Taking advantage of Observables in Angular 2

Some people seem to be confused why Angular 2 seems to favor the Observable abstraction over the Promise abstraction when it comes to dealing with async behavior.

There are pretty good resources about the difference between Observables and Promises already out there. I especially like to highlight this free 7
minutes video by Ben Lesh on egghead.io.
Technically there are a couple of obvious differences like the disposability and lazyness of Observables. In this article we like to focus on some practical advantages that Observables introduce for server communication.

The scenario

Consider you are building a search input mask that should instantly show you results as you type.

If you’ve ever build such a thing before you are probably aware of the challenges that come with that task.

1. Don’t hit the search endpoint on every key stroke

Treat the search endpoint as if you pay for it on a per-request basis. No matter if it’s your own hardware or not. We shouldn’t be hammering the search endpoint more often than needed. Basically we only want to hit it once the user has stopped
typing instead of with every keystroke.

2. Don’t hit the search endpoint with the same query params for subsequent requests

Consider you type foo, stop, type another o, followed by an immediate backspace and rest back at foo. That should be just one request with the term foo and not two even
if we technically stopped twice after we had foo in the search box.

3. Deal with out-of-order responses

When we have multiple requests in-flight at the same time we must account for cases where they come back in unexpected order. Consider we first typed computer, stop, a request goes out, we type car, stop, a request
goes out. Now we have two requests in-flight. Unfortunately the request that carries the results for computer comes back after the request that carries the results for car. This may happen because they are served
by different servers. If we don’t deal with such cases properly we may end up showing results for computer whereas the search box reads car.

Challenge accepted

We will use the free and open wikipedia API to write a little demo.

For simplicity our demo will simply consist of two files: 

app.ts

 and 

wikipedia-service.ts

.
In a real world scenario we would most likely split things further up though.

Let’s start with a Promise-based implementation that doesn’t handle any of the described edge cases.

This is what our 

WikipediaService

 looks like. Despite the fact that the Http/JsonP API still has some little unergonomic parts, there shouldn’t be much of surprise
here.

import {Injectable} from 'angular2/core';
import {URLSearchParams, Jsonp} from 'angular2/http';

@Injectable()
export class WikipediaService {
constructor(private jsonp: Jsonp) {}

search (term: string) {
var search = new URLSearchParams()
search.set('action', 'opensearch');
search.set('search', term);
search.set('format', 'json');
return this.jsonp
.get('http://en.wikipedia.org/w/api.php?callback=JSONP_CALLBACK', { search })
.toPromise()
.then((response) => response.json()[1]);
}
}

Basically we are injecting the 

Jsonp

 service to make a 

GET

 request
against the wikipedia API with a given search term. Notice that we call 

toPromise

 in order to get from an 

Observable<Response>

 to
a

Promise<Response>

. With a little bit of 

then

-chaining we eventually
end up with a

Promise<Array<string>>

 as the return type of our 

search

 method.

So far so good, let’s take a look at the 

app.ts

 file that holds our 

App

 Component.

// check the plnkr for the full list of imports
import {...} from '...';

@Component({
selector: 'my-app',
template: `
<div>
<h2>Wikipedia Search</h2>
<input #term type="text" (keyup)="search(term.value)">
<ul>
<li *ngFor="#item of items">{{item}}</li>
</ul>
</div>
`
})
export class App {
items: Array<string>;
constructor(private wikipediaService: WikipediaService) {
}

search (term) {
this.wikipediaService.search(term)
.then(items => this.items = items);
}
}

bootstrap(App, [WikipediaService, JSONP_PROVIDERS])
.catch(err => console.error(err));

Not much of a surprise here either. We inject our 

WikipediaService

 and expose it’s functionality via a

search

 method
to the template. The template simply binds to 

keyup

 and calls 

search(term.value)

leveraging
Angular 2’s awesome template ref feature.

We unwrap the result of the 

Promise

 that the 

search

 method of the 

WikipediaService

 returns
and expose it as a simple Array of strings to the template so that we can have 

*ngFor

 loop through it and build up a list for us.

You can play with the demo and fiddle with the code through this plnkr.

Unfortunately this implementation doesn’t address any of the described edge cases that we would like to deal with. Let’s refactor our code to make it match the expected behavior.

Taming the user input

Let’s change our code to not hammer the endpoint with every keystroke but instead only send a request when the user stopped typing for 400 ms. This is where Observables really shine. The Reactive Extensions (Rx) offer a broad range of operators that let us
alter the behavior of Observables and create new Observables with the desired semantics.

To unveil such super powers we first need to get an 

Observable<string>

 that carries the search term that the user types in. Instead of manually binding to the 

keyup

 event
we use 

ngFormControl

 from within our template and set it to the name 

"term"

.

<input type="text" [ngFormControl]="term"/>

In our component we create an instance of 

Control

 from 

angular2/common

 and
expose it as a field under the name 

term

 on our component.

Behind the scenes 

term

 automatically exposes an 

Observable<string>

 as
property 

valueChanges

that we can subscribe to. Now that we have an 

Observable<string>

,
taming the user input is as easy as calling 

debounceTime(400)

 on our Observable. This will return a new 

Observable<string>

 that
will only emit a new value when there haven’t been coming new values for 400ms.

export class App {
items: Array<string>;
term = new Control();
constructor(private wikipediaService: WikipediaService) {
this.term.valueChanges
.debounceTime(400)
.subscribe(term => this.wikipediaService.search(term).then(items => this.items = items));
}
}

Don’t hit me twice

As we said, it would be a waste of resources to send out another request for a search term that our app already shows the results for. Fortunately Rx simplifies many operations that it nearly feels unnecessary to mention them. All we have to do to achieve the
desired behavior is to call the

distinctUntilChanged

 operator right after we called 

debounceTime(400)

.
Again, we will get back an 

Observable<string>

 but one that ignores values that are the same as the previous.

Dealing with out-of-order responses

Dealing with out of order responses can be a tricky task. Basically we need a way to express that we aren’t interested anymore in results from previous in-flight requests as soon as we are sending out new requests. In other words: cancel out all previous request
as soon as we start a new one. As I briefly mentioned in the beginning Observables are disposable which means we can unsubscribe from them.

This is where we want to change our 

WikipediaService

 to return an 

Observable<Array<string>>

instead
of an 

Promise<Array<string>>

. That’s as easy as dropping 

toPromise

 and
using 

map

 instead of 

then

.

search (term: string) {
var search = new URLSearchParams()
search.set('action', 'opensearch');
search.set('search', term);
search.set('format', 'json');
return this.jsonp
.get('http://en.wikipedia.org/w/api.php?callback=JSONP_CALLBACK', { search })
.map((response) => response.json()[1]);
}

Now that our 

WikipediaSerice

 returns an Observable instead of a Promise we simply need to replace

then

 with 

subscribe

 in
our 

App

 component.

this.term.valueChanges
.debounceTime(400)
.distinctUntilChanged()
.subscribe(term => this.wikipediaService.search(term).subscribe(items => this.items = items));

But now we have two 

subscribe

 calls. This is needlessly verbose and often a sign for unidiomatic usage. The good news is, now that 

search

 returns
an 

Observable<Array<string>>

 we can simply use 

flatMap

 to project
our 

Observable<string>

 into the desired 

Observable<Array<string>>

 by
composing the Observables.

this.term.valueChanges
.debounceTime(400)
.distinctUntilChanged()
.flatMap(term => this.wikipediaService.search(term))
.subscribe(items => this.items = items);

You may be wondering what 

flatMap

 does and why we can’t use 

map

 here.
The answer is quite simple. The 

map

 operator expects a function that takes a value 

T

 and
returns a value 

U

. For instance a function that takes in a 

string

 and
returns a 

Number

. Hence when you use 

map

 you get from an

Observable<T>

 to
an 

Observable<U>

. However, our 

search

 method produces an

Observable<Array>

 itself.
So coming from an 

Observable<string>

 that we have right after

distinctUntilChanged

,
map would take us to an 

Observable<Observable<Array<string>>

. That’s not quite what we want.

The 

flatMap

 operator on the other hand expects a function that takes a 

T

 and
returns an

Observable<U>

 and produces an 

Observable<U>

 for us.

NOTE: That’s not entirely true, but it helps as a simplification.

That perfectly matches our case. We have an 

Observable<string>

, then call 

flatMap

 with
a function that takes a 

string

 and returns an 

Observable<Array<string>>

.

So does this solve our out-of-order response issues? Unfortunately not. So, why am I bothering you with all this in the first place? Well, now that you understood 

flatMap

 just
replace it with 

switchMap

 and you are done.

What?! You may be wondering if I’m kidding you but no I am not. That’s the beautify of Rx with all it’s useful operators. The 

switchMap

 operator is comparable to 

flatMap

 in
a way. Both operators automatically subscribe to the Observable that the function produces and flatten the result for us. The difference is that the 

switchMap

 operator
automatically unsubscribes from previous subscriptions as soon as the outer Observable emits new values.

Putting some sugar on top

Now that we got the semantics right, there’s one more little trick that we can use to save us some typing. Instead of manually subscribing to the Observable we can let Angular do the unwrapping for us right from within the template. All we have to do to accomplish
that is to use the 

AsyncPipe

 in our template and expose the 

Observable<Array<string>>

 instead
of 

Array<string>

.

@Component({
selector: 'my-app',
template: `
<div>
<h2>Wikipedia Search</h2>
<input type="text" [ngFormControl]="term"/>
<ul>
<li *ngFor="#item of items | async">{{item}}</li>
</ul>
</div>
`
})
export class App {
items: Observable<Array<string>>;
term = new Control();
constructor(private wikipediaService: WikipediaService) {
this.items = this.term.valueChanges
.debounceTime(400)
.distinctUntilChanged()
.switchMap(term => this.wikipediaService.search(term));
}
}

And voilà, here is our final version in a plnkr.

原文链接： http://blog.thoughtram.io/angular/2016/01/06/taking-advantage-of-observables-in-angular2.html