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]

})

export class LazyModule {

// ...

}

... but, here's where Angular's dependency-injection gets really exciting: any element in the component tree can override the LazyViewport provider, thereby causing descendant [lazySrc] instances to be associated with a more local viewport. To facilitate overriding of the LazyViewport service in a subtree of the component graph, I created a [lazyViewport] directive:

// Import the core angular services.

import { Directive } from "@angular/core";

import { ElementRef } from "@angular/core";

import { OnDestroy } from "@angular/core";

import { OnInit } from "@angular/core";

// Import the application components and services.

import { LazyViewport } from "./lazy-viewport.ts";

// ----------------------------------------------------------------------------------- //

// ----------------------------------------------------------------------------------- //

@Directive({

selector: "[lazyViewport]",

inputs: [ "offset: lazyViewportOffset" ],

// The primary role of this directive is to override the default LazyViewport

// instance at this point in the component tree. This way, any lazy-directives

// that are descendants of this element will receive this instance when using

// dependency-injection.

providers: [

{

provide: LazyViewport,

useClass: LazyViewport

}

]

})

export class LazyViewportDirective implements OnInit, OnDestroy {

public offset: number;

private elementRef: ElementRef;

private lazyViewport: LazyViewport;

// I initialize the lazy-viewport directive.

constructor(

elementRef: ElementRef,

lazyViewport: LazyViewport

) {

this.elementRef = elementRef;

this.lazyViewport = lazyViewport;

this.offset = 0;

}

// ---

// PUBLIC METHODS.

// ---

// I get called once when the directive is being destroyed.

public ngOnDestroy() : void {

this.lazyViewport.teardown();

}

// I get called once after the inputs have been bound for the first time.

public ngOnInit() : void {

// Ensure that the offset value is numeric when we go to initialize the viewport.

if ( isNaN( +this.offset ) ) {

console.warn( new Error( `[lazyViewportOffset] must be a number. Currently defined as [${ this.offset }].` ) );

this.offset = 0;

}

// Now that this LazyViewport directive has overridden the instance of

// LazyViewport in the dependency-injection tree, we have to initialize it

// to use the current element as the observer root.

this.lazyViewport.setup( this.elementRef.nativeElement, +this.offset );

}

}

As you can see, this directive defines a providers collection which overrides the injectable service associated with the LazyViewport dependency-injection token. By doing this, any [lazySrc] directive lower down in the component tree will get this LazyService override-instance rather than the global one:

this.lazyViewport.setup( this.elementRef.nativeElement, +this.offset );

At this time, I think it makes more sense to always go with the global instance. But, I just love how Angular's dependency-injection tree makes it so easy to change the viewport. Dependency-injection is just such a win!

Once I import the LazyModule into my AppModule (not worth showing), I can then use the [lazySrc] directive on my IMG tags. In this case, I've created a scrolling list of contacts with avatars. The avatars will only load when the contact is scrolled into view:

// Import the core angular services.

import { Component } from "@angular/core";

// ----------------------------------------------------------------------------------- //

// ----------------------------------------------------------------------------------- //

interface Contact {

id: number;

name: string;

avatarUrl: string;

}

@Component({

selector: "my-app",

styleUrls: [ "./app.component.less" ],

template:

`

<p>

<a (click)="toggleContacts()">Toggle Contacts</a>

</p>

<ul *ngIf="isShowingContacts" class="contacts">

<li *ngFor="let contact of contacts">

<img [lazySrc]="contact.avatarUrl" lazySrcVisible="visible" />

<span>{{ contact.name }} - {{ contact.id }}</span>

</li>

</ul>

<p>

<a (click)="popContact()">Pop Contact</a>

—

<a (click)="pushContact()">Push Contact</a>

</p>

`

})

export class AppComponent {

public contacts: Contact[];

public isShowingContacts: boolean;

public maxID: number;