- var _example = 'toh-6'; block includes include ../_util-fns - var _Http = 'Http'; // Angular `Http` library name. - var _Angular_Http = 'Angular Http' - var _Angular_http_library = 'Angular HTTP library' - var _HttpModule = 'HttpModule' - var _JSON_stringify = 'JSON.stringify' //- Shared var definitions - var _promise = _Promise.toLowerCase() :marked 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 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*. 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 mock 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 !{_appModuleTsVsMainTs} 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 — the supporting service that talks to the remote server — 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.

HTTP !{_Promise}

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 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 `
  • ` tag: +makeExcerpt('app/heroes.component.html', 'delete', '') :marked The `
  • ` 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 — we don't want the `
  • ` 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. #observables :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. Another notable difference: we no longer call `toPromise`, we simply return the *observable* instead. ### HeroSearchComponent Let's create a new `HeroSearchComponent` that calls this new `HeroSearchService`. The component template is simple — 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 #### Initialize the _**heroes**_ property (_**ngOnInit**_) A `Subject` is also an `Observable`. 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 hero-search.component.ts 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 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.module.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 index.html .file package.json .file styles.css .file systemjs.config.js .file tsconfig.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` )