include ../_util-fns

:marked
  # Getting and Saving Data with HTTP

  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.

p Run the #[+liveExampleLink2('', 'toh-6')] 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.

  ### 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
:marked
 ## Prepare for Http

  `Http` 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 (provide) *http* services
  Our app will depend upon the Angular `http` service which itself depends upon other supporting services.
  The `HTTP_PROVIDERS` array from `@angular/http` library holds providers for the complete set of http services.

  We should be able to access these services from anywhere in the application.
  So we register them in the `bootstrap` method of `main.ts` where we
  launch the application and its root `AppComponent`.

+makeExample('toh-6/ts/app/main.ts','v1','app/main.ts (v1)')(format='.')
:marked
  Notice that we supply the `HTTP_PROVIDERS` in an array as the second parameter to the `bootstrap` method.
  This has the same effect the `providers` array in `@Component` metadata.

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

  We generally recommend registering application-wide services in the root `AppComponent` *providers*.
  Here we're registering in `main` for a special reason.

  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 demo/development service, the *in-memory web api*.

  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`.

  Here is a version of `main` that performs this trick
+makeExample('toh-6/ts/app/main.ts', 'final', 'app/main.ts (final)')(format=".")

:marked
  We're replacing the default `XHRBackend`, the service that talks to the remote server,
  with the *in-memory web api* service after priming it with the following `in-memory-data.service.ts` file:
+makeExample('toh-6/ts/app/in-memory-data.service.ts', null, 'app/in-memory-data.service.ts')(format=".")
:marked
  This file replaces the `mock-heroes.ts` which is now safe to delete.

.alert.is-helpful
  :marked
    This chapter is an introduction to the Angular http client.
    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 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
+makeExample('toh-4/ts/app/hero.service.ts', 'get-heroes', 'app/hero.service.ts (getHeroes - old)')(format=".")
: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 Angular's `Http` client:

+makeExample('toh-6/ts/app/hero.service.ts', 'get-heroes', 'app/hero.service.ts (getHeroes using Http)')(format=".")

:marked
  ### Http Promise

  We're still returning a promise but we're creating it differently.

  The Angular `http.get` returns an RxJS `Observable`.
  *Observables* are a powerful way to manage asynchronous data flows.
  We'll learn about `Observables` *later*.

  For *now* we get back on familiar ground by immediately converting that `Observable` to a `Promise` using the `toPromise` operator.
+makeExample('toh-6/ts/app/hero.service.ts', 'to-promise')(format=".")
: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:
+makeExample('toh-6/ts/app/hero.service.ts', 'rxjs')(format=".")

: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.
+makeExample('toh-6/ts/app/hero.service.ts', 'to-data')(format=".")
:marked
  That object returned by `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 server.
  It knows nothing of the twists and turns required to turn the http response into heroes.
  Such is the beauty and purpose of delegating data access to a service like this `HeroService`.
:marked
  ### Error Handling

  At the end of `getHeroes` we `catch` server failures and pass them to an error handler:
+makeExample('toh-6/ts/app/hero.service.ts', 'catch')(format=".")
:marked
  This is a critical step!
  We must anticipate http failures as they happen frequently for reasons beyond our control.

+makeExample('toh-6/ts/app/hero.service.ts', 'error-handler', 'app/hero.service.ts (Error handler)')(format=".")
:marked
  In this demo service we log the error to the console; we should 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.

  ### Promises are Promises
  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()`.

.l-main-section
:marked
  ## Add, Edit, Delete

  Our stakeholders are incredibly pleased with the added flexibility from the api integration, but it doesn't stop there. Next we want to add the capability to add, edit and delete heroes.

  We'll complete `HeroService` by creating `post`, `put` and `delete` http calls to meet our new requirements.

:marked
  ### Post

  We are using `post` to add new heroes. Post requests require a little bit more setup than Get requests, but the format is as follows:

+makeExample('toh-6/ts/app/hero.service.ts', 'post-hero', 'app/hero.service.ts (post hero)')(format=".")

:marked
  Now we create a header and set the content type to `application/json`. We'll call `JSON.stringify` before we post to convert the hero object to a string.

  ### Put

  `put` is used to edit a specific hero, but the structure is very similar to a `post` request. The only difference is that we have to change the url slightly by appending the id of the hero we want to edit.

+makeExample('toh-6/ts/app/hero.service.ts', 'put-hero', 'app/hero.service.ts (put hero)')(format=".")

:marked
  ### Delete
  `delete` is used to delete heroes and the format is identical to `put` except for the function name.

+makeExample('toh-6/ts/app/hero.service.ts', 'delete-hero', 'app/hero.service.ts (delete hero)')(format=".")

:marked
  We add a `catch` to handle our errors for all three cases.

:marked
  ### Save

  We combine the call to the private `post` and `put` methods in a single `save` method. This simplifies the public api and makes the integration with `HeroDetailComponent` easier. `HeroService` determines which method to call based on the state of the `hero` object. If the hero already has an id we know it's an edit. Otherwise we know it's an add.

+makeExample('toh-6/ts/app/hero.service.ts', 'save', 'app/hero.service.ts (save hero)')(format=".")

:marked
  After these additions our `HeroService` looks like this:

+makeExample('toh-6/ts/app/hero.service.ts', null, 'app/hero.service.ts')(format=".")

.l-main-section
:marked
  ## Updating Components

  Loading heroes using `Http` required no changes outside of `HeroService`, but we added a few new features as well.
  In the following section we will update our components to use our new methods to add, edit and delete heroes.

  Before we can add those methods, we need to initialize some variables with their respective imports.

+makeExample('toh-6/ts/app/hero-detail.component.ts', 'variables-imports', 'app/hero-detail.component.ts')(format=".")

:marked
  ### Add/Edit in the *HeroDetailComponent*

  We already have `HeroDetailComponent` for viewing details about a specific hero. 
  Add and Edit are natural extensions of the detail view, so we are able to reuse `HeroDetailComponent` with a few tweaks. 
  The original component was created to render existing data, but to add new data we have to initialize the `hero` property to an empty `Hero` object.

+makeExample('toh-6/ts/app/hero-detail.component.ts', 'ngOnInit', 'app/hero-detail.component.ts (ngOnInit)')(format=".")

:marked
  In order to differentiate between add and edit we are adding a check to see if an id is passed in the url. If the id is absent we bind `HeroDetailComponent` to an empty `Hero` object. In either case, any edits made through the UI will be bound back to the same `hero` property.

  The next step is to add a save method to `HeroDetailComponent` and call the corresponding save method in `HeroesService`.

+makeExample('toh-6/ts/app/hero-detail.component.ts', 'save', 'app/hero-detail.component.ts (save)')(format=".")

:marked
  The same save method is used for both add and edit since `HeroService` will know when to call `post` vs `put` based on the state of the `Hero` object.

  After we save a hero, we redirect the browser back to the previous page using the `goBack()` method.

+makeExample('toh-6/ts/app/hero-detail.component.ts', 'goback', 'app/hero-detail.component.ts (goBack)')(format=".")

:marked
  Here we call `emit` to notify that we just added or modified a hero. `HeroesComponent` is listening for this notification and will automatically refresh the list of heroes to include our recent updates.

.l-sub-section
  :marked
    The `emit` "handshake" between `HeroDetailComponent` and `HeroesComponent` is an example of component to component communication. This is a topic for another day, but we have detailed information in our <a href="/docs/ts/latest/cookbook/component-communication.html#!#child-to-parent">Component Interaction Cookbook</a>

:marked
  Here is `HeroDetailComponent` with its new save button.

figure.image-display
  img(src='/resources/images/devguide/toh/hero-details-save-button.png' alt="Hero Details With Save Button")

:marked
  ### Add/Delete in the *HeroesComponent*

  The user can *add* a new hero by clicking a button and entering a name.

  When the user clicks the *Add New Hero* button, we display the `HeroDetailComponent`.
  We aren't navigating to the component so it won't receive a hero `id`;
  As we noted above, that is the component's cue to create and present an empty hero.

  Add the following HTML to the `heroes.component.html`, just below the hero list (the `*ngFor`).
+makeExample('toh-6/ts/app/heroes.component.html', 'add-hero', 'app/heroes.component.html (add)')(format=".")
:marked
  The user can *delete* an existing hero by clicking a delete button next to the hero's name.

  Add the following HTML to the `heroes.component.html` right after the name in the repeated `<li>` tag:
+makeExample('toh-6/ts/app/heroes.component.html', 'delete-hero', 'app/heroes.component.html (delete)')(format=".")

:marked
  Now let's fix-up the `HeroesComponent` to support the *add* and *delete* actions in the template.
  Let's start with *add*.

  We're using the `HeroDetailComponent` to capture the new hero information.
  We have to tell Angular about that by importing the `HeroDetailComponent` and referencing it in the component metadata `directives` array.
+makeExample('toh-6/ts/app/heroes.component.ts', 'hero-detail-component', 'app/heroes.component.ts (HeroDetailComponent)')(format=".")
.l-sub-section
  :marked
    These are the same lines that we removed in the previous [Routing](toh-pt5.html) chapter.
    We didn't know at the time that we'd need the *HeroDetailComponent* again. So we tidied up.

    Now we *must* put these lines back. If we don't, Angular will ignore the `<my-hero-detail>`
    tag and pushing the *Add New Hero* button will have no visible effect.
:marked
  Next we implement the click handler for the *Add New Hero* button.

+makeExample('toh-6/ts/app/heroes.component.ts', 'add', 'app/heroes.component.ts (add)')(format=".")
:marked
  The `HeroDetailComponent` does most of the work. All we do is toggle an `*ngIf` flag that
  swaps it into the DOM when we add a hero and removes it from the DOM when the user is done.

  The *delete* logic is a bit trickier.
+makeExample('toh-6/ts/app/heroes.component.ts', 'delete', 'app/heroes.component.ts (delete)')(format=".")

:marked
  Of course we delegate the persistence of hero deletion to the `HeroService`.
  But the component is still responsible for updating the display.
  So the *delete* method removes the deleted hero from the list.

:marked
  ### Let's see it
  Here are the fruits of labor in action:
figure.image-display
  img(src='/resources/images/devguide/toh/toh-http.anim.gif' alt="Heroes List Editting w/ HTTP")

:marked
  ### Review the App Structure
  Let’s verify that we have the following structure after all of our good refactoring in this chapter:

.filetree
  .file angular2-tour-of-heroes
  .children
  .file app
  .children
    .file app.component.ts
    .file app.component.css
    .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.service.ts
    .file heroes.component.css
    .file heroes.component.html
    .file heroes.component.ts
    .file main.ts
    .file hero-data.service.ts
  .file node_modules ...
  .file typings ...
  .file index.html
  .file package.json
  .file styles.css
  .file sample.css
  .file systemjs.config.json
  .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 an api.
    - We extended HeroService to support post, put and delete calls.
    - We updated our components to allow adding, editing and deleting of heroes.
    - We configured an in-memory web api.

    Below is a summary of the files we changed.

+makeTabs(
  `toh-6/ts/app/app.component.ts,
   toh-6/ts/app/heroes.component.ts,
   toh-6/ts/app/heroes.component.html,
   toh-6/ts/app/hero-detail.component.ts,
   toh-6/ts/app/hero-detail.component.html,
   toh-6/ts/app/hero.service.ts`,
  null,
  `app.comp...ts,
   heroes.comp...ts,
   heroes.comp...html,
   hero-detail.comp...ts,
   hero-detail.comp...html,
   hero.service.ts`
)
