Javascript to only display animated gif when loade

问题:

I have an animated gif banner on my website that is around 2MB. For people with slow connections, I want to include some Javascript that will only display (and start playing) this gif when it is fully loaded, so that the rest of the website will already display whilst the gif is still loading. Ideally, I would have one image (loading.gif) be displayed first, and then switched to the banner (banner.gif) with javascript when banner.gif is loaded.

How do I do this? (I'm new to Javascript)

Thanks!

回答1:

You can do this using an Image object, like so (do this when you want to start loading the banner, probably in onload):

var banner = new Image();
var loading = new Image();
var bannerElement = document.getElementById("banner"); // assumes an element with id "banner" contains the banner image - you can get the element however you want.
banner.src = "location/of/the/image.gif";
loading.src = "loading.gif";
banner.onload = function() {
     bannerElement.removeChild(bannerElement.lastChild);
     bannerElement.appendChild(banner);
};
bannerElement.removeChild(bannerElement.lastChild);
bannerElement.appendChild(loading);

Your banner element should look like this:

<div id="banner"><img src="location/of/the/image.gif" alt="Banner" /></div>

This is so that 1) The bannerElement.removeChild part will work and 2) To keep with the principles of progressive enhancement so people without JavaScript aren't left out.

回答2:

How about a jquery script like http://jqueryfordesigners.com/image-loading/

So you would do

<style type="text/css" media="screen">
<!--
    BODY { margin: 10px; padding: 0; font: 1em "Trebuchet MS", verdana, arial, sans-serif; font-size: 100%; }
    H1 { margin-bottom: 2px; }

    DIV#loader {
        border: 1px solid #ccc;
        width: 500px;
        height: 500px;
        overflow: hidden;
    }

    DIV#loader.loading {
        background: url(/images/spinner.gif) no-repeat center center;
    }
-->
</style>


$(function () {
    var img = new Image();
    $(img).load(function () {
        //$(this).css('display', 'none'); // .hide() doesn't work in Safari when the element isn't on the DOM already
        $(this).hide();
        $('#loader').removeClass('loading').append(this);
        $(this).fadeIn();
    }).error(function () {
        // notify the user that the image could not be loaded
    }).attr('src', 'myimage.jpg');
});

Note, you dont need to create a new image element if you already have one set. If you create one already then you can just use a selector. something like $('#myimage').load(... which is an image tag with an id called myimage.

回答3:

I don't think you need JavaScript for this - you can set your image area's background image with css (use a very small image so that it appears quickly) and then within that area have your image tag with the 2mb image's src already set. That way it should appear when ready.

example css:

#banner {background: url('/img/banner-background.jpg');}