Rxjs如何调用冷Observables

问题描述：

吹码是来自bookexist护卫ngrx/example 据我所知，它打电话googleBooks服务做出一个http请求，并检查该书是否已经在商店。我无法理解的部分是它不会在护卫服务的任何地方拨打subscribe。我的理解是，Anuglar2中的http Observables被认为是冷观察对象，这意味着它们不会被调用，直到有人订阅它。Rxjs如何调用冷Observables

我的问题是：如何调用下面的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); 
    }); 
}

答

它通过可观察到的是在the CanActivate implementation由被称为：

/** 
* 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是角界面，它是由路由器调用。 CanActivate的实现可以返回一个observable，一个promise或一个boolean。当它返回一个可观察对象时，路由器会订阅它 - 它看到了对它构成的观察值的订阅，并且最终 - 到您在问题中包含的hasBookInApi返回的可观察值。

Rxjs如何调用冷Observables

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