Symfony UX LazyImage

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Symfony UX LazyImage

Symfony UX LazyImage is a Symfony bundle providing utilities to improve image loading performance. It is part of the Symfony UX initiative.

It provides two key features:

  • a Stimulus controller to load lazily heavy images, with a placeholder
  • a BlurHash implementation to create data-uri thumbnails for images

Installation

Before you start, make sure you have Symfony UX configured in your app.

Then install this bundle using Composer and Symfony Flex:

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$ composer require symfony/ux-lazy-image

# Don't forget to install the JavaScript dependencies as well and compile
$ npm install --force
$ npm run watch

# or use yarn
$ yarn install --force
$ yarn watch

Also make sure you have at least version 3.0 of @symfony/stimulus-bridge in your package.json file.

Usage

The default usage of Symfony UX LazyImage is to use its Stimulus controller to first load a small placeholder image that will then be replaced by the high-definition version once the page has been rendered:

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<img
    src="{{ asset('image/small.png') }}"
    {{ stimulus_controller('symfony/ux-lazy-image/lazy-image', {
        src: asset('image/large.png')
    }) }}

    {# Optional but avoids having a page jump when the image is loaded #}
    width="200"
    height="150"
>

With this setup, the user will initially see images/small.png. Then, once the page has loaded and the user’s browser has downloaded the larger image, the src attribute will change to image/large.png.

There is also support for the srcset attribute by passing an srcset value to the controller:

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<img
    src="{{ asset('image/small.png') }}"
    srcset="{{ asset('image/small.png') }} 1x, {{ asset('image/small2x.png') }} 2x"

    {{ stimulus_controller('symfony/ux-lazy-image/lazy-image', {
        src: asset('image/large.png'),
        srcset: {
            '1x': asset('image/large.png'),
            '2x': asset('image/large2x.png')
        }
    }) }}
/>

Note

The stimulus_controller() function comes from WebpackEncoreBundle v1.10.

Instead of using a generated thumbnail that would exist on your filesystem, you can use the BlurHash algorithm to create a light, blurred, data-uri thumbnail of the image:

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<img
    src="{{ data_uri_thumbnail('public/image/large.png', 100, 75) }}"
    {{ stimulus_controller('symfony/ux-lazy-image/lazy-image', {
        src: asset('image/large.png')
    }) }}

    {# Using BlurHash, the size is required #}
    width="200"
    height="150"
/>

The data_uri_thumbnail function receives 3 arguments:

  • the server path to the image to generate the data-uri thumbnail for ;
  • the width of the BlurHash to generate
  • the height of the BlurHash to generate

You should try to generate small BlurHash images as generating the image can be CPU-intensive. Instead, you can rely on the browser scaling abilities by generating a small image and using the width and height HTML attributes to scale up the image.

Extend the default behavior

Symfony UX LazyImage allows you to extend its default behavior using a custom Stimulus controller:

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// mylazyimage_controller.js

import { Controller } from '@hotwired/stimulus';

export default class extends Controller {
    connect() {
        this.element.addEventListener('lazy-image:connect', this._onConnect);
        this.element.addEventListener('lazy-image:ready', this._onReady);
    }

    disconnect() {
        // You should always remove listeners when the controller is disconnected to avoid side-effects
        this.element.removeEventListener('lazy-image:connect', this._onConnect);
        this.element.removeEventListener('lazy-image:ready', this._onReady);
    }

    _onConnect(event) {
        // The lazy-image behavior just started
    }

    _onReady(event) {
        // The HD version has just been loaded
    }
}

Then in your template, add your controller to the HTML attribute:

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<img
    src="{{ data_uri_thumbnail('public/image/large.png', 100, 75) }}"
    {{ stimulus_controller({
        mylazyimage: {},
        'symfony/ux-lazy-image/lazy-image': {
            src: asset('image/large.png')
        }
    }) }}

    {# Using BlurHash, the size is required #}
    width="200"
    height="150"
/>

Note: be careful to add your controller before the LazyImage controller so that it is executed before and can listen on the lazy-image:connect event properly.

Backward Compatibility promise

This bundle aims at following the same Backward Compatibility promise as the Symfony framework: https://symfony.com/doc/current/contributing/code/bc.html

However it is currently considered experimental, meaning it is not bound to Symfony’s BC policy for the moment.

This work, including the code samples, is licensed under a Creative Commons BY-SA 3.0 license.