Async material autocomplete in Angular

Topic

While working on my company project, I get the task of making a country selector. The project is using Angular with Angular Material. This is how I made it.

Prerequisites

For the demo version, I will do a simple angular project with that field only.
To make Angular project type in the command line:

ng new async-autocomplete

I also used the default Angular Material setup by typing.

ng add @angular/material

Now my demo project is ready.

Http Service

To be able to make HTTP calls in my AppModule I imported HttpClientModule from @angular/common/HTTP.
In the app directory, I generated a service which is used for making HTTP call. I typed the command:

ng g service country

which produced the country.service.ts file for me.
In that service, I used HttpClient in the constructor imported from @angular/common/http.
Method for getting countries list

getByName(name: string): Observable<string[]> {
    return this.http
      .get<Country[]>(`https://restcountries.eu/rest/v2/name/${name}`)
      .pipe(map(countryList => countryList.map(({ name }) => name)));
  }

• Country is just a simple interface with the name property.
• Here is the documentation for the URL which I used.
• map is an operator for mapping value inside observable (I am just pulling out country name)

The input

For the field I imported 3 modules in AppModule:

• MatFormFieldModule and MatInputModule is used by the field
• MatAutocompleteModule for autocompletion
• ReactiveFormsModule because the field is used inside reactive form.

The HTML template is quite simple:

<form [formGroup]="form">

  <mat-form-field appearance="fill">
    <mat-label>Name</mat-label>
    <input matInput formControlName="name" [matAutocomplete]="auto">
  </mat-form-field>

</form>

<mat-autocomplete #auto="matAutocomplete">
  <mat-option *ngFor="let countryName of countries$ | async" [value]="countryName">
    {{countryName}}
  </mat-option>
</mat-autocomplete>

There are two important things:

• [matAutocomplete]="auto" is an attribute which connects field with autocompletion list
• async pipe, which subscribes to observable and unsubscribe when the component is destroyed.

My component ts code has two properties:

  countries$: Observable<string[]>;
  form = this.formBuilder.group({
    name: [null],
  });

• countries$ which holds my countries list
• form reactive form definition

In constructor definition:

  constructor(
    private formBuilder: FormBuilder,
    private countryService: CountryService,
  ) {

• formBuilder for reactive form creation
• countryService for using the HTTP method defined in service.

On every input value change, I am switching to service to make GET call for a list and I am assigning it to my observable:

    this.countries$ = this.form.get('name')!.valueChanges.pipe(
      distinctUntilChanged(),
      debounceTime(1000),
      filter((name) => !!name),
      switchMap(name => this.countryService.getByName(name))
    );

• valueChanges which triggers every value change (It is an Observable)
• distinctUntilChanged operator which emits only when the value is different than the previous one (avoid making requests for the same name one after another)
• debounceTime operator to avoid spamming API with too many calls in a short time (It waits 1000ms and if the value is not emitted, then emits last value)
• filter operator which checks if there is the value (avoid HTTP calls with no name)
• switchMap operator which is changing from one observable (valueChanges) to another (getByName from service).

Full TS code:

import { Component, OnInit } from '@angular/core';
import { FormBuilder, FormGroup } from '@angular/forms';
import { Observable } from 'rxjs';
import { debounceTime, distinctUntilChanged, filter, switchMap } from 'rxjs/operators';
import { CountryService } from './country.service';

@Component({
  selector: 'app-root',
  templateUrl: './app.component.html',
  styleUrls: ['./app.component.scss']
})
export class AppComponent {
  countries$: Observable<string[]>;
  form = this.formBuilder.group({
    name: [null],
  });

  constructor(
    private formBuilder: FormBuilder,
    private countryService: CountryService,
  ) {
    this.countries$ = this.form.get('name')!.valueChanges.pipe(
      distinctUntilChanged(),
      debounceTime(1000),
      filter((name) => !!name),
      switchMap(name => this.countryService.getByName(name))
    );
  }
}

Link to repo.