为什么 Ngrx 选择器在没有订阅的情况下触发？

Question

我的问题围绕着代码的和平

@Injectable()
export class TokenInterceptor implements HttpInterceptor {
constructor(private store: Store<fromAuth.State>) {}

intercept(req: HttpRequest<any>, next: HttpHandler) {
  return this.store.select(fromAuth.getToken).pipe(
   first(),
   flatMap(token => {
    const authReq = !!token ? req.clone({
      setHeaders: { Authorization: 'Bearer ' + token },
    }) : req;
    return next.handle(authReq);
   },
  );
 }
}

没看懂operator first()的必要性，作者给出了解释

The observable that we return from the intercept method begins with the store selector. Since this 
observable will form part of the chain when creating a new HttpClient request and subscribing, we 
don’t want to send an update everytime the token changes in the future, else the services using 
HttpClient will appear to get a new value from their request if not unsubscribed. Thus we use the 
first() operator here to only take the first value, then complete

select select 或 returns 一个可观察对象，应该在每次状态改变时触发存储，但是对 select

返回的可观察对象的订阅在哪里

link转至原文：https://antonyderham.me/post/angular-ngrx-auth-interceptor/

Answer 1

它是拦截器处理程序的内部实现。它订阅 intercept 方法的结果并使用发出的值发送请求。

它看起来类似于

const interceptor = new TokenInterceptor(store);
interceptor.intercept(new HttpRequest('POST', '/test', {}), backend).subscribe();

但它在 angular 的幕后。

为什么 Ngrx 选择器在没有订阅的情况下触发？

why the Ngrx selector fires without subscription?

angular

observable

ngrx

subscription

ngrx-store