问题
Some context, I am running a script on a website's home page to swap background images on a timer. We decided it would be better to attempt to implement preloading of the images, which prompts the following issue in Firefox:
Preloading images on the first page load will not prevent the browser from loading the image from the original source again instead of the cache. Oddly though, refreshing the page will successfully cause the image to be loaded in from the cache.
The JavaScript that runs on page load takes all of the image URLs, and attempts to preload them via calling (new Image()).src = 'http:// ...'; for each one.
Inspecting the page load revealed that the images would be loaded in on page load, but then the image would also be loaded in again when the slide was revealed.
Test Image Link (SO reputation restrictions): http://i.stack.imgur.com/E9KLM.png
In the image, the images -66.png, -21.png, -63.png, and -83.png were preloaded from the JavaScript, but are then requested again when the slider reveals that slide.
What's also strange is that the bottom images look like they were queued to be loading in since the page was created. Maybe it's because this takes priority over the script that was loaded once the document was ready?
To finish off, if I was to refresh the page and jump to a slide that was preloaded with the images, but never revealed, it is shown to be loaded from the cache like it should have been originally.
My theory is that the original background images are maybe declared to needing to be loaded from the server when the page is first loaded, but aren't actually loaded until the slide is revealed. On document ready, when the javascript preloads the images, they're not cached yet so they need to be loaded from the server. Then a slide is revealed and the browser tells that image that it needs to be loaded as originally declared.
Does anyone know why this situation is occuring? If so, are there any solutions to resolve?
I have an idea that involves adding the image URLs as a data-url attribute instead, and then having javascript preload them and add them as background images at that point, but I haven't tested this yet.
回答1:
For those interested, we were not able to find a perfect solution for this.
What we did notice, however, is that the images were being pulled again based on their size. The larger the image size, the more likely the image was not fetched from the cache when the image was shown to the user.
We semi-resolved this issue by compressing our background images even further, and then greatly limiting the amount of images preloaded on the first page load. We found that these two steps greatly increased the changes of the images being pulled in through the cache when needed. It also saved more bandwidth and improved page loading times in general by a significant amount.
来源：https://stackoverflow.com/questions/20681115/preloading-images-in-firefox-arent-being-retrieved-from-the-cache-on-the-same-p