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How to Lazy Load Scripts and Styles

You can use the LazyLoadService in @abp/ng.core package in order to lazy load scripts and styles in an easy and explicit way.

Getting Started

You do not have to provide the LazyLoadService at module or component level, because it is already provided in root. You can inject and start using it immediately in your components, directives, or services.

import { LazyLoadService } from '@abp/ng.core';

@Component({
  /* class metadata here */
})
class DemoComponent {
  constructor(private lazyLoadService: LazyLoadService) {}
}

Usage

You can use the load method of LazyLoadService to create a <script> or <link> element in the DOM at the desired position and force the browser to download the target resource.

How to Load Scripts

The first parameter of load method expects a LoadingStrategy. If you pass a ScriptLoadingStrategy instance, the LazyLoadService will create a <script> element with given src and place it in the designated DOM position.

import { LazyLoadService, LOADING_STRATEGY } from '@abp/ng.core';

@Component({
  template: `
    <some-component *ngIf="libraryLoaded$ | async"></some-component>
  `
})
class DemoComponent {
  libraryLoaded$ = this.lazyLoad.load(
    LOADING_STRATEGY.AppendAnonymousScriptToHead('/assets/some-library.js'),
  );

  constructor(private lazyLoadService: LazyLoadService) {}
}

The load method returns an observable to which you can subscibe in your component or with an async pipe. In the example above, the NgIf directive will render <some-component> only if the script gets successfully loaded or is already loaded before.

You can subscribe multiple times in your template with async pipe. The styles will only be loaded once.

Please refer to LoadingStrategy to see all available loading strategies and how you can build your own loading strategy.

How to Load Styles

If you pass a StyleLoadingStrategy instance as the first parameter of load method, the LazyLoadService will create a <link> element with given href and place it in the designated DOM position.

import { LazyLoadService, LOADING_STRATEGY } from '@abp/ng.core';

@Component({
  template: `
    <some-component *ngIf="stylesLoaded$ | async"></some-component>
  `
})
class DemoComponent {
  stylesLoaded$ = this.lazyLoad.load(
    LOADING_STRATEGY.AppendAnonymousStyleToHead('/assets/some-styles.css'),
  );

  constructor(private lazyLoadService: LazyLoadService) {}
}

The load method returns an observable to which you can subscibe in your component or with an AsyncPipe. In the example above, the NgIf directive will render <some-component> only if the style gets successfully loaded or is already loaded before.

You can subscribe multiple times in your template with async pipe. The styles will only be loaded once.

Please refer to LoadingStrategy to see all available loading strategies and how you can build your own loading strategy.

Advanced Usage

You have quite a bit of freedom to define how your lazy load will be implemented. Here is an example:

const domStrategy = DOM_STRATEGY.PrependToHead();

const crossOriginStrategy = CROSS_ORIGIN_STRATEGY.Anonymous(
  'sha384-Vkoo8x4CGsO3+Hhxv8T/Q5PaXtkKtu6ug5TOeNV6gBiFeWPGFN9MuhOf23Q9Ifjh',
);

const loadingStrategy = new StyleLoadingStrategy(
  'https://stackpath.bootstrapcdn.com/bootstrap/4.4.1/css/bootstrap.min.css',
  domStrategy,
  crossOriginStrategy,
);

this.lazyLoad.load(loadingStrategy, 1, 2000);

This code will create a <link> element with given url and integrity hash, insert it to to top of the <head> element, and retry once after 2 seconds if first try fails.

API

loaded: Set

All previously loaded paths are available via this property. It is a simple JavaScript Set.

load(strategy: LoadingStrategy, retryTimes?: number, retryDelay?: number): Observable

strategy parameter is the primary focus here and is explained above.
retryTimes defines how many times the loading will be tried again before fail (default: 2).
retryDelay defines how much delay there will be between retries (default: 1000).

What's Next?

TrackByService

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