1b-brief-introduction-to-angular.md

description
In this section we will use angular services and review observables with RxJS

1b - Brief Introduction To Angular

1. Add a user service

ng g service services/user/user

2. Add HttpClientModule to AppModule

{% code title="src/app/app.module.ts" %}

import { BrowserModule } from '@angular/platform-browser';
import { NgModule } from '@angular/core';

import { AppRoutingModule } from './app-routing.module';
import { AppComponent } from './app.component';
import { HomeComponent } from './home/home.component';
import { HttpClientModule } from '@angular/common/http'; // Added

@NgModule({
  declarations: [
    AppComponent,
    HomeComponent
  ],
  imports: [
    BrowserModule,
    AppRoutingModule,
    HttpClientModule     // Added
  ],
  providers: [],
  bootstrap: [AppComponent]
})
export class AppModule { }

{% endcode %}

3. Make a User service

Add some fake data in a user.json file in the assets folder of your new app. This is in place of a URL to hit a real API, for no.

{% code title="src/assets/users.json" %}

[{ "name": "duncan" }, { "name": "sarah" }, { "name": "peter" }]

{% endcode %}

Make the service and inject in the HttpClient

{% code title="src/app/services/user/user.service.ts" %}

import { Injectable } from "@angular/core";
import { HttpClient } from "@angular/common/http";

@Injectable({
  providedIn: 'root'
})
export class UserService {
  apiUrl = './../../../assets/users.json';

  constructor(private httpClient: HttpClient) {}

  getUsers(): Observable<any> { {
    return this.httpClient.get(this.apiUrl);
  }
}

{% endcode %}

4. Add a UserList component and inject in the User service

Make a user-list component, 'c' is short hand for component.

ng g c user-list

Add a new eagerly loaded route for the UserList component

{% code title="src/app/app-routing.module.ts" %}

import { NgModule } from '@angular/core';
import { Routes, RouterModule } from '@angular/router';
import { HomeComponent } from './home/home.component';
import { UserListComponent } from './user-list/user-list.component'; //added

const routes: Routes = [
  { path: '', component: HomeComponent },
  { path: 'user-list', component: UserListComponent }, //added
];

@NgModule({
  imports: [RouterModule.forRoot(routes)],
  exports: [RouterModule]
})
export class AppRoutingModule { }

{% endcode %}

Check the route works by navigating to http://localhost:4200/user-list

{% hint style="info" %} Here we do the simple async pipe
We will stop and discuss observables and their operators in slides here https://docs.google.com/presentation/d/1xf8aPIvQjgjUVGH_1sRkikvh5H73x2xvX7PnN4AjYt4/edit?usp=sharing {% endhint %}

Inject user service and call its getUsers method and bind in to a local variable

{% code title="src/app/user-list/user-list.component.ts" %}

import { Component, OnInit } from '@angular/core';
import { UserService } from '../services/user/user.service';

@Component({
  selector: 'app-user-list',
  templateUrl: './user-list.component.html',
  styleUrls: ['./user-list.component.css']
})
export class UserListComponent implements OnInit {
  users$: any;
  constructor(private userService: UserService) {}

  ngOnInit() {
    this.users$ = this.userService.getUsers();
  }
}

{% endcode %}

{% hint style="info" %} The below is an example of subscribing in the component. Try it out but we will be using the async pipe. We bind to the observable in the rest of the demo so make sure to change back. {% endhint %}

{% code title="src/app/user-list/user-list.component.ts" %}

import { Component, OnInit, OnDestroy } from '@angular/core';
import { UserService } from '../services/user/user.service';
import { takeWhile } from 'rxjs/operators';
import { User } from '../models/user';

@Component({
  selector: 'app-user-list',
  templateUrl: './user-list.component.html',
  styleUrls: ['./user-list.component.css']
})
export class UserListComponent implements OnInit, OnDestroy {
  users: User[];
  componentActive = true;

  constructor(private userService: UserService) {}

  ngOnInit() {
    this.userService
      .getUsers()
      .pipe(takeWhile(() => this.componentActive))
      .subscribe((users: User[]) => (this.users = users));
  }

  ngOnDestroy(): void {
    this.componentActive = false;
  }
}

{% endcode %}

Use the async pipe to and an ngFor template directive to subscribe and iterate over the users.

5. Use Angular ngFor to bind users

{% code title="src/app/user-list/user-list.component.html" %}

<p>
  user-list works!
</p>
<div *ngFor="let user of (users$ | async)"> {{user.name}}</div>

{% endcode %}

6. Add strong typing

Add User type to a models folder

ng g interface models/user

{% code title="models/user.ts" %}

export interface User {
  name: string;
}

{% endcode %}

Add User type to the service

{% code title="src/app/services/user/user.service.ts" %}

export class UserService {
  apiUrl = './../../../assets/users.json';            //changed
​
  constructor(private httpClient: HttpClient) {}
​
  getUsers(): Observable<User[]> {                    //changed
    return this.httpClient.get<User[]>(this.apiUrl);  //changed
  }
}

{% endcode %}

Add User type to the UserList component

{% code title="src/app/user-list/user-list.component.ts" %}

import { Component, OnInit } from '@angular/core';
import { UserService } from '../services/user/user.service';
import { Observable } from 'rxjs';     //changed
import { User } from '../models/user'; //changed

@Component({
  selector: 'app-user-list',
  templateUrl: './user-list.component.html',
  styleUrls: ['./user-list.component.css']
})
export class UserListComponent implements OnInit {
  users$: Observable<User[]>;                      //changed

  constructor(private userService: UserService) {}

  ngOnInit() {
    this.users$ = this.userService.getUsers();
  }
}

{% endcode %}

Extra reading

Naming observables - https://medium.com/@benlesh/observables-are-just-functions-but-also-collections-how-do-i-name-them-918c5ce2f64