6: Get Data from a Server

[examples/angular-tour-of-heroes] / src / app / hero.service.ts

diff --git a/src/app/hero.service.ts b/src/app/hero.service.ts
index 649b856..f8374b1 100644 (file)
--- a/src/app/hero.service.ts
+++ b/src/app/hero.service.ts
@@ -2,7 +2,9 @@ import { Injectable } from '@angular/core';
 import { Observable, of, EMPTY } from 'rxjs';
 import { Hero } from './hero';
 import { HEROES } from './mock-heroes';
+import { HttpClient, HttpHeaders } from '@angular/common/http';
 import { MessageService } from './message.service';
+import { catchError, tap } from 'rxjs/operators';
 
 
 @Injectable({
@@ -10,15 +12,24 @@ import { MessageService } from './message.service';
 })
 export class HeroService {
 
-  constructor(private messageService : MessageService) { }
+  private heroesUrl = 'api/heroes';  // URL to web api
 
-  getHeroes() : Observable<Hero[]> {
-    this.messageService.add('HeroService: fetching heroes...');
-    return of(HEROES);
+  constructor(
+    private http: HttpClient,
+    private messageService: MessageService) { }
+
+  getHeroes(): Observable<Hero[]> {
+    this.log('fetching heroes...');
+    return this.http
+      .get<Hero[]>(this.heroesUrl)
+      .pipe(
+        tap((heroes: Hero[]) => this.log(`fetched ${heroes.length} heroes`)),
+        catchError(this.handleError<Hero[]>('getHeroes', []))
+      );
   }
 
   getHero(id: number): Observable<Hero> {
-    this.messageService.add(`HeroService: requested hero id=${id}`);
+    this.log(`requested hero id=${id}`);
     const found: Hero | undefined = HEROES.find(hero => hero.id === id);
     if (found === undefined) {
       return EMPTY;
@@ -26,4 +37,29 @@ export class HeroService {
       return of(found);
     }
   }
+
+  /**
+   * Handle Http operation that failed.
+   * Let the app continue.
+   * @param operation - name of the operation that failed
+   * @param result - optional value to return as the observable result
+   */
+  private handleError<T>(operation = 'operation', result?: T) {
+    return (error: any): Observable<T> => {
+
+      // TODO: send the error to remote logging infrastructure
+      console.error(error); // log to console instead
+
+      // TODO: better job of transforming error for user consumption
+      this.log(`${operation} failed: ${error.message}`);
+
+      // Let the app keep running by returning an empty result.
+      return of(result as T);
+    };
+  }
+
+  /** Log a HeroService message with the MessageService */
+  private log(message: string) {
+    this.messageService.add(`HeroService: ${message}`);
+  }
 }