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# ImageLoader

The ImageLoader Utility allows you as an implementer to delay the loading of images on your web page until such a time as your user is likely to see them. This can improve your overall pageload performance by deferring the loading of some images that are not immediately visible at the time the page first renders. Because images are often the heaviest components of a given page, deferring the loading of some images can yield a marked improvement in the way the page "feels" to your user.

The ImageLoader Utility lets you determine triggers and time limits to initiate image loading. This allows you to ensure that the images are loaded before your user is likely to see them. This technique, obviously, is appropriate only for images that are not immediately visible to your user at initial page load.

## Getting Started

To include the source files for ImageLoader and its dependencies, first load the YUI seed file if you haven't already loaded it.

    <script src="http://yui.yahooapis.com/3.10.3/build/yui/yui-min.js"></script>

Next, create a new YUI instance for your application and populate it with the modules you need by specifying them as arguments to the `YUI().use()` method. YUI will automatically load any dependencies required by the modules you specify.

    <script>
    // Create a new YUI instance and populate it with the required modules.
    YUI().use('imageloader', function (Y) {
        // ImageLoader is available and ready for use. Add implementation
        // code here.
    });
    </script>

For more information on creating YUI instances and on the `use()` method, see the documentation for the YUI Global Object.

### Creating an Image Group

Images are organized into groups. Each group has one or more triggers. A trigger is simply any DOM event, such as the `mouseover` of a specific `<div>`, a button click, or a window scroll. The images in a group won't load into the page until this trigger fires. Groups also have an optional time limit; if none of the group's triggers are activated before the time limit expires, the images are fetched anyway.

A group is defined as an instance of `Y.ImgLoadGroup`. It comprises a collection of images that will show up on the page based on the same triggers and time limit.

    var myFirstGroup = new Y.ImgLoadGroup({ timeLimit: 2 });
    myFirstGroup.addTrigger('#someDivId', 'mouseover');

This defines a group triggered by a `mouseover` of the `<div>` with the HTML ID of `someDivId`; with the `timeLimit` attribute we are specifying that all images in `myFirstGroup` should load two seconds after the window's `load` event fires even if our trigger hasn't been tripped.

### Adding Images to the Group

Register one or more images with the group using the HTML ID of the image element and the URL for the image:

    myFirstGroup.registerImage({
        domId: 'idOfDivWaitingForImage',
        bgUrl: 'http://www.example.com/image/url'
    });

This will set the image at `http://www.example.com/image/url` as the background image of the `<div>` with the ID `idOfDivWaitingForImage`. There are three kinds of images you can register with an ImageLoader group instance; see Adding Images below for more details on this process.

## Using the ImageLoader Utility

This section describes how to use the ImageLoader Utility in further detail. It contains these subsections:

- Performance Considerations
- Approach
- Triggers
- Custom Event Triggers
- Adding Images
- Loading Images Below the Fold
- Image Visibility
- Using Class Names

### Performance Considerations

The images on your page require possible DNS lookups, new HTTP transactions, and ultimately the transmission of image data in packets over the wire. While all this is happening, the user is often left waiting for the page to become fully functional. All of your `onload` JavaScript, for example, is deferred until after all the page's images have finished loading.

Should the user have to wait for all of these images? If the images are front and center on the page, then yes, suffering the load time is necessary. But what about images that the user doesn't see right away — the images below the fold; the images hidden towards the end of a carousel; the images that won't appear until a non-default tab of some module is clicked? ImageLoader allows you to delay the load of these images until after page load so that the page is fully functional more quickly. And, by using triggers, you can ensure that the images are loaded just before the user needs them so that there's no degradation of user experience.

### Approach

How can you anticipate when the user will be able to see images? Well, you as a developer know your page, and you know what actions are available to the user. You can utilize your knowledge to identify user events that indicate what the user is about to see.

For example, you know that any image lying below the fold won't be visible until the user either scrolls the page or resizes the browser window. In a tabbed module, you know that the user can't click one of the tabs until she mouses over that tab.

Consequently, you can use scroll events, mouseover events, or other indicators of user intent to stay one step ahead of the user and decide when to load images. The ImageLoader Utility lets you do exactly this.

### Triggers

Images are grouped together in terms of which user action(s) should trigger their loading. A trigger is simply any DOM event. Your first step is to create an ImageLoader group object and define its trigger:

    var myFirstGroup = new Y.ImgLoadGroup({
        timeLimit: 2
    });
    myFirstGroup.addTrigger('#someDivId', 'mouseover');

The `timeLimit` attribute is a time limit for the group. If the user has not performed the trigger event within the specified time limit, the images are fetched anyway. You can elect to not specify either the time limit or the trigger (indicating the user must perform the trigger event, or there should only be a simple time delay, respectively.)

You can have as many triggers as you wish for a group. Just add them with the `group` class's `addTrigger` method:

    myFirstGroup.addTrigger('#someOtherDivId', 'click');

The trigger conditionals are disjunctive; the first one to fire initiates the image fetching.

### Custom Event Triggers

You can also specify custom events as triggers. If the event belongs to the `Y` instance, call `addCustomTrigger` with the event name:

    myFirstGroup.addCustomTrigger('mycustomevent:imgloadevent');

Alternatively, if you have a custom event attached to a local object, you can specify this in the `addCustomTrigger` call:

    myFirstGroup.addCustomTrigger('mycustomevent:imgloadevent2', myCustomEvent);

And this group's images will be fetched upon `myCustomEvent.fire('mycustomevent:imgloadevent2');`.

### Adding Images

Once a group is created, you can add as many images as you'd like to it. There are three types of images:

1. background image (a `<div>` with a background image; URL set in `style.backgroundImage`). Use the `bgUrl` attribute to register this kind of image.
2. source image (an `<img>` tag; URL set in a `url` attribute). Use the `srcUrl` attribute to register this kind of image.
3. png background image (a `<div>` with a png background image; for IE6, sets an alpha filter `src`; for other browsers sets a background image). Use the `isPng` and `bgUrl` attributes to register this kind of image.

To add an image to a group, register the DOM ID of the image element and the image URL with the `registerImage` method:

    myFirstGroup.registerImage({
        domId: 'idOfDivWaitingForImage',
        bgUrl: 'http://www.example.com/image/url'
    });
    myFirstGroup.registerImage({
        domId: 'idOfImgWaitingForImage',
        srcUrl: 'http://www.example.com/other/image/url'
    });
    myFirstGroup.registerImage({
        domId: 'idOfDivWaitingForPngImage',
        bgUrl: 'http://www.example.com/png/image/url',
        isPng: true
    });

This will set the image at `http://www.example.com/image/url` as the background-image of the `<div>` with the ID `idOfDivWaitingForImage` and likewise with the two other image elements.

### Loading Images Below the Fold

A group can check its images at the DOM ready state and immediately begin loading those that are above the fold (i.e., inside the current viewport) while delaying the load of those that aren't. Just set a value for the `foldDistance` property of the group. Images are checked and loaded in a cascading fashion. That is, each image will be loaded only when it comes within `foldDistance` pixels of the bottom of the viewport. The effect is that images are loaded as needed as the user scrolls down the page. When you set a `foldDistance`, the group automatically gets window `scroll` and window `resize` triggers.

    var foldGroup = new Y.ImgLoadGroup({
        foldDistance: 30
    });
    foldGroup.registerImage({
        domId: 'partwayDownPageImage',
        bgUrl: 'http://www.example.com/image/url'
    });

### Image Visibility

You can set your `<img>` tags to have the CSS property `visibility:hidden`. This will allow your page to keep its structure until the image is actually loaded. Since these images are probably out of the viewport anyway, this may not make a perceptible difference, but it will help some browsers avoid reflowing the page when deferred images are loaded. To accomplish this using ImageLoader, set the `setVisible` attribute of the image to `true` when you register the image; ImageLoader will then set the `visibility` property to `visible` when the image is fetched.

### Using Class Names

As an alternative to registering each image with a group, you can use CSS class names to group images together. When using this approach, images that are part of the same group should all share a common class name. Each should also have its image set as the element's background image via CSS in the element's `style` attribute. To prevent the image from loading immediately when the element renders, create a CSS style definition for that class overriding the background image to `none`. Lastly, set the `className` attribute of the ImageLoader group.

The following combination of CSS, HTML, and JavaScript illustrates this approach:

    <div
      class='yui-imgload-somegroup'
      style='background-image:url("http://www.example.com/image/url");'>
    </div>

    <style>
      .yui-imgload-somegroup {
        background:none !important;
      }
    </style>

    <script>
      someGroup.set('className', 'yui-imgload-somegroup');
    </script>

## Important Usage Requirements

There are some important things to keep in mind while using the ImageLoader Utility. Otherwise it may not work the way you expect, or it may have some undesired side effects.

### "src" Attribute of Source Images

When using ImageLoader with `<img>` elements (via the `srcUrl` attribute), leave the `src` attribute out of the HTML element altogether. Do not set an empty string for the value of `src`. Some browsers react to this by assuming the empty string means "/", and consequently the browser re-requests the current HTML page and tries to stuff it into the `<img>` element. This is bad news for performance.

`<img id="anImgEl" />
<img id="anImgEl" src="" />`

### Resizing Images

When resizing `<img>` elements (via `height` and `width` attributes) on the fly, the height and width of the image must be specified in the JavaScript. Do this by setting `width`/`height` attributes in the `registerImage` call. Failure to do this will result in no resizing. Browsers ignore width/height set in the HTML when there is no `src` attribute. And when the `src` is finally set, the width/height end up being the image's natural size.

`someGroup.registerImage({ domId: "someImgDiv", srcUrl: "http://www.example.com/image/url", width: W, height: H });
someGroup.registerImage({ domId: "someImgDiv", srcUrl: "http://www.example.com/image/url" });`