Guides / Building Search UI / UI & UX patterns

Infinite scrolling is a very common pattern to display a list of results. This pattern has two variants:

  • With a button to click on at the end of the list of results
  • With a listener on the scroll event that is called when the list of results reaches the end

Angular InstantSearch covers those two different implementations. The former is covered by the built-in ais-infinite-hits widget and the latter is covered by the infiniteHits connector. This guide focuses on the second implementation using the Intersection Observer API. A browser API is used in the example, but the concepts can be applied to any kind of infinite scroll library. You can find the complete example on GitHub.

The Intersection Observer API isn’t yet widely supported. You may want to consider using a polyfill.

Display a list of hits

The first step to creating the infinite scroll component is to render the results with the ais-infinite-hits connector. There’s an external Hit component but it’s not the point of this guide, the intent is to keep the code simple. You can find more information about the connectors API in the dedicated guide about widget customisation.

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<!-- app.component.html -->
<ais-instantsearch [config]="config">
    <ais-search-box></ais-search-box>
    <ais-infinite-hits>
        <ng-template let-hits="hits" let-results="results" let-refine="showMore" >
            <div *ngFor="let hit of hits">
                <ais-highlight attribute="name" [hit]="hit"></ais-highlight>
            </div>
            <button (click)="refine()">Show More</button>
        </ng-template>
    </ais-infinite-hits>
</ais-instantsearch>
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// app.component.ts
const searchClient = algoliasearch(
  "latency",
  "6be0576ff61c053d5f9a3225e2a90f76"
);

@Component({
  selector: "app-root",
  templateUrl: "./app.component.html",
  styleUrls: ["./app.component.css"]
})
export class AppComponent {
  public config = {
    indexName: "instant_search",
    searchClient
  };
}
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import { BrowserModule } from "@angular/platform-browser";
import { NgModule } from "@angular/core";
import { NgAisModule } from "angular-instantsearch";

import { AppComponent } from "./app.component";

@NgModule({
  declarations: [AppComponent],
  imports: [NgAisModule.forRoot(), BrowserModule],
  providers: [],
  bootstrap: [AppComponent]
})
export class AppModule {}

Track the scroll position

Once you have your list of results, the next step is to track the scroll position to determine when the rest of the content needs to be loaded. For this purpose, the Intersection Observer API is used. To track when the bottom of the list enters the viewport, you observe a “sentinel” element. This trick is used to avoid observing all the items of the results. You can reuse the same element across the different renders. You can find more information about this pattern on the Web Fundamentals website.

Now create a simple directive that detects when an element is visible inside the viewport using the Intersection Observer API.

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import { AfterViewInit, Directive, ElementRef, EventEmitter, OnDestroy, Output } from "@angular/core";

@Directive({ selector: "[onVisible]" })
export class OnVisibleDirective implements AfterViewInit, OnDestroy {
  @Output() public onVisible: EventEmitter<any> = new EventEmitter();

  private _intersectionObserver?: IntersectionObserver;

  constructor(private _element: ElementRef) {}

  public ngAfterViewInit() {
    this._intersectionObserver = new IntersectionObserver(entries => {
      this.checkForIntersection(entries);
    }, {});
    this._intersectionObserver.observe(<Element>this._element.nativeElement);
  }

  public ngOnDestroy() {
    this._intersectionObserver.disconnect();
  }

  private checkForIntersection = (
    entries: Array<IntersectionObserverEntry>
  ) => {
    entries.forEach((entry: IntersectionObserverEntry) => {
      const isIntersecting =
        (<any>entry).isIntersecting &&
        entry.target === this._element.nativeElement;

      if (isIntersecting) {
        this.onVisible.emit();
      }
    });
  };
}

Now declare it in the main app module and use it.

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import { BrowserModule } from "@angular/platform-browser";
import { NgModule } from "@angular/core";
import { NgAisModule } from "angular-instantsearch";

import { AppComponent } from "./app.component";

@NgModule({
  declarations: [AppComponent],
  imports: [NgAisModule.forRoot(), BrowserModule],
  providers: [],
  bootstrap: [AppComponent]
})
export class AppModule {}

Retrieve more results

Now that you can track when you reach the end of the results, you can use the showMore function exposed in the template to the onVisible directive.

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<!-- app.component.html -->
<ais-instantsearch [config]="config">
    <ais-search-box></ais-search-box>
    <ais-infinite-hits>
        <ng-template let-hits="hits" let-results="results" let-refine="showMore" >
            <div *ngFor="let hit of hits">
                <ais-highlight attribute="name" [hit]="hit"></ais-highlight>
            </div>
            <div (onVisible)="refine()"></div>
        </ng-template>
    </ais-infinite-hits>
</ais-instantsearch>

Go further than 1000 hits

By default, Algolia limits the number of hits you can retrieve for a query to 1000; when doing an infinite scroll, you usually don’t want to go over this limit.

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$index->setSettings([
  'paginationLimitedTo' => 1000
]);

Disabling the limit doesn’t mean that you can go until the end of the hits, but just that Algolia will go as far as possible in the index to retrieve results in a reasonable time.

Now you should have a complete infinite scroll experience.

The complete source code of the example is on GitHub.

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