into the DOM, otherwise focusin is not fired for some reason.

var result = (function() {
    var hasIt = false;

    function swap() {
        hasIt = true; // when fired, set hasIt to true
    }

    var a = document.createElement('a'); // create test element
    a.href = "#"; // to make it focusable
    a.addEventListener('focusin', swap, false); // bind focusin

    document.body.appendChild(a); // append
    a.focus(); // focus
    document.body.removeChild(a); // remove again

    return hasIt; // should be true if focusin is fired
})();

回答2:

focusin & focusout should be fired BEFORE target element receives focus, event order is also seems buggy

http://www.w3.org/TR/DOM-Level-3-Events/#events-focusevent-event-order

currently, only IE works according to spec:

Chrome/Safari:
focus
focusin
DOMFocusIn
blur
focusout
DOMFocusOut
focus
focusin
DOMFocusIn

Opera 12:
focus
DOMFocusIn
focusin
blur
DOMFocusOut
focusout
focus
DOMFocusIn
focusin

IE 8:
focusin
focus
focusout
focusin
blur
focus

Firefox 14:
focus
blur
focus

回答3:

You can test for ("onfocusin" in document).

This method has the advantage of being light and unobtrusive and it will tell you whether the browser supports the focusin event. (not on Chrome, sorry)

You may use the following code to get the same behaviour as the focusin event on all browsers (IE9+, Chrome, Firefox, Edge):

var eventName, useCapture;
if ("onfocusin" in document) {
  eventName = "focusin";
  useCapture = false;
} else {      
  eventName = "focus";
  useCapture = true;
}

document.body.addEventListener(eventName, function( event ) {
    event.target.style.background = "pink";    
  }, useCapture);

JS fiddle here: https://jsfiddle.net/d306qm92/2/

More information about the Firefox workaround here: https://developer.mozilla.org/en-US/docs/Web/Events/focus#Event_delegation

UPDATE: As said above, the test will wrongly say "false" on Chrome, nonetheless the code sample will work as intended as Chrome supports both methods (focusin and focus with useCapture).