@impactdk/ngx-lazyload

ngx-lazyload

A module to easily add lazyloading of images and picture tags in your Angular application

Installation

Import the NgxLazyloadModule from @impactdk/ngx-lazyload in the module you want to use the directive / LazyPicture component in. If you wish you can add configs in the forRoot/forFeature method on the module.

You also need to provide the ObserverService from @impactdk/ngx-lazyload in your app.module

import { NgModule } from '@angular/core';
import { AppComponent } from './app.component';
import { BrowserModule } from '@angular/platform-browser';

import { NgxLazyloadModule, ObserverService } from '@impact/ngx-lazyload';

@NgModule({
  imports: [BrowserModule, NgxLazyloadModule],
  declarations: [AppComponent],
  bootstrap: [AppComponent],
  providers: [ObserverService],
})
export class AppModule {}

The config has a few options:

interface ObserverServiceConfig {
    root?: Element | null;
    rootMargin?: string;
    threshold?: number | number[];
}

root: The element that is used as the viewport for checking visiblity of the target. Must be the ancestor of the target. Defaults to the browser viewport if not specified or if null.

rootMargin: Margin around the root. Can have values similar to the CSS margin property, e.g. "10px 20px 30px 40px" (top, right, bottom, left). If the root element is specified, the values can be percentages. This set of values serves to grow or shrink each side of the root element's bounding box before computing intersections. Defaults to all zeros.

threshold: Either a single number or an array of numbers which indicate at what percentage of the target's visibility the observer's callback should be executed. If you only want to detect when visibility passes the 50% mark, you can use a value of 0.5. If you want the callback run every time visibility passes another 25%, you would specify the array [0, 0.25, 0.5, 0.75, 1]. The default is 0 (meaning as soon as even one pixel is visible, the callback will be run). A value of 1.0 means that the threshold isn't considered passed until every pixel is visible.

Usage

This module provides lazy loading for 2 use cases, a normal img tag and a component to generate picture tags with lazy loading of sources.

The img tag

We have created a LazeLoad directive that you can apply to any img tag. It has two inputs LazyLoadSrc and LazyLoadSrcSet (Optional).

Useage:

<img Lazyload [LazyLoadSrc]="src" [LazyLoadSrcSet]="srcSet" alt="A fancy image" >

The picture component

We have also created a component to generate lazy loaded picture tags with sources within. To use it simply add the LazyPicture component and supply the the inputs 3 inputs:

• LazyLoadSrc - The source of the default image
• LazyLoadAlt - The alt tag of the image (Optional)
• LazyLoadSources - An array of sources (Optional)

The sources should have the following format:

interface LazyLoadSource {
    media?: string; // "(max-width: 700px)"
    srcSet?: string; // "stick-figure-narrow.png 138w, stick-figure-hd-narrow.png 138w"
    sizes?: string; // "(max-width: 500px) 50vw, 10vw"
}

Useage:

<LazyPicture *ngFor="let picture of pictures"
             [LazyLoadSrc]="picture.src"
             [LazyLoadAlt]="picture.alt"
             [LazyLoadSources]="picture.sources">
</LazyPicture>

SSR Support

The module does support Server side rendering, however by default it will simply apply the sources on the server rendering.

This ofcourse means that any pictures that should have been lazy loaded, will on the first request not be and have all the sources set in place.

It is however possible to fix this by injecting a boolean called "isBot" in your application, if its false it will not apply the sources on the server, if its true it will assume that its a crawler or bot from a search engine or social media that should be provided with the image, and apply the sources right away.

We recommend that you use isBot and inject it in your Express server like this:

const {
    provideModuleMap,
} = require('@nguniversal/module-map-ngfactory-loader');
app.engine('html', (_, options, callback) => {
    const isCrawler = isBot(options.req.headers['user-agent']);
    renderModuleFactory(AppServerModuleNgFactory, {
        // Our index.html
        document: template,
        url: options.req.url,
        // DI so that we can get lazy-loading to work differently (since we need it to just instantly render it)
        extraProviders: [
            provideModuleMap(LAZY_MODULE_MAP),
            {
                provide: 'isBot',
                useFactory: () => isCrawler,
                deps: [],
            },
        ],
    }).then(html => {
        callback(null, html);
    });
});

Further help

Reach out to MHO (Martin Hobert)