angular-docs-cn/aio/content/tutorial/toh-pt6.md

@title HTTP

@intro We convert our service and components to use Angular's HTTP service

@description 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.

Where We Left Off

In the previous chapter, 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. The application runs and updates automatically as we continue to build the Tour of Heroes.

Providing HTTP Services

Register for HTTP services

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:

{@example 'toh-pt6/ts/src/app/in-memory-data.service.ts' region='init'}

This file replaces the which is now safe to delete.

Heroes and HTTP

Look at our current HeroService implementation 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. Our updated import statements are now: 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. 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.

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*.

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

Get hero by id

The HeroDetailComponent asks the HeroService to fetch a single hero to edit.

The HeroService currently fetches all heroes and then finds the desired hero by filtering for the one with the matching id. That's fine in a simulation. It's wasteful to ask a real server for all heroes when we only want one. Most web APIs support a get-by-id request in the form api/hero/:id (e.g., api/hero/11).

Update the HeroService.getHero method to make a get-by-id request, applying what we just learned to write getHeroes:It's almost the same as getHeroes. The URL identifies which hero the server should update by encoding the hero id into the URL to match the api/hero/:id pattern.

We also adjust to the fact that the data in the response is a single hero object rather than !{_an} !{_array}.

Unchanged getHeroes API

Although we made significant internal changes to getHeroes() and getHero(), the public signatures did not change. We still return a !{_Promise} from both methods. We won't have to update any of the components that call them.

Our stakeholders are thrilled with the web API integration so far. 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.

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!

Updates weren't lost before. What changed? When the app used a list of mock heroes, updates were applied directly to the hero objects within 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.

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

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: 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.

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: 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. 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.Refresh the browser and create some new heroes!

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: The <li> element should now look like this: 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 <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: 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. We want our delete button to be placed at the far right of the hero entry. This extra CSS accomplishes that:

Hero service delete method

The hero service's delete method uses the delete HTTP method to remove the hero from the server: Refresh the browser and try the new delete functionality.

!{_Observable}s

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.

{@example 'toh-pt6/ts/src/app/hero-search.service.ts'}

The !{_priv}http.get() call in HeroSearchService is similar to the one in the HeroService, although the URL now has a query string.

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.

{@example 'toh-pt6/ts/src/app/hero-search.component.html'}

We'll also want to add styles for the new component.

{@example 'toh-pt6/ts/src/app/hero-search.component.css'}

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.

{@example 'toh-pt6/ts/src/app/hero-search.component.ts'}

Search terms

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

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. 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.

Add the search component to the dashboard

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

{@example 'toh-pt6/ts/src/app/dashboard.component.html'}

Finally, we import HeroSearchComponent from hero-search.component.ts and add it to the !{_declarations} !{_array}: Run the app again, go to the Dashboard, and enter some text in the search box. At some point it might look like this.

Application structure and code

Review the sample source code in the for this chapter. Verify that we have the following structure:

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.

Next Step

Return to the learning path where you can read about the concepts and practices you discovered in this tutorial.