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# The mouseenter, mouseleave, and hover Events

The `event-mouseenter` module adds support for "mouseenter" and "mouseleave" events that are often preferable alternatives to "mouseover" and "mouseout".

The `event-hover` module adds support for a "hover" event that combines subscriptions to each of these two events, taking two callback functions, one for when the mouse enters the subscribed node, and another for when it leaves the node.

## `mouseover` and `mouseout` are noisy

The `mouseover` and `mouseout` events bubble. That means with the following DOM structure...

    <div id="hover-me">
        <div class="header">
            <button class="close">Close</button>
        </div>
        <div class="body">
            ... more markup
        </div>
    </div>

...a `mouseover` subscription on `#hover-me` would be called three times when a user moved the mouse over the close button because

1. The user moused over #hover-me
2. The user moused over the <div> in #hover-me, and that `mouseover` event bubbled to #hover-me
3. The user moused over the close button, and that `mouseover` event bubbled to #hover-me

Chances are, you don't care about any of these except the first. That's where `mouseenter` and `mouseleave` come in. They only fire when the `e.target` of the event is the node subscribed to. That means no noise.

    YUI().use('event-mouseenter', 'transition', function (Y) {
        var hoverMe = Y.one('#hover-me');

        hoverMe.on('mouseenter', function () {
            this.one('.header').transition('fadeIn');
        });

        hoverMe.on('mouseleave', function () {
            this.one('.header').transition('fadeOut');
        });
    });

Though these events work by essentially not bubbling, you can still delegate `mouseenter` and `mouseleave` using `node.delegate(...)`.

    // The overHandler callback will be executed when any .hoverable
    // element is moused over.
    container.delegate('mouseenter', overHandler, '.hoverable');

## `mouseenter` + `mouseleave` == `hover`

It's common to pair `mouseenter` and `mouseleave` subscriptions (or `mouseover` and `mouseout` for the uninitiated) to create an effect that only lasts as long as the mouse is over an element. To make that easier, the `event-hover` module adds a `hover` event. It accepts a second callback and attaches the first to `mouseenter` and the second to `mouseleave`.

    YUI().use('event-hover', 'transition', function (Y) {
        function over() {
            this.one('.header').transition('fadeIn');
        }
        function out() {
            this.one('.header').transition('fadeOut');
        }

        // Two callbacks, `mouseenter` and `mouseleave`
        Y.one('#hover-me').on('hover', over, out);
    });

If you need to override the default `this` object assignment or bind arguments to a `hover` subscription, just add those arguments after the second callback.

Similarly, if you want to delegate the `hover` event, pass a CSS filter after the second callback.

    // Default `this` to racerX
    mach5.on('hover', racerX.oilSlick, racerX.tireSpikes, racerX);

    // The delegate filter comes before the `this` override
    ruralTown.delegate('hover', ufo.scan, ufo.memoryWipe, '.person', ufo);