Implementing lazy loading in Virtuemart

Let’s start this a quick refresh on lazy loading:

https://developers.google.com/web/fundamentals/performance/lazy-loading-guidance/images-and-video/

To implement lazy loading (with this method) in virtuemart category page, you will need to add a class to the thumbnail <img> element, which is easily done in /com_virtuemart/sublayouts/products.php

Look for the line:

echo $product->images[0]->displayMediaThumb('class="browseProductImage"', false);

and add the desired class to the displayMediaThumb() function:

echo $product->images[0]->displayMediaThumb('class="browseProductImage lazy"', false);

In your theme template, create an override for the index.php and add the javascript in the footer:

 document.addEventListener("DOMContentLoaded", function() {
  var lazyImages = [].slice.call(document.querySelectorAll("img.lazy"));

  if ("IntersectionObserver" in window) {
    let lazyImageObserver = new IntersectionObserver(function(entries, observer) {
      entries.forEach(function(entry) {
        if (entry.isIntersecting) {
          let lazyImage = entry.target;
          lazyImage.src = lazyImage.dataset.src;
          lazyImage.srcset = lazyImage.dataset.srcset;
          lazyImage.classList.remove("lazy");
          lazyImageObserver.unobserve(lazyImage);
        }
      });
    });

    lazyImages.forEach(function(lazyImage) {
      lazyImageObserver.observe(lazyImage);
    });
  } else {
    // Possibly fall back to a more compatible method here
  }
}); 

At this stage your code won’t work and no image will be displayed. For the code to work you need to have a data-src defined in the <img> element.

One way to do this is to locate where the <img> html for the thumbnail is generated and create an override to define a data-src instead of the usual src.

I didn’t find any documentation as to where this code is generated, so I add to search for it. I used eclipse IDE and added a local copy of my virtuemart installation as a workspace in the IDE then search the function in the folder. It was located in:

/administrator/components/com_virtuemart/helpers/mediahandler.php

The html per say is generated by the displayIt() function. We change src to data-src in the file for the moment as it is not clear to me how I’ll be able to override this function in my theme.

My first test after settings all this is not running properly. Actually it does even seem to act the opposite it should, with the picture first loaded and that disappear when it crawls into view.

Let’s debug that. First, I checked that all my code is properly loaded, namely the class for the image and the data-src to prevent the picture to load before it come into view.

At that stage the “lazy” class is properly added to the source code but the image source is not changed to data-src as expected.

Changing this in the developer console does not solve the issue (picture still not showing) unless the srcset=”undefined” is deleted. This srcset attribute is added by the lazyload js.

By searching for <img in the mediahandler.php file I found the function displayImage(). I changed the src attribute to data-src and remove the srcset line from the lazyload javascript before running another test.

Changed src to data-src in evey img element in displayImage() and displayIt() function. Now the lazy loading function is working except for some css issue (product name appearing on overlay on the image instead of displaying below the image). Some pictures do not appear at all.

Product img elements have now data-src attribute instead of src but , they don’t have the “lazy” class so they are not displayed when they come into the view port. This might be changed in the single product view.

Product image has no class set but they don’t load. The lazy load script should not prevent image without the lazy class to load, so the issue might be that I changed the src attribute to data-src.

So the displayIt() function in /administrator/components/com_virtuemart/helpers/mediahandler.php handles picture in the product view as well as in the category view.

To add the lazy class to the product image and additional thumbnails, we need to modify default_images.php and default_images_additional.php in templates/your_template_name/html/com_virtuemart/productdetails/

in defaut_images.php look for the following code:

if(!empty($width) or !empty($height)){

            echo $image->displayMediaThumb("",true,"rel='vm-additional-images' class='block'", true, true, false, $width, $height);

        } else {

            echo $image->displayMediaFull("",true,"rel='vm-additional-images' class='block'");

        }

and change it to:

if(!empty($width) or !empty($height)){
echo $image->displayMediaThumb("class='lazy'",true,"rel='vm-additional-images' class='block'", true, true, false, $width, $height);
} else {
echo $image->displayMediaFull("class='lazy'",true,"rel='vm-additional-images' class='block'");
}

And in default:images_additional.php

 if(VmConfig::get('add_img_main', 1)) {
            echo $image->displayMediaThumb('class="product-image thumbnail" style="cursor: pointer"',false,false);
            echo '';
        } else {
            echo $image->displayMediaThumb("class='thumbnail'",true,"rel='vm-additional-images'",true,false);
        }

becomes:

if(VmConfig::get('add_img_main', 1)) {
            echo $image->displayMediaThumb('class="product-image thumbnail lazy" style="cursor: pointer"',false,false);
            echo '';
        } else {
            echo $image->displayMediaThumb("class='thumbnail lazy'",true,"rel='vm-additional-images'",true,false);
        }

Note: that you must override these two files in your template if you don’t want to see your changes deleted by the next update.

At that point we are left with the issue that thumbnails div does not have the appropriate height which leads to the product name to overlay on the picture, which in our case is not desirable. It seems to be handled in the products.php sublayouts and the class of the div we are looking for is vm-product-media-container. The div is indeed located in this file but it does not have the style attribute that causes the issue, nor does it call a variable for this, that suppose that the inline style is added through a filter on the server side or through js or jquery on the client side.

In our case, the following inline style is injected and cause display issue: style=”height: 42px;”

 Line 1165  $imgHeight = VmConfig::get('img_height','');

        if(!empty($imgHeight)){

            $imgHeight = 'height:'.VmConfig::get('img_height',90).'px;';

        } else {

            $imgHeight = '';

        }

To be specific the issue in on the div wrapping the image:

<div class=”vm-product-media-container” data-mh=”media-container” style=”height: 63px;”>

This code is generated in : /html/com_virtuemart/sublayouts/products.php

function displayMediaThumb($imageArgs='',$lightbox=true,$effect="class='modal' rel='group'",$return = true,$withDescr = false,$absUrl = false, $width=0,$height=0)

Let’s see where the display MediaThumb() is called and find out where the 42px height is defined.

in default_images.php

if (!empty($this->product->images)) {
    $image = $this->product->images[0];
?>
displayMediaThumb("",true,"rel='vm-additional-images'", true, true, false, $width, $height);
} else {
echo $image->displayMediaFull("",true,"rel='vm-additional-images'");
        }
?>

This div height is related to the thumbnails height settings in the template configuration panel. In the current state, the height takes various value each time the page is refreshed. Setting the width and height to zero or empty in the configuration panel leads to no image being displayed, dynamic resizing being off.

With dynamic resizing on and a value of 100 we got a good rendering, without issue.

200 leads to some issue. This might have to do with the dynamic resizing. Let’s try to locate this code to check what’s happening there.

The style does not appear in the source code. It appears in the DOM only (chrome and firefox). This tend to show that the inline style is generated by javascript. To try to spot the script causing the issue, we will add <script>debugger</script> after the element to debug. Reload the page and add a breakpoint by following the instruction here:

https://developers.google.com/web/tools/chrome-devtools/javascript/breakpoints

Results: Paused on attribute modification: jquery.min.jsmedia/jui/js/jquery.min.js

Stack frame:

style (jquery.min.js?105ec84b263645980d07d0cf26df4607:2)
(anonymous) (jquery.min.js?105ec84b263645980d07d0cf26df4607:2)
Q (jquery.min.js?105ec84b263645980d07d0cf26df4607:2)
Q (jquery.min.js?105ec84b263645980d07d0cf26df4607:2)
css (jquery.min.js?105ec84b263645980d07d0cf26df4607:2)
(anonymous) (jquery.matchHeight-min.js:8)
each (jquery.min.js?105ec84b263645980d07d0cf26df4607:2)
each (jquery.min.js?105ec84b263645980d07d0cf26df4607:2)
r.apply (jquery.matchHeight-min.js:8) t.fn.matchHeight (jquery.matchHeight-min.js:7) (anonymous) (jquery.matchHeight-min.js:11) each (jquery.min.js?105ec84b263645980d07d0cf26df4607:2) r._applyDataApi (jquery.matchHeight-min.js:11) u (jquery.min.js?105ec84b263645980d07d0cf26df4607:2) fireWith (jquery.min.js?105ec84b263645980d07d0cf26df4607:2) ready (jquery.min.js?105ec84b263645980d07d0cf26df4607:2) (jquery.min.js?105ec84b263645980d07d0cf26df4607:2)

jquery.matchHeight is a jQuery library to achieve equal height between div. In modern browsers, there are css solutions that achieve the same results: check Flexbox and CSS Grid.

Horme3 template uses /js/jquery.matchHeight-min.js

So matchHeight library does not handle properly the height of the div with the lazy loading technique I am using at the moment. The script is called in the index.php file of the template. Let’s have a look how the site behave without this script.

jquery.matchHeight catch the data-mh=”media-container” attributes to perform its operation. I tried to remove it from the thumbnail wrapper and add it to the product wrapper but the result is a mess. I will check if I can rewrite the template to support Flexbox instead.

So the issue of height not being set when the src is missing or invalid has also caught the attention or css-tricks which wrote a nice article about the topic. Let’s see if we can implement their options.

Lazy loading draw back: Content Reflow

The solution I will try is to give a svg source at the right size and aspect ratio encoded in base64 (or not: https://css-tricks.com/probably-dont-base64-svg/ )

I got to work out how to configure the viewport attribute in image to get the right aspect ratio.

Also the number of line of the short description is now an issue with the div height. Got to trouble shoot that.

Maybe best would be to restore all changes and reimplement lazy loading and psvg place holder from the ground up to avoid errors introduced when fiddling in the code to find out how to fix the height :)

This is my base64 placeholder (<svg></svg> method did not work):

src=”data:image/svg+xml;base64,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”

Current issue is due to single line description not being correctly adjusted by matchHeight. Otherwise all the rest is working.

Issue was caused by some script being disabled in virtuemart configuration panel. Virtuemart CSS, jquery and Fancybox were disabled.

The place holder should have a height matching the height of the highest thumbnails. Working with normalized thumbnail picture (in terms of dimension) would make things way more easier and cleaner.

Overriding templates of admin component takes place in administrator/templates/html/com_virtuemart/helpers/mediahandler.php

Check which admin theme you are using.

Log in the admin dashboard and check the page source, search for

/administrator/templates/your template name/css/template.css?105ec84b263645980d07d0cf26df4607

So at that point all issues have been solved. We need to check why plain SVG failed to load correctly. Specially even with encoding an inkscape file saved as plain svg in base64, we have a file size that have nearly double compared with our current page size. Off course we don’t need to load pictures that are not in the view port which is saving bandwidth and we even gained a smoother experience as the element are already placed properly on the page thanks to the svg place holder which is inline which means no loading time and no recalculation.

That seems to be a win. We need to ensure that the file that needs changes can be overriden properly and also browser compatibility to avoid bad experience.