angular-cn/public/docs/ts/_cache/guide/server-communication.jade

block includes
  include ../_util-fns
  - var _Http = 'Http'; // Angular `Http` library name.
  - var _Angular_Http = 'Angular <code>Http</code>'
  - var _Angular_http_library = 'Angular HTTP library'

:marked
  [HTTP](https://tools.ietf.org/html/rfc2616) is the primary protocol for browser/server communication.
.l-sub-section
  :marked
    The [`WebSocket`](https://tools.ietf.org/html/rfc6455) protocol is another important communication technology;
    we won't cover it in this chapter.
:marked
  Modern browsers support two HTTP-based APIs: 
  [XMLHttpRequest (XHR)](https://developer.mozilla.org/en-US/docs/Web/API/XMLHttpRequest) and 
  [JSONP](https://en.wikipedia.org/wiki/JSONP). A few browsers also support
  [Fetch](https://developer.mozilla.org/en-US/docs/Web/API/Fetch_API). 
  
  The !{_Angular_http_library} simplifies application programming of the **XHR** and **JSONP** APIs
  as we'll learn in this chapter covering:

  - [HTTP client sample overview](#http-client)
  - [Fetch data with http.get](#fetch-data)
  <li if-docs="ts"> [RxJS Observable of HTTP Responses](#rxjs)</li>
  <li if-docs="ts"> [Enabling RxJS Operators](#enable-rxjs-operators)</li>
  - [Extract JSON data](#extract-data)
  - [Error handling](#error-handling)
  - [Send data to the server](#update)
  <li if-docs="ts"> [Promises instead of observables](#promises)</li>
  - [Cross-origin requests: Wikipedia example](#cors)
    <ul if-docs="ts"> 
      <li> [Set query string parameters](#search-parameters)</li>
      <li> [Debounce search term input](#more-observables)</li>
    </ul>
  - [Appendix: the in-memory web api service](#in-mem-web-api)

  We illustrate these topics with code that you can <live-example>run live</live-example>.
  
.l-main-section
:marked
  ## Demos

  This chapter describes server communication with the help of the following demos

block demos-list
  :marked
    - [HTTP client: Tour of Heroes with Observables](#http-client)
    - [HTTP client: Tour of Heroes with !{_Promise}s](#promises)
    - [JSONP client: Wikipedia to fetch data from a service that does not support CORS](#cors)
    - [JSONP client: Wikipedia using observable operators to reduce server calls](#more-observables)

:marked
  These demos are orchestrated by the root `AppComponent`
+makeExample('server-communication/ts/app/app.component.ts', null, 'app/app.component.ts')

+ifDocsFor('ts')
  :marked
    There is nothing remarkable here _except_ for the import of RxJS operators.
  +makeExample('server-communication/ts/app/app.component.ts', 'import-rxjs')(format='.')
  :marked
    We'll talk about that [below](#rxjs) when we're ready to explore observables.
:marked
  First, we have to configure our application to use server communication facilities.

.l-main-section#http-providers
:marked
  # Providing HTTP Services

  We use the !{_Angular_Http} client to communicate with a server using a familiar HTTP request/response protocol.
  The `!{_Http}` client is one of a family of services in the !{_Angular_http_library}.

+ifDocsFor('ts')
  .l-sub-section
    :marked
      SystemJS knows how to load services from the !{_Angular_http_library} when we import from the `@angular/http` module
      because we registered that module name in the `system.config` file.

:marked
  Before we can use the `!{_Http}` client , we'll have to register it as a service provider with the Dependency Injection system.

.l-sub-section
  :marked
    Learn about providers in the [Dependency Injection](dependency-injection.html) chapter.

:marked
  In this demo, we register providers in the `bootstrap()` method of 
  <span ngio-ex>app/main.ts</span>.

+makeExample('server-communication/ts/app/main.ts', 'v1', 'app/main.ts (v1)')(format='.')

block http-providers
  :marked
    We begin by importing the symbols we need, most of them familiar by now. The newcomer is `HTTP_PROVIDERS`, 
    a collection of service providers from the !{_Angular_http_library}.
 
    We register HTTP providers in the bootstrap method by passing them in an array as the second parameter after the root component.

    ### Why register in *bootstrap*?
    
    We prefer to register application-wide providers in the metadata `providers` array
    of the root `AppComponent` like this:
  +makeExample('server-communication/ts/app/app.component.ts','http-providers')(format='.')
  :marked
    Here we register the providers in the `bootstrap` method in the `main.ts` file. Why?
    
    This is a *sample application* that doesn't talk to a real server.
    We're going to reconfigure the (typically-hidden) `XhrBackend` service with a fake provider
    that fetches and saves sample data from an in-memory data store.
    This replacement service is called the [*in-memory web api*](#in-mem-web-api).
    
    Such sleight-of-hand is something the root application component should *not* know about.
    For this reason, and this reason *only*, we hide it *above* the `AppComponent` in `main.ts`.

.l-main-section#http-client
:marked
  # The Tour of Heroes _HTTP_ Client Demo

  Our first demo is a mini-version of the [tutorial](../tutorial)'s "Tour of Heroes" (ToH) application.
  This version gets some heroes from the server, displays them in a list, lets us add new heroes, and saves them to the server.
  We use the !{_Angular_Http} client to communicate via `XMLHttpRequest (XHR)`.

  It works like this.
figure.image-display
  img(src='/resources/images/devguide/server-communication/http-toh.gif' alt="ToH mini app" width="250")
:marked
  This demo has a single component, the `HeroListComponent`.  Here's its template:
+makeExample('server-communication/ts/app/toh/hero-list.component.html', null, 'app/toh/hero-list.component.html (template)')
:marked
  It presents the list of heroes with an `ngFor`. 
  Below the list is an input box and an *Add Hero* button where we can enter the names of new heroes
  and add them to the database. 
  We use a [template reference variable](template-syntax.html#ref-vars), `newHeroName`, to access the 
  value of the input box in the `(click)` event binding.
  When the user clicks the button, we pass that value to the component's `addHero` method and then
  clear it to make it ready for a new hero name.
  
  Below the button is an area for an error message.

a#oninit
a#HeroListComponent
:marked
  ### The *HeroListComponent* class
  Here's the component class:
+makeExample('server-communication/ts/app/toh/hero-list.component.ts','component', 'app/toh/hero-list.component.ts (class)')  
:marked
  Angular [injects](dependency-injection.html) a `HeroService` into the constructor
  and the component calls that service to fetch and save data.

  The component **does not talk directly to the !{_Angular_Http} client**!
  The component doesn't know or care how we get the data. 
  It delegates to the `HeroService`.
  
  This is a golden rule: **always delegate data access to a supporting service class**.

  Although _at runtime_ the component requests heroes immediately after creation, 
  we do **not** call the service's `get` method in the component's constructor.
  We call it inside the `ngOnInit` [lifecycle hook](lifecycle-hooks.html) instead
  and count on Angular to call `ngOnInit` when it instantiates this component. 
.l-sub-section
  :marked
    This is a *best practice*. 
    Components are easier to test and debug when their constructors are simple and all real work 
    (especially calling a remote server) is handled in a separate method.
block getheroes-and-addhero
  :marked
    The service's `getHeroes()` and `addHero()` methods return an `Observable` of hero data that the !{_Angular_Http} client fetched from the server.
    
    *Observables* are a big topic, beyond the scope of this chapter. 
    But we need to know a little about them to appreciate what is going on here.
    
    We should think of an `Observable` as a stream of events published by some source.
    We listen for events in this stream by ***subscribing*** to the `Observable`. 
    In these subscriptions we specify the actions to take when the web request
    produces a success event (with the hero data in the event payload) or a fail event (with the error in the payload).

:marked
  With our basic intuitions about the component squared away, we're ready to look inside the `HeroService`.

a#HeroService
.l-main-section#fetch-data
:marked
  ## Fetch data with the **HeroService**

  In many of our previous samples we faked the interaction with the server by
  returning mock heroes in a service like this one:
+makeExample('toh-4/ts/app/hero.service.ts', 'just-get-heroes')(format=".")
:marked
  In this chapter, we revise that `HeroService` to get the heroes from the server using the !{_Angular_Http} client service: 
+makeExample('server-communication/ts/app/toh/hero.service.ts', 'v1', 'app/toh/hero.service.ts (revised)')

:marked
  Notice that the !{_Angular_Http} client service is
  [injected](dependency-injection.html) into the `HeroService` constructor.
+makeExample('server-communication/ts/app/toh/hero.service.ts', 'ctor')
:marked
  Look closely at how we call `!{_priv}http.get`
+makeExample('server-communication/ts/app/toh/hero.service.ts', 'http-get', 'app/toh/hero.service.ts (getHeroes)')(format=".")
:marked
  We pass the resource URL to `get` and it calls the server which should return heroes.
.l-sub-section
  :marked
    It *will* return heroes once we've set up the [in-memory web api](#in-mem-web-api)
    described in the appendix below.
    Alternatively, we can (temporarily) target a JSON file by changing the endpoint URL:
  +makeExample('server-communication/ts/app/toh/hero.service.ts', 'endpoint-json')(format=".")

+ifDocsFor('ts')
  :marked
    <a id="rxjs"></a>
    The return value may surprise us. 
    Many of us who are familiar with asynchronous methods in modern JavaScript would expect the `get` method to return a 
    [promise](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Promise).
    We'd expect to chain a call to `then()` and extract the heroes.
    Instead we're calling a `map()` method. 
    Clearly this is not a promise.

    In fact, the `http.get` method returns an **Observable** of HTTP Responses (`Observable<Response>`) from the RxJS library
    and `map` is one of the RxJS *operators*.

  .l-main-section
  :marked
    ## RxJS Library
    [RxJS](https://github.com/ReactiveX/RxJS) ("Reactive Extensions") is a 3rd party library, endorsed by Angular,
    that implements the [*asynchronous observable*](https://www.youtube.com/watch?v=UHI0AzD_WfY "Rob Wormald on observables") pattern.

    All of our Developer Guide samples have installed the RxJS npm package and loaded via `system.js`
    because observables are used widely in Angular applications.
    We certainly need it now when working with the HTTP client.
    And we must take a critical extra step to make RxJS observables usable.

    ### Enable RxJS Operators
    The RxJS library is quite large. 
    Size matters when we build a production application and deploy it to mobile devices.
    We should include only those features that we actually need.

    Accordingly, Angular exposes a stripped down version of `Observable` in the `rxjs/Observable` module, 
    a version that lacks most of the operators including some we'd like to use here
    such as the `map` method we called above in `getHeroes`.

    It's up to us to add the operators we need. 
    
    We could add _every_ RxJS operators with a single import statement. 
    While that is the easiest thing to do, we'd pay a penalty in extended launch time and application size
    because the full library is so big. We only use a few operators in our app.
    
    Instead, we'll import each `Observable` operator and static class method, one-by-one, until we have a custom *Observable* implementation tuned
    precisely to our requirements. We'll put the `import` statements in one `app/rxjs-operators.ts` file.
  +makeExample('server-communication/ts/app/rxjs-operators.ts', null, 'app/rxjs-operators.ts')(format=".")
  :marked
    If we forget an operator, the TypeScript compiler will warn that it's missing and we'll update this file.
  .l-sub-section
    :marked
      We don't need _all_ of these particular operators in the `HeroService` &mdash; just `map`, `catch` and `throw`.
      We'll need the other operators later, in a *Wiki* example [below](#more-observables).
  :marked
    Finally, we import `rxjs-operator`_itself_ in our `app.component.ts`:
  +makeExample('server-communication/ts/app/app.component.ts', 'import-rxjs', 'app/app.component.ts (import rxjs)')(format=".")
  :marked
    Let's return to our study of the `HeroService`.
    
l-main-section
a#extract-data
:marked
  ## Process the response object
  Remember that our `getHeroes()` method mapped the `!{_priv}http.get` response object to heroes with an `!{_priv}extractData` helper method:
+makeExample('server-communication/ts/app/toh/hero.service.ts', 'extract-data', 'app/toh/hero.service.ts (excerpt)')(format=".")
:marked
  The `response` object does not hold our data in a form we can use directly. 
  To make it useful in our application we must parse the response data into a JSON object

  #### Parse to JSON
block parse-json
  :marked
    The response data are in JSON string form.
    We must parse that string into JavaScript objects which we do by calling `response.json()`.

  .l-sub-section
    :marked
      This is not Angular's own design. 
      The Angular HTTP client follows the ES2015 specification for the
      [response object](https://fetch.spec.whatwg.org/#response-class) returned by the `Fetch` function.
      That spec defines a `json()` method that parses the response body into a JavaScript object.

.l-sub-section
  :marked
    We shouldn't expect the decoded JSON to be the heroes !{_array} directly.
    The server we're calling always wraps JSON results in an object with a `data`
    property. We have to unwrap it to get the heroes.
    This is conventional web api behavior, driven by 
    [security concerns](https://www.owasp.org/index.php/OWASP_AJAX_Security_Guidelines#Always_return_JSON_with_an_Object_on_the_outside).
.alert.is-important
  :marked
     Make no assumptions about the server API. 
     Not all servers return an object with a `data` property.
:marked
  ### Do not return the response object
  Our `getHeroes()` could have returned the HTTP response. Bad idea! 
  The point of a data service is to hide the server interaction details from consumers.
  The component that calls the `HeroService` wants heroes. 
  It has no interest in what we do to get them.
  It doesn't care where they come from.
  And it certainly doesn't want to deal with a response object.

+ifDocsFor('ts')
  .callout.is-important
    header HTTP GET is delayed 
    :marked
      The `!{_priv}http.get` does **not send the request just yet!** This observable is
      [*cold*](https://github.com/Reactive-Extensions/RxJS/blob/master/doc/gettingstarted/creating.md#cold-vs-hot-observables)
      which means the request won't go out until something *subscribes* to the observable.
      That *something* is the [HeroListComponent](#subscribe).

a#error-handling
:marked
  ### Always handle errors

  Whenever we deal with I/O we must be prepared for something to go wrong as it surely will. 
  We should catch errors in the `HeroService` and do something with them. 
  We may also pass an error message back to the component for presentation to the user
  but only if we can say something the user can understand and act upon.

  In this simple app we provide rudimentary error handling in both the service and the component.
block error-handling
  :marked
    The eagle-eyed reader may have spotted our use of the `catch` operator in conjunction with a `handleError` method.
    We haven't discussed so far how that actually works. 

    We use the Observable `catch` operator on the service level.
    It takes an error handling function with an error object as the argument.
    Our service handler, `handleError`, logs the response to the console, 
    transforms the error into a user-friendly message, and returns the message in a new, failed observable via `Observable.throw`.

+makeExample('server-communication/ts/app/toh/hero.service.ts', 'error-handling', 'app/toh/hero.service.ts (excerpt)')(format=".")

a#subscribe
a#hero-list-component
h4 #[b HeroListComponent] error handling
block hlc-error-handling
  :marked
    Back in the `HeroListComponent`, where we called `!{_priv}heroService.getHeroes()`, 
    we supply the `subscribe` function with a second function parameter to handle the error message.
    It sets an `errorMessage` variable which we've bound conditionally in the `HeroListComponent` template.

+makeExample('server-communication/ts/app/toh/hero-list.component.ts', 'getHeroes', 'app/toh/hero-list.component.ts (getHeroes)')(format=".")
  
.l-sub-section
  :marked
    Want to see it fail? Reset the api endpoint in the `HeroService` to a bad value. Remember to restore it!

  
<a id="update"></a>
<a id="post"></a>
.l-main-section
:marked
  ## Send data to the server
  
  So far we've seen how to retrieve data from a remote location using an HTTP service. 
  Let's add the ability to create new heroes and save them in the backend.

  We'll create an easy method for the `HeroListComponent` to call, an `addHero()` method that takes
  just the name of a new hero:

+makeExample('server-communication/ts/app/toh/hero.service.ts', 'addhero-sig')(format=".")

:marked
  To implement it, we need to know some details about the server's api for creating heroes.
  
  [Our data server](#server) follows typical REST guidelines.
  It expects a [`POST`](http://www.w3.org/Protocols/rfc2616/rfc2616-sec9.html#sec9.5) request
  at the same endpoint where we `GET` heroes.
  It expects the new hero data to arrive in the body of the request, 
  structured like a `Hero` entity but without the `id` property.
  The body of the request should look like this:
  
code-example(format="." language="javascript").
  { "name": "Windstorm" }
:marked
  The server will generate the `id` and return the entire `JSON` representation
  of the new hero including its generated id. The hero arrives tucked inside a response object
  with its own `data` property.
  
  Now that we know how the API works, we implement `addHero()`like this:

+ifDocsFor('ts')
  +makeExample('server-communication/ts/app/toh/hero.service.ts', 'import-request-options', 'app/toh/hero.service.ts (additional imports)')(format=".")  
+makeExample('server-communication/ts/app/toh/hero.service.ts', 'addhero', 'app/toh/hero.service.ts (addHero)')(format=".")

:marked
  ### Headers

  The `Content-Type` header allows us to inform the server that the body will represent JSON.

+ifDocsFor('ts')
  :marked
    [Headers](../api/http/index/Headers-class.html) are one of the [RequestOptions](../api/http/index/RequestOptions-class.html).
    Compose the options object and pass it in as the *third* parameter of the `post` method, as shown above.

:marked
  ### Body

  Despite the content type being specified as JSON, the POST body must actually be a *string*.
  Hence, we explicitly encode the JSON hero content before passing it in as the body argument.

+ifDocsFor('ts')
  .l-sub-section
    :marked
      We may be able to skip the `JSON.stringify` step in the near future.

:marked
  ### JSON results

  As with `getHeroes()`, we [extract the data](#extract-data) from the response using the
  `!{_priv}extractData()` helper.

block hero-list-comp-add-hero
  :marked
    Back in the `HeroListComponent`, we see that *its* `addHero()` method subscribes to the observable returned by the *service's* `addHero()` method.
    When the data, arrive it pushes the new hero object into its `heroes` array for presentation to the user.
+makeExample('server-communication/ts/app/toh/hero-list.component.ts', 'addHero', 'app/toh/hero-list.component.ts (addHero)')(format=".")

+ifDocsFor('ts')
  h2#promises Fall back to Promises
  :marked
    Although the Angular `http` client API returns an `Observable<Response>` we can turn it into a 
    [Promise<Response>](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Promise) if we prefer.
    It's easy to do and a promise-based version looks much like the observable-based version in simple cases.
  .l-sub-section
    :marked
      While promises may be more familiar, observables have many advantages. 
      Don't rush to promises until you give observables a chance.
  :marked
    Let's rewrite the `HeroService` using promises , highlighting just the parts that are different.
  +makeTabs(
    'server-communication/ts/app/toh/hero.service.promise.ts,server-communication/ts/app/toh/hero.service.ts', 
    'methods, methods', 
    'app/toh/hero.service.promise.ts (promise-based), app/toh/hero.service.ts (observable-based)')
  :marked
    Converting from an observable to a promise is as simple as calling `toPromise(success, fail)`.

    We move the observable's `map` callback to the first *success* parameter and its `catch` callback to the second *fail* parameter
    and we're done!
    Or we can follow the promise `then.catch` pattern as we do in the second `addHero` example.

    Our `errorHandler` forwards an error message as a failed promise instead of a failed Observable.

    The diagnostic *log to console* is just one more `then` in the promise chain.

    We have to adjust the calling component to expect a `Promise` instead of an `Observable`.

  +makeTabs(
    'server-communication/ts/app/toh/hero-list.component.promise.ts, server-communication/ts/app/toh/hero-list.component.ts', 
    'methods, methods', 
    'app/toh/hero-list.component.promise.ts (promise-based), app/toh/hero-list.component.ts (observable-based)')
  :marked
    The only obvious difference is that we call `then` on the returned promise instead of `subscribe`.
    We give both methods the same functional arguments. 
  .l-sub-section
    :marked
      The less obvious but critical difference is that these two methods return very different results!

      The promise-based `then` returns another promise. We can keep chaining more `then` and `catch` calls, getting a new promise each time.

      The `subscribe` method returns a `Subscription`. A `Subscription` is not another `Observable`. 
      It's the end of the line for observables. We can't call `map` on it or call `subscribe` again.
      The `Subscription` object has a different purpose, signified by its primary method, `unsubscribe`.

      Learn more about observables to understand the implications and consequences of subscriptions.

h2#cors Cross-origin requests: Wikipedia example
:marked
  We just learned how to make `XMLHttpRequests` using the !{_Angular_Http} service. 
  This is the most common approach for server communication. 
  It doesn't work in all scenarios.

  For security reasons, web browsers block `XHR` calls to a remote server whose origin is different from the origin of the web page.
  The *origin* is the combination of URI scheme, hostname and port number. 
  This is called the [Same-origin Policy](https://en.wikipedia.org/wiki/Same-origin_policy).

.l-sub-section
  :marked
    Modern browsers do allow `XHR` requests to servers from a different origin if the server supports the 
    [CORS](https://en.wikipedia.org/wiki/Cross-origin_resource_sharing) protocol.
    If the server requires user credentials, we'll enable them in the [request headers](#headers).

:marked
  Some servers do not support CORS but do support an older, read-only alternative called [JSONP](https://en.wikipedia.org/wiki/JSONP).
  Wikipedia is one such server.
.l-sub-section
  :marked
    This [StackOverflow answer](http://stackoverflow.com/questions/2067472/what-is-jsonp-all-about/2067584#2067584) covers many details of JSONP.
:marked
  ### Search wikipedia
  
  Let's build a simple search that shows suggestions from wikipedia as we type in a text box.

figure.image-display
  img(src='/resources/images/devguide/server-communication/wiki-1.gif' alt="Wikipedia search app (v.1)" width="250")

block wikipedia-jsonp+
  :marked
    Wikipedia offers a modern `CORS` API and a legacy `JSONP` search API. Let's use the latter for this example.
    The Angular `Jsonp` service both extends the `!{_Http}` service for JSONP and restricts us to `GET` requests. 
    All other HTTP methods throw an error because JSONP is a read-only facility. 

    As always, we wrap our interaction with an Angular data access client service inside a dedicated service, here called `WikipediaService`.

  +makeExample('server-communication/ts/app/wiki/wikipedia.service.ts',null,'app/wiki/wikipedia.service.ts')
  :marked
    The constructor expects Angular to inject its `jsonp` service. 
    We register that service with `JSONP_PROVIDERS` in the  [component below](#wikicomponent) that calls our `WikipediaService`.

  <a id="query-parameters"></a>
  :marked
    ### Search parameters
    The [Wikipedia 'opensearch' API](https://www.mediawiki.org/wiki/API:Opensearch)
    expects four parameters (key/value pairs) to arrive in the request URL's query string.
    The keys are `search`, `action`, `format`, and `callback`.
    The value of the `search` key is the user-supplied search term to find in Wikipedia.
    The other three are the fixed values "opensearch", "json", and "JSONP_CALLBACK" respectively.
  .l-sub-section
    :marked
      The `JSONP` technique requires that we pass a callback function name to the server in the query string: `callback=JSONP_CALLBACK`.
      The server uses that name to build a JavaScript wrapper function in its response which Angular ultimately calls to extract the data.
      All of this happens under the hood.
  :marked
    If we're looking for articles with the word "Angular", we could construct the query string by hand and call `jsonp` like this:
  +makeExample('server-communication/ts/app/wiki/wikipedia.service.1.ts','query-string')(format='.')
  :marked
    In more parameterized examples we might prefer to build the query string with the Angular `URLSearchParams` helper as shown here:
  +makeExample('server-communication/ts/app/wiki/wikipedia.service.ts','search-parameters','app/wiki/wikipedia.service.ts (search parameters)')(format=".")
  :marked
    This time we call `jsonp` with *two* arguments: the `wikiUrl` and an options object whose `search` property is the `params` object.
  +makeExample('server-communication/ts/app/wiki/wikipedia.service.ts','call-jsonp','app/wiki/wikipedia.service.ts (call jsonp)')(format=".")
  :marked
    `Jsonp` flattens the `params` object into the same query string we saw earlier before putting the request on the wire.

  <a id="wikicomponent"></a>
  :marked
    ### The WikiComponent

    Now that we have a service that can query the Wikipedia API, 
    we turn to the component that takes user input and displays search results.

  +makeExample('server-communication/ts/app/wiki/wiki.component.ts', null, 'app/wiki/wiki.component.ts')
  :marked
    The `providers` array in the component metadata specifies the Angular `JSONP_PROVIDERS` collection that supports the `Jsonp` service.
    We register that collection at the component level to make `Jsonp` injectable in the `WikipediaService`.

    The component presents an `<input>` element *search box* to gather search terms from the user. 
    and calls a `search(term)` method after each `keyup` event.

    The `search(term)` method delegates to our `WikipediaService` which returns an observable array of string results (`Observable<string[]>`). 
    Instead of subscribing to the observable inside the component as we did in the `HeroListComponent`, 
    we forward the observable result to the template (via `items`) where the [async pipe](pipes.html#async-pipe)
    in the `ngFor` handles the subscription.
  .l-sub-section
    :marked
      We often use the [async pipe](pipes.html#async-pipe) in read-only components where the component has no need to interact with the data.
      We couldn't use the pipe in the `HeroListComponent` because the "add hero" feature pushes newly created heroes into the list.

  :marked
    ## Our wasteful app

    Our wikipedia search makes too many calls to the server. 
    It is inefficient and potentially expensive on mobile devices with limited data plans.

    ### 1. Wait for the user to stop typing
    At the moment we call the server after every key stroke.
    The app should only make requests when the user *stops typing* .
    Here's how it *should* work &mdash; and *will* work  &mdash;  when we're done refactoring:
  figure.image-display
    img(src='/resources/images/devguide/server-communication/wiki-2.gif' alt="Wikipedia search app (v.2)" width="250")
  :marked
    ### 2. Search when the search term changes

    Suppose the user enters the word *angular* in the search box and pauses for a while. 
    The application issues a search request for *Angular*.

    Then the user backspaces over the last three letters, *lar*, and immediately re-types *lar* before pausing once more.
    The search term is still "angular". The app shouldn't make another request.

    ### 3. Cope with out-of-order responses

    The user enters *angular*, pauses, clears the search box, and enters *http*. 
    The application issues two search requests, one for *angular* and one for *http*. 

    Which response will arrive first? We can't be sure. 
    A load balancer could dispatch the requests to two different servers with different response times.
    The results from the first *angular* request might arrive after the later *http* results.
    The user will be confused if we display the *angular* results to the *http* query.

    When there are multiple requests in-flight, the app should present the responses
    in the original request order. That won't happen if *angular* results arrive last.

    <a id="more-observables"></a>
    ## More fun with Observables
    We can address these problems and improve our app with the help of some nifty observable operators. 

    We could make our changes to the `WikipediaService`. 
    But we sense that our concerns are driven by the user experience so we update the component class instead.

  +makeExample('server-communication/ts/app/wiki/wiki-smart.component.ts', null, 'app/wiki/wiki-smart.component.ts')
  :marked
    We made no changes to the template or metadata, confining them all to the component class.
    Let's review those changes.

    ### Create a stream of search terms

    We're binding to the search box `keyup` event and calling the component's `search` method after each keystroke.

    We turn these events into an observable stream of search terms using a `Subject` 
    which we import from the RxJS observable library:
  +makeExample('server-communication/ts/app/wiki/wiki-smart.component.ts', 'import-subject')
  :marked
    Each search term is a string, so we create a new `Subject` of type `string` called `searchTermStream`.
    After every keystroke, the `search` method adds the search box value to that stream
    via the subject's `next` method.
  +makeExample('server-communication/ts/app/wiki/wiki-smart.component.ts', 'subject')(format='.')
  :marked
    ### Listen for search terms

    Earlier, we passed each search term directly to the service and bound the template to the service results.
    Now we listen to the *stream of terms*, manipulating the stream before it reaches the `WikipediaService`.
  +makeExample('server-communication/ts/app/wiki/wiki-smart.component.ts', 'observable-operators')(format='.')
  :marked
    We wait for the user to stop typing for at least 300 milliseconds 
    ([debounceTime](https://github.com/Reactive-Extensions/RxJS/blob/master/doc/api/core/operators/debounce.md)).
    Only changed search values make it through to the service 
    ([distinctUntilChanged](https://github.com/Reactive-Extensions/RxJS/blob/master/doc/api/core/operators/distinctuntilchanged.md)).

    The `WikipediaService` returns a separate observable of string arrays (`Observable<string[]>`) for each request.
    We could have multiple requests *in flight*, all awaiting the server's reply,
    which means multiple *observables-of-strings* could arrive at any moment in any order.

    The [switchMap](https://github.com/Reactive-Extensions/RxJS/blob/master/doc/api/core/operators/flatmaplatest.md)
    (formerly known as `flatMapLatest`) returns a new observable that combines these `WikipediaService` observables, 
    re-arranges them in their original request order,
    and delivers to subscribers only the most recent search results. 

    The displayed list of search results stays in sync with the user's sequence of search terms.
  .l-sub-section
    :marked
      We added the `debounceTime`, `distinctUntilChanged`, and `switchMap` operators to the RxJS `Observable` class
      in `rxjs-operators` as [described above](#rxjs)

a#in-mem-web-api
.l-main-section
:marked
  ## Appendix: Tour of Heroes in-memory server

  If we only cared to retrieve data, we could tell Angular to get the heroes from a `heroes.json` file like this one:
+makeJson('server-communication/ts/app/heroes.json', null, 'app/heroes.json')(format=".")
.l-sub-section
  :marked
    We wrap the heroes array in an object with a `data` property for the same reason that a data server does:
    to mitigate the [security risk](http://stackoverflow.com/questions/3503102/what-are-top-level-json-arrays-and-why-are-they-a-security-risk)
    posed by top-level JSON arrays. 
:marked
  We'd set the endpoint to the JSON file like this:
+makeExample('server-communication/ts/app/toh/hero.service.ts', 'endpoint-json')(format=".")

- var _a_ca_class_with = _docsFor === 'ts' ? 'a custom application class with' : ''
:marked
  The *get heroes* scenario would work.
  But we want to *save* data too. We can't save changes to a JSON file. We need a web API server.
  We didn't want the hassle of setting up and maintaining a real server for this chapter.
  So we turned to an *in-memory web API simulator* instead.

.l-sub-section
  :marked
    The in-memory web api is not part of the Angular core. 
    It's an optional service in its own `angular-in-memory-web-api` library
    that we installed with npm (see `package.json`) and 
    registered for module loading by SystemJS (see `systemjs.config.js`)

:marked
  The in-memory web API gets its data from !{_a_ca_class_with} a `createDb()`
  method that returns a map whose keys are collection names and whose values 
  are !{_array}s of objects in those collections.
  
  Here's the class we created for this sample based on the JSON data:
+makeExample('server-communication/ts/app/hero-data.ts', null, 'app/hero-data.ts')(format=".")
:marked
  Ensure that the `HeroService` endpoint refers to the web API:
+makeExample('server-communication/ts/app/toh/hero.service.ts', 'endpoint')(format=".")
:marked
  Finally, we need to redirect client HTTP requests to the in-memory web API.
block redirect-to-web-api
  :marked
    This redirection is easy to configure because Angular's `http` service delegates the client/server communication tasks
    to a helper service called the `XHRBackend`. 

    To enable our server simulation, we replace the default `XHRBackend` service with 
    the in-memory web API service using standard Angular provider registration techniques. 
    We initialize the in-memory web API with *seed data* from the mock hero dataset at the same time.
:marked
  Here is the revised (and final) version of <span ngio-ex>app/main.ts></span> demonstrating these steps.

+makeExcerpt('app/main.ts', 'final')

:marked
  See the full source code in the <live-example></live-example>.