How to properly preload images, js and css files?_问答_开发者

I'm creating a website from scratch and I was really into this in the late 90's but the web has changed alot since then! And I'm more of a designer so when I started putting this site together, I basically did a system of php includes to make the site more "dynamic"

When you first visit the site, you'll be presented to a logon screen, if you're not already logged on (cookies). If you're not logged on, a page called access.php is introdused. I thought I'd preload the most heavy images at this point. So that when the user is done logging on, the images are already cached. And this is working as I want. But I still notice that the biggest image still isn't rendered immediatly anyway. So it's seems kinda pointless.

All of this has made me rethink how the site is structured and how scripts and css files are loaded. Using FireBug and YSlow with Firefox I see a few pointers like expires headers and reducing the size of each script. But is this really the culprit?

For example, would this be really really stupid in the main index.php? The entire site is basically structured like this

<?php
require("dbconnect.php");
?>

<?php
include ("head.php");
?>

And below this is basically just the body and the content of the site. Head.php however consists of the doctype, head portions, linking of two css style sheets, jQuery library, jQuery validation engine, Cufon and Cufon font file, and then the small Cufon.Replace snippet.

The rest of the body comes with the index.php file, but at the bottom of this again is an include of a file called "footer.php" which basically consists of loading of a couple of jsLoader scripts and a slidepanel and then a js function. All of this makes the end page source loo开发者_开发知识库k like a typical complete webpage, but I'm wondering if any of you can see immediatly that "this is really really stupid" and "don't do that, do this instead" etc. :) Are includes a bad way to go?

This site is also pretty image intensive and I can probably do a little more optimization. But I don't think that's its the primary culprit. YSlow gives me a report of what takes up the most space:

doc(1) - 5.8K
js(5) - 198.7K
css(2) - 5.6K
cssimage(8) - 634.7K
image(6) - 110.8K

I know it looks like it's cssimage(8) that weighs the most, but I've already preloaded these images from before and it doesn't really affect the rendering.

To speed a little, you could assemble all your images on the same image sprite, so that you have only 1 request to download all the images. But that requires you to fine tune your css to let display just the small subset of your image. To have a better explanation, check out : http://css-tricks.com/css-sprites/

Another answer that could seem a little stupid but I like to think of this when I make a website : Just Keep It Simple. I mean do all your JS add real value, do all this images are fine, could you display less, make a lighter design ? I'm not criticizing your work at all, just suggest you...

I used the following approach on an extranet project:

Using jQuery and a array of file names, I ajax in all the images, .js and .css files so that they are preloaded in the cache. As I iterate through the array, I update a progress bar on the screen that indicates that the site is loading - much like a flash loader.

It worked well.

What I will do is show by default the loading page with pure CSS and HTML then wait for the jQuery to load and preload the images with ImageLoader. Once you are done redirect to the normal website since the images will be already in the cache they won't be loaded again.

Another optimization you can do is minify all JS files and combine all except the jquery.js. Put the jquery.js first into your HTML so it loads first. Also put your SCRIPT tags at the bottom of the HTML.

It sounds like you have pretty much nailed preloading, if you have loaded it once, and the expiry header is set correctly, you have preloaded it, no matter what kind of content it is.

File combination can be key to a quick website, each extra file will add load time, in the worst cases of network and server lag you might add up to a second extra for each separate file. More commonly it will be around 100 - 200 milliseconds per file.

If not already minified, minify the scripts, and put them in the same file, just remember to keep the order. I have no idea why Ivo Sabev wouldn't include jQuery.

Same thing with the CSS files.

How much have you done about testing image compression? There can really be a gain from trying out different compression settings and comparing size vs. quality. For PNG images IrfanView with PNGOUT can often make files 25% smaller than other programs, on top of that, a very big gain in size reduction can be achieved by reducing the image to 8 bit colour, with a lot of graphic elements you simply can't tell the difference. Right here on Stack Overflow there is a great example of well compressed and stacked images in the editor control buttons: http://sstatic.net/so/Img/wmd-buttons.png