<style>
/* Hide overlay markup while loading, if js is enabled */
.yui3-js-enabled .yui3-overlay-loading {
    top:-1000em;
    left:-1000em;
    position:absolute;
}

{{>overlay-css}}
.yui3-overlay-content .yui3-widget-bd {
    position: relative;
}

.yui3-overlay-content .yui3-widget-bd .yui3-ovr-z-i{
    position: absolute;
    bottom: 1em;
}

.yui3-overlay-content .yui3-widget-bd .yui3-ovr-z-i em{
    color: #FF8800;
    font-size: 160%;
    font-style: normal;
    font-weight: bold;
}

.yui3-overlay-content .yui3-widget-bd {
    padding-top:6em;
}

/* Example Layout CSS */
.overlay-example {
    padding:5px;
    height:25em;
}

.overlay-example select {
    width:35em;
}
</style>

<div class="intro">
    <p>This example shows how you can work with the `zIndex` attribute which the Overlay supports, to implement a simple `bringToTop` function. The example code also listens for changes in the `zIndex` attribute, which it uses to update the content of the overlay, when it is brought to the top of the stack.</p>
</div>

<div class="example">
    {{>overlay-stack-source}}
</div>

<h2>Basic Overlay Stackability (zIndex and shim support)</h2>

<h3>Setting Up The YUI Instance</h3>

<p>As with the <a href="overlay-xy.html">"Basic XY Positioning"</a> example, to create an instance of an Overlay on your page, the only module you need to request is the `overlay` module. The `overlay` module will pull in the `widget`, `widget-stack`, `widget-position`, `widget-position-align`, `widget-position-constrain` and `widget-stdmod` extensions it uses.</p>

```
YUI({...}).use("overlay", function(Y) {
    // We'll write example code here, where we have a Y.Overlay class available.
});
```

<p>Using the `overlay` module will also pull down the default CSS required for overlay, on top of which we only need to add our required look/feel CSS for the example.</p>

{{>need-skin-note}}
```
<body class="yui3-skin-sam"> {{>need-skin-comment}}
```


<h3>Instantiating The Overlay</h3>

<p>For this example we'll instantiate Overlays from script, as we did for the <a href="overlay-align.html">"Alignment Support"</a> example. We'll create 6 Overlay instances and give them increasing zIndex and xy attribute values:</p>

```
function getOverlayXY(xy, i) {
    return [xy[0] + i * 60, xy[1] + i * 40];
}

for (var i = 0; i < n; ++i) {

    ovrXY = getOverlayXY(xy, i);
    ovrZIndex = i+1;

    // Setup n Overlays, with increasing zIndex values and xy positions
    overlay = new Y.Overlay({
        zIndex:ovrZIndex,
        xy:ovrXY,

        width:"8em",
        height:"8em",
        headerContent: 'Overlay <span class="yui3-ovr-number">' + i + '</span>',
        bodyContent: "z-index = " + ovrZIndex
    });

    overlay.render("#overlay-stack");

    ...

}
```

<p>We store the Overlay instances in an `overlays` array, which we'll later use to sort the Overlays by their zIndex values. We also setup a listener for the `zIndex` attribute change event, which will update the body section of the Overlay to display its new zIndex value.</p>

```
overlays.push(overlay);

// Update body whenever zIndex changes
overlay.after("zIndexChange", function(e) {
    this.set("bodyContent", "z-index = " + e.newVal);
});
```

<h3>Handling MouseDown Using Widget.getByNode</h3>

<p>The `Widget` class has a static `getByNode` method which can be used to retrieve Widget instances based on a node reference. The method will return the closest Widget which contains the given node.</p>

<p>
We'll use this method in a click listener bound to the container of the example ("#overlay-stack"). Target nodes of click events bubbled up to this example container, will be passed to the `Widget.getByNode` method, to see if an Overlay was clicked on.
</p>

```
function onStackMouseDown(e) {
    var widget = Y.Widget.getByNode(e.target);

    // If user clicked on an Overlay, bring it to the top of the stack
    if (widget && widget instanceof Y.Overlay) {
        bringToTop(widget);
    }
}

Y.on("mousedown", onStackMouseDown, "#overlay-stack");
```

<p>If an Overlay was clicked on, we invoke the simple bringToTop method which will set the zIndex of the clicked Overlay to the highest current Overlay zIndex value.</p>

<h3>The `bringToTop` Implementation</h3>

<p>We use a basic comparator function to sort the array of Overlays we have. The comparator makes sure the array element we're dealing with <a href="../widget/widget-build.html">has a `WidgetStack` implementation</a> (which Overlays do) and if so, sorts them in descending `zIndex` attribute value order:</p>

```
// zIndex comparator
function byZIndexDesc(a, b) {
    if (!a || !b || !a.hasImpl(Y.WidgetStack) || !b.hasImpl(Y.WidgetStack)) {
        return 0;
    } else {
        var aZ = a.get("zIndex");
        var bZ = b.get("zIndex");

        if (aZ > bZ) {
            return -1;
        } else if (aZ < bZ) {
            return 1;
        } else {
            return 0;
        }
    }
}
```

<p>Once sorted, for the purposes of the example, we simply switch the `zIndex` of the "highest" Overlay, with the Overlay which was clicked on, giving the selected Overlay the highest zIndex value:</p>

```
function bringToTop(overlay) {

    // Sort overlays by their numerical zIndex values
    overlays.sort(byZIndexDesc);

    // Get the highest one
    var highest = overlays[0];

    // If the overlay is not the highest one, switch zIndices
    if (highest !== overlay) {
        var highestZ = highest.get("zIndex");
        var overlayZ = overlay.get("zIndex");

        overlay.set("zIndex", highestZ);
        highest.set("zIndex", overlayZ);
    }
}
```

<h3>CSS: Overlay Look/Feel</h3>

<p>As mentioned in the <a href="overlay-xy.html">"Basic XY Positioning"</a> example, the overlay.css Sam Skin file (build/overlay/assets/skins/sam/overlay.css) provides the default functional CSS for the overlay. Namely the CSS rules to hide the overlay, and position it absolutely. However there's no default out-of-the-box look/feel applied to the Overlay widget.</p>

<p>The example provides it's own look and feel for the Overlay, by defining rules for the content box, header and body sections:</p>

```
{{>overlay-css}}
.yui3-overlay-content .yui3-widget-bd .yui3-ovr-z-i{
    line-height: 3.5em;
    font-size:190%;
}
```

<h2>Complete Example Source</h2>
```
{{>overlay-stack-source}}
```
