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

block includes
  include ../_util-fns
: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 client library simplifies application programming of the **XHR** and **JSONP** APIs
  as we'll learn in this chapter covering:

ul
  li #[a(href="#http-client") Http client sample overview]
  li #[a(href="#fetch-data") Fetch data with http.get]
  +ifDocsFor('ts')
    li #[a(href="#rxjs") RxJS Observable of HTTP Responses]
    li #[a(href="#enable-rxjs-operators") Enabling RxJS Operators]
  li #[a(href="#extract-data") Extract JSON data]
  li #[a(href="#error-handling") Error handling]
  li #[a(href="#update") Send data to the server]
  +ifDocsFor('ts')
    li #[a(href="#promises") Promises instead of observables]
  li #[a(href="#cross-origin-requests") Cross-origin requests: Wikipedia example]
  +ifDocsFor('ts')
    ul
      li #[a(href="#search-parameters") Set query string parameters]
      li #[a(href="#more-observables") Debounce search term input]
  li #[a(href="#in-mem-web-api") Appendix: the in-memory web api service]
p.  
  We illustrate these topics with code that you can 
  #[+liveExampleLink2('run live in a browser', 'server-communication')].

.l-main-section
a#http-client
:marked
  ## The *Http* Client Demo
  
  We use the Angular `Http` client to communicate via `XMLHttpRequest (XHR)`.
  
  We'll demonstrate with 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.
  
  It works like this.
figure.image-display
  img(src='/resources/images/devguide/server-communication/http-toh.gif' alt="ToH mini app" width="250")
:marked
  It's implemented with two components — a parent `TohComponent` shell and the `HeroListComponent` child.
  We've seen these kinds of component in many other documentation samples. 
  Let's see how they change to support communication with a server.
.l-sub-section
  :marked
    We're overdoing the "separation of concerns" by creating two components for a tiny demo. 
    We're making a point about application structure that is easier to justify when the app grows.
:marked
  Here is the `TohComponent` shell:
+makeExample('server-communication/ts/app/toh/toh.component.ts', '', 'app/toh/toh.component.ts')
block http-providers
  :marked
    As usual, we import the symbols we need. The newcomer is `HTTP_PROVIDERS`, 
    an array of service providers from the Angular HTTP library.
    We'll be using that library to access the server.
    We also import a `HeroService` that we'll look at shortly.

    The component specifies both the ``HTTP_PROVIDERS` and the `HeroService` in the metadata `providers` array,
    making them available to the child components of this "Tour of Heroes" application.

  .l-sub-section
    :marked
      Alternatively, we may choose to add the `HTTP_PROVIDERS` while bootstrapping the app:
    +makeExample('server-communication/ts/app/main.ts','http-providers','app/main.ts')(format='.')
    :marked
      Learn about providers in the [Dependency Injection](dependency-injection.html) chapter.
  
:marked
  This sample only has one child, 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
  The component template displays a list of heroes with the `ngFor` repeater directive.
figure.image-display
  img(src='/resources/images/devguide/server-communication/hero-list.png' alt="Hero List")
:marked
  Beneath the heroes 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 ready for a new hero name.
  
  Below the button is an area for an error message.

a(id="oninit")
a(id="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
  We [inject](dependency-injection.html) the `HeroService` into the constructor.
  That's the instance of the `HeroService` that we provided in the parent shell `TohComponent`.

  Notice that the component **does not talk to the server directly!** 
  The component doesn't know or care how we get the data. 
  Those details it delegates to the `heroService` class (which we'll get to in a moment).
  
  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 HTTP hero data.
    We subscribe to this `Observable`, 
    specifying the actions to take when the request succeeds or fails. 
    We'll get to observables and subscription shortly.

:marked
  With our basic intuitions about the component squared away, we can turn to development of the backend data source
  and the client-side `HeroService` that talks to it.
  
  ### Fetch data
  
  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 get the heroes from the server using a (browser-based) HTTP client service.
  Here's the new `HeroService`: 
+makeExample('server-communication/ts/app/toh/hero.service.ts', 'v1', 'app/toh/hero.service.ts')

block http-client-service
  :marked
    The imported `Http` client service gets
    [injected](dependency-injection.html) into the `HeroService` constructor.
  .l-sub-section
    :marked
      `Http` is not part of the Angular core. It's an optional service in its own `@angular/http` library
      that we installed with npm (see the `package.json`) and 
      registered for module loading by SystemJS (see `systemjs.config.js`)

: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=".")

block rxjs
  :marked
    <a id="rxjs"></a>
    The return value may surprise us. Many of us would expect 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 almost all operators including the ones 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 each operator, one-by-one, until we had a custom *Observable* implementation tuned
    precisely to our requirements.

    That would be a distraction today. We're learning HTTP, not counting bytes.
    So we'll make it easy on ourselves and enrich *Observable* with the full set of operators.
    It only takes one `import` statement. 
    It's best to add that statement early when we're bootstrapping the application. 
    :
  +makeExample('server-communication/ts/app/main.ts', 'import-rxjs', 'app/main.ts (import rxjs)')(format=".")

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
  * check the response status,
  * parse the response data into a JSON object

+ifDocsFor('ts')
  .alert.is-important
    :marked
      *Beta alert*: error status interception and parsing may be absorbed within `http` when Angular is released. 

:marked
  #### Non-success status codes
  A status code outside the 200-299 range denotes an error from the _application point of view_
  but it is not an error from the _HTTP point of view_. 
  For example, a `404 - Not Found` is a response like any other. 
  The request went out; a response came back; here it is, thank you very much.
  We'd have an exception only if `#{_priv}http` failed to operate (e.g., it errored internally).

block non-success-status-codes
  :marked
    Because a status code outside the 200-299 range _is an error_ from the application point of view, 
    we intercept it and throw, moving the observable chain to the error path.

    The `catch` operator that is next in the `getHeroes` observable chain will handle our thrown error.

:marked
  #### 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 to handle the error message.
    It sets an `errorMessage` variable which we've bound conditionally in the 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/Headers-class.html) are one of the [RequestOptions](../api/http/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=".")

block promises
  a#promises
  :marked
    ## Fall back to Promises

    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.1.ts,server-communication/ts/app/toh/hero.service.ts', 
    'methods, methods', 
    'app/toh/hero.service.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.1.ts, server-communication/ts/app/toh/hero-list.component.ts', 
    'methods, methods', 
    'app/toh/hero-list.component.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.

a#cross-origin-requests
:marked
  ## Cross-origin requests: Wikipedia example

  We just learned how to make `XMLHttpRequests` using Angular's built-in `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 both `CORS` and `JSONP` search APIs, 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 
    ([debounce](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.

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 `angular2-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 by copy-and-pasting 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
    in `TohComponent`. We initialize the in-memory web API with mock hero data at the same time.

    Here are the pertinent details, excerpt from `TohComponent`, starting with the imports:
  +makeExample('server-communication/ts/app/toh/toh.component.ts', 'in-mem-web-api-imports', 'toh.component.ts (web API imports)')(format=".")
  :marked
    Then we add the following two provider definitions to the `providers` array in component metadata:
  +makeExample('server-communication/ts/app/toh/toh.component.ts', 'in-mem-web-api-providers', 'toh.component.ts (web API providers)')(format=".")

p See the full source code in the #[+liveExampleLink2('live example', 'server-communication')].