为什么在这个 Angular 2 组件中该服务被调用两次？

Question

我这里有组件代码，当我订阅可观察对象时，服务被调用两次，但是如果我订阅Behavioursubject，它只会触发一次，

我可以在我的日志中看到这些结果，请参阅下面我的组件代码，在 ngOninit 上调用 subscribeToMap() 方法。

import { Component, OnInit } from '@angular/core';
import { Router }            from '@angular/router';

import { Observable }        from 'rxjs/Observable';
import { Subject }           from 'rxjs/Subject';

// Observable class extensions
import 'rxjs/add/observable/of';

// Observable operators
import 'rxjs/add/operator/catch';
import 'rxjs/add/operator/debounceTime';
import 'rxjs/add/operator/distinctUntilChanged';

import { HeroSearchService } from './hero-search-service';
import { Hero } from './../hero';

@Component({
  selector: 'hero-search',
  templateUrl: './hero-search.component.html',
  styleUrls: [ './hero-search.component.css' ],
  providers: [HeroSearchService]
})
export class HeroSearchComponent implements OnInit {
  heroes: Observable<Hero[]>;
  private searchTerms = new Subject<string>();

  constructor(
    private heroSearchService: HeroSearchService,
    private router: Router) {}

  // Push a search term into the observable stream.
  search(term: string): void {
    this.searchTerms.next(term);
    console.log("new " + term);
  }



  ngOnInit(): void {
    this.heroes = this.searchTerms
      .debounceTime(300)        // wait 300ms after each keystroke before considering the term
      .distinctUntilChanged()   // ignore if next search term is same as previous
      .switchMap(term => {
        return term   // switch to new observable each time the term changes
        // return the http search observable
        ? this.heroSearchService.search(term)
        // or the observable of empty heroes if there was no search term
        : Observable.of<Hero[]>([])})
      .catch(error => {
        // TODO: add real error handling
        console.log(error);
        return Observable.of<Hero[]>([]);
      });
      this.subscribeToMap();
  }

  subscribeToMap(): void{
     this.heroes.subscribe(() => console.log("called twice"));
     this.searchTerms.subscribe(() => console.log("called once"));
  }


  gotoDetail(hero: Hero): void {
    let link = ['/detail', hero.id];
    this.router.navigate(link);
  }
}

这是我的服务的代码

import { Injectable } from '@angular/core';
import { Http }       from '@angular/http';

import { Observable }     from 'rxjs/Observable';
import 'rxjs/add/operator/map';

import { Hero }           from './../hero';

@Injectable()
export class HeroSearchService {

  constructor(private http: Http) {}

  search(term: string): Observable<Hero[]> {
    console.log("service is called");
    return this.http
               .get(`api/heroes/?name=${term}`)
               .map(response => response.json().data as Hero[]);
  }
}

非常感谢！

Answer 1

当订阅正确实现时，它与“取消订阅”方法、Observable 等无关。这种行为是 Angular 本身的设计。

https://www.reddit.com/r/Angular2/comments/59532r/function_being_used_multiple_times/d95vjlz/

如果您在开发模式下运行，它将至少运行该函数两次。因为在开发模式下，它会进行检查、更改，然后重新检查以进行验证，而生产模式仅进行第一次检查，假设您已完成质量保证并解决了检查后更改的任何值。

PS 这可能是您在开发模式中将面临的下一个问题:)

Angular2更改检测“表达式在检查后已更改”