I'm reading the Angular Guide about Routing & Navigation.
They use this code for retrieving the router's param 'id'
and using it to get a hero with the service
service:
ngOnInit() {
this.route.params
.switchMap((params: Params) => this.service.getHero(+params['id']))
.subscribe((hero: Hero) => this.hero = hero);
}
But I do not well understand what is the purpose of using the switchMap
operator in the above code.
The following code would not be the same?
ngOnInit() {
this.route.params
// NOTE: I do not use switchMap here, but subscribe directly
.subscribe((params: Params) => {
this.service.getHero(+params['id']).then(hero => this.hero = hero)
});
}
switchMap
is usually used when you have some async operation that is triggered by some prepended "event/stream".
The difference to e.g. flatMap
or concatMap
is, that as soon as the next trigger emits, the current async operation is canceled and re-triggered.
In your case this means, that as soon as the route-params change, your hero-service is automatically called again with the changed params and the previous call is canceled so you won't receive outdated data.
This is especially helpful for search-queries that might take longer then 200-300ms and are triggered while a user is typing.
The following code would not be the same?
No. While it might behave the same in many cases, if you imagine the following scenario:
getHero(4)
(a very slow request)getHero(1)
(a fast request)getHero(1)
completes -> hero is "1"getHero(4)
completes -> hero is now "4" but the last used param was "1"In such a case switchMap
would just discard the getHero(4)
-call since it is outdated as soon as a new trigger happens.