toh-pt6.jade

- var _example = 'toh-6';

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'
  - var _HttpModule = 'HttpModule'
  - var _JSON_stringify = 'JSON.stringify'

//- Shared var definitions
- var _promise = _Promise.toLowerCase()

:marked
  # Getting and Saving Data

  Our stakeholders appreciate our progress.
  Now they want to get the hero data from a server, let users add, edit, and delete heroes,
  and save these changes back to the server.

  In this chapter we teach our application to make the corresponding HTTP calls to a remote server's web API.

  Run the <live-example></live-example> for this part.

.l-main-section
:marked
  ## Where We Left Off

  In the [previous chapter](toh-pt5.html), we learned to navigate between the dashboard and the fixed heroes list, editing a selected hero along the way.
  That's our starting point for this chapter.

block start-server-and-watch
  :marked
    ### Keep the app transpiling and running

    Open a terminal/console window and enter the following command to
    start the TypeScript compiler, start the server, and watch for changes:

  code-example(language="bash").
    npm start

:marked
  The application runs and updates automatically as we continue to build the Tour of Heroes.

.l-main-section#http-providers
h1 Providing HTTP Services
block http-library
  :marked
    The `HttpModule` is ***not*** a core Angular module.
    It's Angular's optional approach to web access and it exists as a separate add-on module called `@angular/http`,
    shipped in a separate script file as part of the Angular npm package.

    Fortunately we're ready to import from `@angular/http` because `systemjs.config` configured *SystemJS* to load that library when we need it.

:marked
  ### Register for HTTP services

block http-providers
  :marked
    Our app will depend upon the Angular `http` service which itself depends upon other supporting services.
    The `HttpModule` from `@angular/http` library holds providers for a complete set of HTTP services.

    We should be able to access these services from anywhere in the application.
    So we register them all by adding `HttpModule` to the `imports` list of the `AppModule` where we
    bootstrap the application and its root `AppComponent`.

  +makeExample('app/app.module.ts', 'v1','app/app.module.ts (v1)')

  :marked
    Notice that we supply `!{_HttpModule}` as part of the *imports* !{_array} in root NgModule `AppModule`.

.l-main-section
:marked
  ## Simulating the web API

  We recommend registering application-wide services in the root
  `!{_AppModuleVsAppComp}` *providers*.  <span if-docs="dart">Here we're
  registering in `main` for a special reason.</span>

  Our application is in the early stages of development and far from ready for production.
  We don't even have a web server that can handle requests for heroes.
  Until we do, *we'll have to fake it*.

  We're going to *trick* the HTTP client into fetching and saving data from
  a mock service, the *in-memory web API*.
  <span if-docs="dart"> The application itself doesn't need to know and
  shouldn't know about this.  So we'll slip the in-memory web API into the
  configuration *above* the `AppComponent`.</span>

  Here is a version of <span ngio-ex>!{_appModuleTsVsMainTs}</span> that performs this trick:

+makeExcerpt(_appModuleTsVsMainTs, 'v2')

block backend
  :marked
    We're importing the  `InMemoryWebApiModule` and adding it to the module `imports`.
    The  `InMemoryWebApiModule` replaces the default `Http` client backend &mdash;
    the supporting service that talks to the remote server &mdash;
    with an  _in-memory web API alternative service_.

  +makeExcerpt(_appModuleTsVsMainTs, 'in-mem-web-api', '')

  :marked
    The `forRoot` configuration method takes an `InMemoryDataService` class
    that primes the in-memory database as follows:

+makeExample('app/in-memory-data.service.ts', 'init')(format='.')

p This file replaces the #[code #[+adjExPath('mock-heroes.ts')]] which is now safe to delete.

block dont-be-distracted-by-backend-subst
  .alert.is-helpful
    :marked
      This chapter is an introduction to the !{_Angular_http_library}.
      Please don't be distracted by the details of this backend substitution. Just follow along with the example.

      Learn more later about the in-memory web API in the [HTTP client chapter](../guide/server-communication.html#!#in-mem-web-api).
      Remember, the in-memory web API is only useful in the early stages of development and for demonstrations such as this Tour of Heroes.
      Skip it when you have a real web API server.

.l-main-section
:marked
  ## Heroes and HTTP

  Look at our current `HeroService` implementation

+makeExcerpt('toh-4/ts/app/hero.service.ts (old getHeroes)', 'get-heroes')

:marked
  We returned a !{_Promise} resolved with mock heroes.
  It may have seemed like overkill at the time, but we were anticipating the
  day when we fetched heroes with an HTTP client and we knew that would have to be an asynchronous operation.

  That day has arrived! Let's convert `getHeroes()` to use HTTP.

+makeExcerpt('app/hero.service.ts (updated getHeroes and new class members)', 'getHeroes')

:marked
  Our updated import statements are now:

+makeExcerpt('app/hero.service.ts (updated imports)', 'imports')

- var _h3id = `http-${_promise}`
:marked
  Refresh the browser, and the hero data should be successfully loaded from the
  mock server.

  <h3 id="!{_h3id}">HTTP !{_Promise}</h3>

  We're still returning a !{_Promise} but we're creating it differently.

block get-heroes-details
  :marked
    The Angular `http.get` returns an RxJS `Observable`.
    *Observables* are a powerful way to manage asynchronous data flows.
    We'll learn about [Observables](#observables) later in this chapter.

    For *now* we get back on familiar ground by immediately by
    converting that `Observable` to a `Promise` using the `toPromise` operator.

  +makeExcerpt('app/hero.service.ts', 'to-promise', '')

  :marked
    Unfortunately, the Angular `Observable` doesn't have a `toPromise` operator ...
    not out of the box.
    The Angular `Observable` is a bare-bones implementation.

    There are scores of operators like `toPromise` that extend `Observable` with useful capabilities.
    If we want those capabilities, we have to add the operators ourselves.
    That's as easy as importing them from the RxJS library like this:

  +makeExcerpt('app/hero.service.ts', 'rxjs', '')

  :marked
    ### Extracting the data in the *then* callback

    In the *promise*'s `then` callback we call the `json` method of the HTTP `Response` to extract the
    data within the response.
  +makeExcerpt('app/hero.service.ts', 'to-data', '')

:marked
  That response JSON has a single `data` property.
  The `data` property holds the !{_array} of *heroes* that the caller really wants.
  So we grab that !{_array} and return it as the resolved !{_Promise} value.

.alert.is-important
  :marked
    Pay close attention to the shape of the data returned by the server.
    This particular *in-memory web API* example happens to return an object with a `data` property.
    Your API might return something else. Adjust the code to match *your web API*.

:marked
  The caller is unaware of these machinations. It receives a !{_Promise} of *heroes* just as it did before.
  It has no idea that we fetched the heroes from the (mock) server.
  It knows nothing of the twists and turns required to convert the HTTP response into heroes.
  Such is the beauty and purpose of delegating data access to a service like this `HeroService`.

  ### Error Handling

  At the end of `getHeroes()` we `catch` server failures and pass them to an error handler:

+makeExcerpt('app/hero.service.ts', 'catch', '')

:marked
  This is a critical step!
  We must anticipate HTTP failures as they happen frequently for reasons beyond our control.

+makeExcerpt('app/hero.service.ts', 'handleError', '')

- var rejected_promise = _docsFor == 'dart' ? 'propagated exception' : 'rejected promise';
:marked
  In this demo service we log the error to the console; we would do better in real life.

  We've also decided to return a user friendly form of the error to
  the caller in a !{rejected_promise} so that the caller can display a proper error message to the user.

  ### Unchanged `getHeroes` API

  Although we made significant *internal* changes to `getHeroes()`, the public signature did not change.
  We still return a !{_Promise}. We won't have to update any of the components that call `getHeroes()`.

  Our stakeholders are thrilled with the added flexibility from the API integration. 
  Now they want the ability to create and delete heroes.

  Let's see first what happens when we try to update a hero's details.

.l-main-section
:marked
  ## Update hero details

  We can edit a hero's name already in the hero detail view. Go ahead and try
  it. As we type, the hero name is updated in the view heading. 
  But when we hit the `Back` button, the changes are lost!

.l-sub-section
  :marked
    Updates weren't lost before, what's happening?
    When the app used a list of mock heroes, changes were made directly to the
    hero objects in the single, app-wide shared list. Now that we are fetching data
    from a server, if we want changes to persist, we'll need to write them back to
    the server.

:marked
  ### Save hero details

  Let's ensure that edits to a hero's name aren't lost. Start by adding,
  to the end of the hero detail template, a save button with a `click` event
  binding that invokes a new component method named `save`:

+makeExcerpt('app/hero-detail.component.html', 'save')

:marked
  The `save` method persists hero name changes using the hero service
  `update` method and then navigates back to the previous view:

+makeExcerpt('app/hero-detail.component.ts', 'save')

:marked
  ### Hero service `update` method

  The overall structure of the `update` method is similar to that of
  `getHeroes`, although we'll use an HTTP _put_ to persist changes
  server-side:

+makeExcerpt('app/hero.service.ts', 'update')

:marked
  We identify _which_ hero the server should update by encoding the hero id in
  the URL. The put body is the JSON string encoding of the hero, obtained by
  calling `!{_JSON_stringify}`.  We identify the body content type
  (`application/json`) in the request header.

  Refresh the browser and give it a try. Changes to hero names should now persist.

.l-main-section
:marked
  ## Add a hero

  To add a new hero we need to know the hero's name. Let's use an input
  element for that, paired with an add button.

  Insert the following into the heroes component HTML, first thing after
  the heading:

+makeExcerpt('app/heroes.component.html', 'add')

:marked
  In response to a click event, we call the component's click handler and then
  clear the input field so that it will be ready to use for another name.

+makeExcerpt('app/heroes.component.ts', 'add')

:marked
  When the given name is non-blank, the handler delegates creation of the
  named hero to the hero service, and then adds the new hero to our !{_array}.

  Finally, we implement the `create` method in the `HeroService` class.
+makeExcerpt('app/hero.service.ts', 'create')

:marked
  Refresh the browser and create some new heroes!

.l-main-section
:marked
  ## Delete a hero

  Too many heroes?
  Let's add a delete button to each hero in the heroes view.

  Add this button element to the heroes component HTML, right after the hero
  name in the repeated `<li>` tag:

+makeExcerpt('app/heroes.component.html', 'delete', '')

:marked
  The `<li>` element should now look like this:

+makeExcerpt('app/heroes.component.html', 'li-element')

:marked
  In addition to calling the component's `delete` method, the delete button
  click handling code stops the propagation of the click event &mdash; we
  don't want the `<li>` click handler to be triggered because that would
  select the hero that we are going to delete!

  The logic of the `delete` handler is a bit trickier:

+makeExcerpt('app/heroes.component.ts', 'delete')

:marked
  Of course, we delegate hero deletion to the hero service, but the component
  is still responsible for updating the display: it removes the deleted hero
  from the !{_array} and resets the selected hero if necessary.

:marked
  We want our delete button to be placed at the far right of the hero entry.
  This extra CSS accomplishes that:

+makeExcerpt('app/heroes.component.css', 'additions')

:marked
  ### Hero service `delete` method

  The hero service's `delete` method uses the _delete_ HTTP method to remove the hero from the server:

+makeExcerpt('app/hero.service.ts', 'delete')

:marked
  Refresh the browser and try the new delete functionality.

:marked
  ## !{_Observable}s

block observables-section-intro
  :marked
    Each `Http` service method  returns an `Observable` of HTTP `Response` objects.

    Our `HeroService` converts that `Observable` into a `Promise` and returns the promise to the caller.
    In this section we learn to return the `Observable` directly and discuss when and why that might be
    a good thing to do.

    ### Background
    An *observable* is a stream of events that we can process with array-like operators.

    Angular core has basic support for observables. We developers augment that support with
    operators and extensions from the [RxJS Observables](http://reactivex.io/rxjs/) library.
    We'll see how shortly.

    Recall that our `HeroService` quickly chained the `toPromise` operator to the `Observable` result of `http.get`.
    That operator converted the `Observable` into a `Promise` and we passed that promise back to the caller.

    Converting to a promise is often a good choice. We typically ask `http.get` to fetch a single chunk of data.
    When we receive the data, we're done.
    A single result in the form of a promise is easy for the calling component to consume
    and it helps that promises are widely understood by JavaScript programmers.

:marked
  But requests aren't always "one and done". We may start one request,
  then cancel it, and make a different request before the server has responded to the first request.
  Such a _request-cancel-new-request_ sequence is difficult to implement with *!{_Promise}s*.
  It's easy with *!{_Observable}s* as we'll see.

  ### Search-by-name
  We're going to add a *hero search* feature to the Tour of Heroes.
  As the user types a name into a search box, we'll make repeated HTTP requests for heroes filtered by that name.

  We start by creating `HeroSearchService` that sends search queries to our server's web api.

+makeExample('app/hero-search.service.ts')

:marked
  The `!{_priv}http.get()` call in `HeroSearchService` is similar to the one
  in the `HeroService`, although the URL now has a query string.
  <span if-docs="ts">Another notable difference: we no longer call `toPromise`,
  we simply return the *observable* instead.</span>

  ### HeroSearchComponent

  Let's create a new `HeroSearchComponent` that calls this new `HeroSearchService`.

  The component template is simple &mdash; just a text box and a list of matching search results.

+makeExample('app/hero-search.component.html')
:marked
  We'll also want to add styles for the new component.
+makeExample('app/hero-search.component.css')
:marked
  As the user types in the search box, a *keyup* event binding calls the component's `search` method with the new search box value.

  The `*ngFor` repeats *hero* objects from the component's `heroes` property. No surprise there.

  But, as we'll soon see, the `heroes` property is now !{_an} *!{_Observable}* of hero !{_array}s, rather than just a hero !{_array}.
  The `*ngFor` can't do anything with !{_an} `!{_Observable}` until we flow it through the `async` pipe (`AsyncPipe`).
  The `async` pipe subscribes to the `!{_Observable}` and produces the !{_array} of heroes to `*ngFor`.

  Time to create the `HeroSearchComponent` class and metadata.

+makeExample('app/hero-search.component.ts')

:marked
  #### Search terms

  Let's focus on the `!{_priv}searchTerms`:

+makeExcerpt('app/hero-search.component.ts', 'searchTerms', '')

block search-criteria-intro
  :marked
    A `Subject` is a producer of an _observable_ event stream;
    `searchTerms` produces an `Observable` of strings, the filter criteria for the name search.

    Each call to `search` puts a new string into this subject's _observable_ stream by calling `next`.

:marked
  <a id="ngoninit"></a>
  #### Initialize the _**heroes**_ property (_**ngOnInit**_)

  <span if-docs="ts">A `Subject` is also an `Observable`.</span>
  We're going to turn the stream
  of search terms into a stream of `Hero` !{_array}s and assign the result to the `heroes` property.

+makeExcerpt('app/hero-search.component.ts', 'search', '')

:marked
  If we passed every user keystroke directly to the `HeroSearchService`, we'd unleash a storm of HTTP requests.
  Bad idea. We don't want to tax our server resources and burn through our cellular network data plan.

block observable-transformers
  :marked
    Fortunately, we can chain `Observable` operators to the string `Observable` that reduce the request flow.
    We'll make fewer calls to the `HeroSearchService` and still get timely results. Here's how:

    * `debounceTime(300)` waits until the flow of new string events pauses for 300 milliseconds
    before passing along the latest string. We'll never make requests more frequently than 300ms.

    * `distinctUntilChanged` ensures that we only send a request if the filter text changed.
    There's no point in repeating a request for the same search term.

    * `switchMap` calls our search service for each search term that makes it through the `debounce` and `distinctUntilChanged` gauntlet.
    It cancels and discards previous search observables, returning only the latest search service observable.

  .l-sub-section
    :marked
      The [switchMap operator](http://www.learnrxjs.io/operators/transformation/switchmap.html)
      (formerly known as "flatMapLatest") is very clever.

      Every qualifying key event can trigger an `http` method call.
      Even with a 300ms pause between requests, we could have multiple HTTP requests in flight
      and they may not return in the order sent.

      `switchMap` preserves the original request order while returning
       only the observable from the most recent `http` method call.
      Results from prior calls are canceled and discarded.

      We also short-circuit the `http` method call and return an observable containing an empty array
      if the search text is empty.

      Note that _canceling_ the `HeroSearchService` observable won't actually abort a pending HTTP request
      until the service supports that feature, a topic for another day.
      We are content for now to discard unwanted results.
  :marked
    * `catch` intercepts a failed observable.
    Our simple example prints the error to the console; a real life application should do better.
    Then we return an observable containing an empty array to clear the search result.

    ### Import RxJS operators
    The RxJS operators are not available in Angular's base `Observable` implementation.
    We have to extend  `Observable` by *importing* them.

    We could extend `Observable` with just the operators we need here by
    including the pertinent `import` statements at the top of this file.

  .l-sub-section
    :marked
      Many authorities say we should do just that.
  :marked
    We take a different approach in this example.
    We combine all of the RxJS `Observable` extensions that _our entire app_ requires into a single RxJS imports file.

  +makeExample('app/rxjs-extensions.ts')(format='.')

  :marked
    We load them all at once by importing `rxjs-extensions` at the top of `AppModule`.

  +makeExcerpt('app/app.module.ts', 'rxjs-extensions')(format='.')

:marked
  ### Add the search component to the dashboard

  We add the hero search HTML element to the bottom of the `DashboardComponent` template.

+makeExample('app/dashboard.component.html')(format='.')

- var _declarations = _docsFor == 'dart' ? 'directives' : 'declarations'
- var declFile = _docsFor == 'dart' ? 'app/dashboard.component.ts' : 'app/app.module.ts'
:marked
  Finally, we import `HeroSearchComponent` from 
  <span ngio-ex>hero-search.component.ts</span>
  and add it to the `!{_declarations}` !{_array}:

+makeExcerpt(declFile, 'search')

:marked
  Run the app again, go to the *Dashboard*, and enter some text in the search box.
  At some point it might look like this.

figure.image-display
  img(src='/resources/images/devguide/toh/toh-hero-search.png' alt="Hero Search Component")

.l-main-section
:marked
  ## Application structure and code

  Review the sample source code in the <live-example></live-example> for this chapter.
  Verify that we have the following structure:

block filetree
  .filetree
    .file angular-tour-of-heroes
    .children
      .file app
      .children
        .file app.component.ts
        .file app.component.css
        .file app.module.ts
        .file app.routing.ts
        .file dashboard.component.css
        .file dashboard.component.html
        .file dashboard.component.ts
        .file hero.ts
        .file hero-detail.component.css
        .file hero-detail.component.html
        .file hero-detail.component.ts
        .file hero-search.component.html (new)
        .file hero-search.component.css (new)
        .file hero-search.component.ts (new)
        .file hero-search.service.ts (new)
        .file rxjs-extensions.ts
        .file hero.service.ts
        .file heroes.component.css
        .file heroes.component.html
        .file heroes.component.ts
        .file main.ts
        .file in-memory-data.service.ts (new)
      .file node_modules ...
      .file typings ...
      .file index.html
      .file package.json
      .file styles.css
      .file systemjs.config.js
      .file tsconfig.json
      .file typings.json

.l-main-section
:marked
  ## Home Stretch

  We are at the end of our journey for now, but we have accomplished a lot.
  - We added the necessary dependencies to use HTTP in our application.
  - We refactored `HeroService` to load heroes from a web API.
  - We extended `HeroService` to support post, put and delete methods.
  - We updated our components to allow adding, editing and deleting of heroes.
  - We configured an in-memory web API.
  - We learned how to use !{_Observable}s.

  Here are the files we added or changed in this chapter.

block file-summary
  +makeTabs(
    `toh-6/ts/app/app.component.ts,
     toh-6/ts/app/app.module.ts,
     toh-6/ts/app/heroes.component.ts,
     toh-6/ts/app/heroes.component.html,
     toh-6/ts/app/heroes.component.css,
     toh-6/ts/app/hero-detail.component.ts,
     toh-6/ts/app/hero-detail.component.html,
     toh-6/ts/app/hero.service.ts,
     toh-6/ts/app/in-memory-data.service.ts`,
    ',,,,,,,,',
    `app.comp...ts,
     app.mod...ts,
     heroes.comp...ts,
     heroes.comp...html,
     heroes.comp...css,
     hero-detail.comp...ts,
     hero-detail.comp...html,
     hero.service.ts,
     in-memory-data.service.ts`
  )

  +makeTabs(
    `toh-6/ts/app/hero-search.service.ts,
    toh-6/ts/app/hero-search.component.ts,
    toh-6/ts/app/hero-search.component.html,
    toh-6/ts/app/hero-search.component.css,
    toh-6/ts/app/rxjs-extensions.ts`,
    null,
    `hero-search.service.ts,
    hero-search.component.ts,
    hero-search.component.html,
    hero-search.component.css,
    rxjs-extensions.ts`
  )