Question

打击代码来自ngrx/example中的bookexist路线卫我知道它正在调用googleBooks服务来发出http请求并检查该书是否已经存储在商店中。我无法理解的部分是它没有在警卫服务中的任何地方调用subscribe。我的理解是，Anuglar2中的http Observables被认为是冷可观察量，这意味着在有人订阅它之前它们不会被调用。

我的问题是：如何调用以下googleBooks服务？

hasBookInApi(id: string): Observable<boolean> {
  return this.googleBooks.retrieveBook(id)
    .map(bookEntity => new book.LoadAction(bookEntity))
    .do((action: book.LoadAction) => this.store.dispatch(action))
    .map(book => !!book)
    .catch(() => {
      this.router.navigate(['/404']);
      return of(false);
    });
}

Answer 1

通过the CanActivate implementation中组成的观察者来调用它：

/**
 * This is the actual method the router will call when our guard is run.
 *
 * Our guard waits for the collection to load, then it checks if we need
 * to request a book from the API or if we already have it in our cache.
 * If it finds it in the cache or in the API, it returns an Observable
 * of `true` and the route is rendered successfully.
 *
 * If it was unable to find it in our cache or in the API, this guard
 * will return an Observable of `false`, causing the router to move
 * on to the next candidate route. In this case, it will move on
 * to the 404 page.
 */
canActivate(route: ActivatedRouteSnapshot): Observable<boolean> {
  return this.waitForCollectionToLoad()
    .switchMap(() => this.hasBook(route.params['id']));
}

CanActivate是一个Angular接口，由路由器调用。 CanActivate的实现可以返回一个observable，一个promise或一个boolean。当它返回一个observable时，路由器会订阅它 - 它会看到对它内部组成的observable进行的订阅，并最终 - 对你在问题中包含的hasBookInApi返回的可观察对象进行订阅。

Rxjs如何调用冷Observables

1 个答案: