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Step 2: Implementation of preloading images. Manage image uploads. Lazy loading of images to reduce server load

When you create a gallery on a website, you need to load images in advance so that there are no errors in the scripts or the user does not see pieces of underloaded images.

At one time, I was looking for a good preloader for such purposes, but I could not find a solution that would completely satisfy me. I had to write my own.

It has basic functionality that allows you to perform two main functions: upload images and then call a callback. But it also has its own features. It can load pictures one by one in the order in which they were placed in the array. Not just one at a time, but two-three-N. And for each loading iteration a callback can be executed. But that’s not all: after loading the last image, the final callback will be executed, indicating that all images have been loaded.

Usage

There are two options for using jQuery.preload.
The first is to pass the address(es) of the images:

$.preload(images, , );

  • images
    The image address can be input as a string (for one image) or as an array.
  • part
    Next comes an optional parameter indicating how many images to load at a time. 0 means everything at once, the default value.
  • callback
    And finally, the callback is a function that will be executed once the image is loaded. It can be given its own last parameter, which takes the value true if this part is the last.

The second use case is to download all images and background pictures in element:

$("#elem").preload();

  • callback
    A function that will be executed after all images have been loaded.
Examples
  • — the images will be loaded one at a time, after loading each one the callback will fire, and after loading the last one the final callback will fire.
  • — the callback displays previously hidden but already loaded images.

For correct operation plugin required jQuery versions 1.6 or higher.

When you are engaged in layout and creating all sorts of tricks with pictures (usually with substitution on hover or even their animation), such a nuance always comes up as delayed loading of a picture/image. At first glance, the problem is not that serious and, as soon as the pictures are loaded, everything works fine, but this is the first hover effect, which tries to load an image and replace it at the same time, creates a jerky effect that often does not look very nice and can ruin the first impression of users of your site.

With animation, things are even worse. Imagine, for example, that you need to implement in JS (JQuery) a passing car, which is replaced after 2 seconds by a bus. The images with the car and the bus are different. At the same time, they will begin to load only at the moment when the browser receives a link to them with the ability to load ( that is, at the beginning of the animation). It will take a few seconds to load such a picture. That is, it turns out that for half of the two-second animation this image will only be loaded - this is not the order.

But all this can be fixed by preloading ( preload) images, which allows you to load the image into the buffer immediately when loading the main page ( or even before that). This is what I want to talk about in this post.

Preloading images in JavaScript (JQuery)The option with loading in JS, in my opinion, is one of the best, especially if you are dealing with animation. Its essence is that you simply create a copy of the image using JavaScript and load it into the browser buffer, thereby, if necessary, you will no longer need to load the image, since it will already be preloaded.

Sounds complicated? =) But it just looks like this code, where you need to replace the path to the image with your own and everything will work:

//create jQuery function, which will load images into the buffer jQuery.preloadImages = function() ( for(var i = 0; i< arguments.length; i++) { jQuery("").attr("src", arguments[ i ]); ) ); //specify the path to the image that needs to be loaded $.preloadImages("/tpl/testimg.png");


If you need to load several images, then you can simply list them separated by commas, like this:

$.preloadImages("imageone.jpg", "dirname/imageok.jpg", "/tpl/testimg.png");


There can be any number of pictures. The main thing is not to forget about the JQuery function, without it preloading will not work.

This method also has disadvantages, such as the fact that it depends on whether JS is enabled in the user's browser. But on personal experience I will say that if you compare yourself to those who have JS turned off, then modern technologies A lot of website building will not be possible to implement. And I think there are not so many such users. In general, you need to follow the majority, and the majority do not customize the browser to such an extent.

I personally prefer to use this loading method in many projects due to the fact that it is convenient to implement.

Preloading images using new HTML5 features This method is relatively new due to the fact that HTML5 technology itself was launched not so long ago. Its essence is that you put a link to the picture through the link tag, and in rel attribute (type of loaded element) you indicate that this is a preload. Thus, the browser sees this tag and automatically loads the image into the buffer.

This is done by adding to html code like this tag ( just change the link to the image to your own):


Here rel contains 2 parameters: prerender- for His Majesty Chrome and prefetch- for other browsers. If you want to upload larger number images, then copy the tag and replace the link required quantity once:

The advantage of this method is that disabling JS will not affect preloading in any way, but personally I see 2 obvious disadvantages:
1) HTML5 technology, and therefore this preloading method, is not supported by all browsers. Now, however, browsers are being updated and most modern developing browsers recognize HTML5, but this is still far from ideal.
2) More cumbersome in contrast to the JS implementation, because each image will have to be described with a separate tag ( and with different parameters rel to achieve something close to cross-browser compatibility).

In general, I think that this method is promising, but it takes time. Right now it lacks versatility.

Preload images in time-tested HTMLThe very first idea that usually comes to mind is pure html- this is to create a div block with CSS parameter "display:none;", there is a picture in it and this should become a preload. This doesn't actually work, the browser will only start loading into the buffer when display is different from none.

But I came up with a way using CSS tricks. I'll tell you more about him.

Placement in an invisible block via CSS opacity and position (positioning) CSS has such a property - opacity. Its purpose is to change the transparency and you can change the transparency down to zero. In addition, CSS also has position property, which is needed to position the element on the page ( you can move a block of information pixel-by-pixel anywhere within and beyond the visibility of the page). All this will be useful to us:

Thus, we get an invisible block with a picture, which is also actually located outside visible to the user information. By the way, this type of positioning outside the screen is often used if they want to create some kind of invisible block; I have often seen how links to free templates exactly this way. So you know everything =)

If you want to preload several images in this way, then just indicate them inside block div, like this:

These are the methods for preloading pictures/images that can be used in website development. If you have any other ideas, I’ll be glad to read them in the comments.
Good luck to everyone in the layout and creation of animation! =)

Why do you need images at all? Let's imagine that you have an element on a website page, and you want its background image to show or change when you hover over it with the mouse. In principle, problems should not arise; everything can be solved simply: when creating a site in the style markup for the pseudo-class:hover of the desired element, we specify the path to the required image in the background property.

But there is one caveat: if the hover event occurs for the first time, then it may take time to load the image. Although this time interval may be small, sometimes you may notice a short, annoying delay between hovering over an element and the image being fully displayed. It is clear that in the future there will be no such delays, since browsers will cache the image in good faith. But, as you know, the first impression of your website should only be positive, and there is no need to spoil it with such trifles, especially since they can be avoided. How to solve such a small but unique problem? There are many methods, here our web studio will introduce you only to those that can be solved using css.

The first technique is to immediately load the graphic component, and then remove it from the visible area using the background-position property, for example by setting the following x and y offsets - -9999px -9999px. When the hover event occurs, we already apply real values, for example, bottom left.

What to do if the element already has background image, and when you hover the mouse cursor it must be changed. The above technique is no longer suitable as a solution. Typically, in such cases, sprites (combined background images) are used when the position of the background is shifted. If this method is not available to you, try the following. Apply an image to another existing element on the page that does not have a background image.

#random-unsuspecting-element ( background: url(images/grass.png) no-repeat -9999px -9999px; )

#grass:hover ( background: url(images/grass.png) no-repeat; )

The idea is to create new elements that are only needed to preload images. You will say that this is not the best The best way, because the number of such dummies can be huge, but the solution will not be elegant. And you will be absolutely right. We don't need additional elements, since we can use existing ones with peace of mind. All we need is to use the:after or:before pseudo-classes.

#something:before (

content: url("./img.jpg");

visibility:hidden;

This option gives us the opportunity to preload images without creating additional elements.

Of course, you can use javascript for these purposes, but if the above solutions are enough for you to create a website, then why not use them.

In order to be able to upload one or more files to the server, a special field is used in the form. IN Firefox browsers, IE and Opera, such an element is displayed as a text field, next to which there is a button labeled “Browse...” (Fig. 1). In Safari and Chrome, only the “Select file” button is available (Fig. 2).

Rice. 1. View of the field for uploading a file in Firefox

When you click the button, a file selection window opens, where you can specify which file the user wants to use.

The syntax for the file upload field is as follows.

The attributes are listed in table. 1.

Before using this field, you must do the following on the form:

  • set sending method POST data(method="post" );
  • set the enctype attribute to multipart/form-data .

    • The file upload form is demonstrated in example 1.

    Example 1: Creating a field to send a file

    HTML5 IE Cr Op Sa Fx

    Sending a file to the server

    Although you can set the width of a field using the size attribute, the width actually has no effect on the form's output. IN Safari browsers and in Chrome this attribute has no effect at all.

    The multiple attribute is more important; it allows you not to be limited to one file for selection, but to specify several of them at once for simultaneous loading.

    If the accept attribute is not specified, then files of any type are added and loaded. The presence of accept allows you to limit the file selection, which is especially important when you only need to upload an image or video. The value is , several values ​​are separated by a comma. You can also use the following keywords:

    In table Figure 2 shows some valid values ​​for the accept attribute.

    The use of additional attributes is shown in example 2.

    HTML5 IE 10+ Cr Op Sa Fx

    Upload your photos to the server

    Not all browsers support the new attributes. IE completely ignores multiple and accept , Safari doesn't support accept , and Firefox doesn't work with MIME type, only with keywords. Therefore, in the example above, the value is set specifically for Firefox to image/*,image/jpeg . Also note a strange bug in Opera that doesn't allow spaces after commas inside accept .

    The result of the example is shown in Fig. 3. Please note that due to the presence of multiple, the appearance of the field has changed slightly.

    • Translation
    • Tutorial

    Fast and smooth loading of images is one of the important components of a good web interface. In addition, more and more sites are appearing that use large photographs in design; for such projects it is especially important to monitor the correct loading of graphics. This article describes several techniques that will help you control image loading.

    Using a container for each image A simple method that can be applied to any image on the site. The trick is that each image is wrapped in a DIV, which prevents row-by-row loading:


    Using the container, you can control the aspect ratio of the image and also use the loading indicator, which is very convenient if the images are heavy.

    For example, to set the aspect ratio to 4:3, you could use the following CSS:

    Img_wrapper( position: relative; padding-top: 75%; overflow: hidden; ) .img_wrapper img( position: absolute; top: 0; width: 100%; opacity: 0; )
    To ensure that the image is displayed in the browser only after it has been fully loaded, you need to add an onload event for the image and use JavaScript to handle the event:


    function imgLoaded(img)( var $img = $(img); $img.parent().addClass("loaded"); );
    Function code inside HEAD tag should be located at the very end, after any jQuery or other plugin. After the image is fully loaded, it must be shown on the page:

    Img_wrapper.loaded img( opacity: 1; )
    For the effect of a smooth appearance of an image, you can use CSS3 transition:

    Img_wrapper img( position: absolute; top: 0; width: 100%; opacity: 0; -webkit-transition: opacity 150ms; -moz-transition: opacity 150ms; -ms-transition: opacity 150ms; transition: opacity 150ms; )
    A live example of this method is available.

    Using a container for many images The previous method works well for individual images, but what if there are a lot of them on the page, for example a photo gallery or a slider? It is not advisable to upload everything at once - the pictures can weigh a lot. To solve this problem, you can force JavaScript to load only what you need into this moment image time. Example HTML markup for a slideshow:


    We use the slideLoaded() function to control the process:

    Function slideLoaded(img)( var $img = $(img), $slideWrapper = $img.parent(), total = $slideWrapper.find("img").length, percentLoaded = null; $img.addClass("loaded "); var loaded = $slideWrapper.find(".loaded").length; if(loaded == total)( percentLoaded = 100; // INSTANTIATE PLUGIN $slideWrapper.easyFader(); ) else ( // TRACK PROGRESS percentLoaded = loaded/total * 100);
    Loaded images are assigned the loaded class, and the overall progress is displayed. Again, JavaScript should be placed at the end of the HEAD tag, after everything else.

    Caching On graphically heavy sites you can background, unnoticed by the user, load images into the browser cache. For example, there is a multi-page website, on one of internal pages which has a lot of graphic content. In this case, it would be advisable to load images into the cache even before the user switches to desired page. addresses of images in the array:

    var heroArray = [ "/uploads/hero_about.jpg", "/uploads/hero_history.jpg", "/uploads/hero_contact.jpg", "/uploads/hero_services.jpg" ]
    When a visitor enters the site, after loading home page, images begin to be loaded into the cache. To ensure that caching does not interfere with the display of current content, you need to add JavaScript functionality to the window load event:

    Function preCacheHeros())( $.each(heroArray, function())( var img = new Image(); img.src = this; )); ); $(window).load(function())( preCacheHeros(); ));
    This method improves the usability of the site, but puts additional load on the server. This must be kept in mind when implementing such functionality. In addition, it is necessary to take into account possible ways visitors to the site and cache images located on pages that the user is most likely to visit. To understand such paths through the site, it is necessary to analyze traffic statistics.

    The event-based loading method is that images begin to be loaded after a certain event. This increases productivity and saves user traffic. HTML markup:


    It's worth noting that the image URL is specified in data-src, not src. This is necessary so that the browser does not download the image immediately. Instead, src is loaded with a base64 transparent GIF pixel, which reduces the number of round trips to the server.

    All that remains is to change the src value to data-src when the desired event occurs. JavaScript allows you to load images incrementally:

    Function lazyLoad())( var $images = $(".lazy_load"); $images.each(function())( var $img = $(this), src = $img.attr("data-src"); $ img .on("load",imgLoaded($img)) .attr("src",src )); $(window).load(function())( lazyLoad(); );

    Conclusion There is not one the best way in order to manage the loading of images on the site. In each specific case, it is necessary to choose the appropriate method, and also combine several, if appropriate. One of the main points that you need to pay attention to is performance and traffic. Simply put, first of all you should think about how it will be more convenient for the user site.