docs(http-client): edit copy and true TOC to doc shape (#3360)
This commit is contained in:
Kapunahele Wong
Jules Kremer
parent
a8ad96eb16
commit
4dbd920d46
@ -57,7 +57,7 @@ export class HeroService {
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// #docregion error-handling
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private handleError (error: Response | any) {
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// In a real world app, we might use a remote logging infrastructure
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// In a real world app, you might use a remote logging infrastructure
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let errMsg: string;
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if (error instanceof Response) {
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const body = error.json() || '';
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@ -17,27 +17,48 @@ block includes
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[Fetch](https://developer.mozilla.org/en-US/docs/Web/API/Fetch_API).
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The !{_Angular_http_library} simplifies application programming with the **XHR** and **JSONP** APIs.
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This page covers:
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- [The Tour of Heroes *HTTP* client demo](#http-client).
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- [Fetch data with http.get](#fetch-data).
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<li if-docs="ts"> [RxJS library](#rxjs).</li>
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<li if-docs="ts"> [Enable RxJS operators](#enable-rxjs-operators).</li>
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- [Process the response object](#extract-data).
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- [Always handle errors](#error-handling).
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- [Send data to the server](#update).
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<li if-docs="ts"> [Fall back to promises](#promises).</li>
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- [Cross-Origin Requests: Wikipedia example](#cors).
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<ul if-docs="ts">
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<li> [Search parameters](#search-parameters).</li>
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<li> [More fun with observables](#more-observables).</li>
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# Contents
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* [Demos](#demos)
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* [Providing HTTP Services](#http-providers)
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* [The Tour of Heroes *HTTP* client demo](#http-client)
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- [The `HeroListComponent` class](#HeroListComponent)
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* [Fetch data with `http.get()`](#fetch-data)
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<li if-docs="ts"> [RxJS library](#rxjs-library)
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<ul>
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<li> [Enable RxJS operators](#enable-rxjs-operators)</li>
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</ul>
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- [Guarding against Cross-Site Request Forgery](#xsrf).
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- [Override default request headers (and other request options)](#override-default-request-options).
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- [Appendix: Tour of Heroes _in-memory web api_](#in-mem-web-api).
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</li>
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* [Process the response object](#extract-data)
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- [Parse to `JSON`](#parse-to-json)
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- [Do not return the response object](#no-return-response-object)
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- [Always handle errors](#error-handling)
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- [`HeroListComponent` error handling](#hero-list-component)
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* [Send data to the server](#update)
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- [Headers](#headers)
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- [JSON results](#json-results)
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<ul><li if-docs="ts"> [Fall back to promises](#promises)</ul>
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* [Cross-Origin Requests: Wikipedia example](#cors)
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<ul if-docs="ts">
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<li> [Search Wikipedia](#search-wikipedia)</li>
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<li> [Search parameters](#search-parameters)</li>
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<li> [The WikiComponent](#wikicomponent)</li>
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</ul>
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* [A wasteful app](#wasteful-app)
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<li if-docs="ts"> [More fun with Observables](#more-observables)
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<ul>
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<li> [Create a stream of search terms](#create-stream)</li>
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<li> [Listen for search terms](#listen-for-search-terms)</li>
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</ul>
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</li>
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* [Guarding against Cross-Site Request Forgery](#xsrf)
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* [Override default request headers (and other request options)](#override-default-request-options)
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* [Appendix: Tour of Heroes _in-memory web api_](#in-mem-web-api)
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A <live-example>live example</live-example> illustrates these topics.
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a#demos
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.l-main-section
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:marked
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# Demos
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@ -49,7 +70,7 @@ block demos-list
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- [The Tour of Heroes *HTTP* client demo](#http-client).
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- [Fall back to !{_Promise}s](#promises).
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- [Cross-Origin Requests: Wikipedia example](#cors).
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- [More fun with observables](#more-observables).
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- [More fun with Observables](#more-observables).
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:marked
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The root `AppComponent` orchestrates these demos:
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@ -92,7 +113,7 @@ block http-providers
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.l-sub-section
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:marked
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The `HttpModule` is necessary for making HTTP calls.
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Though the JsonpModule isn't necessary for plain HTTP,
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Though the `JsonpModule` isn't necessary for plain HTTP,
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there is a JSONP demo later in this page.
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Loading its module now saves time.
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.l-main-section#http-client
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@ -160,7 +181,7 @@ block getheroes-and-addhero
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a#HeroService
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.l-main-section#fetch-data
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:marked
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## Fetch data with http.get
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## Fetch data with _http.get()_
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In many of the previous samples the app faked the interaction with the server by
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returning mock heroes in a service like this one:
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@ -196,20 +217,21 @@ a#HeroService
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Clearly this is not a promise.
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In fact, the `http.get` method returns an **Observable** of HTTP Responses (`Observable<Response>`) from the RxJS library
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and `map` is one of the RxJS *operators*.
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and `map()` is one of the RxJS *operators*.
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a#rxjs-library
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.l-main-section
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:marked
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## RxJS library
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<a href="http://reactivex.io/rxjs" target="_blank" title="RxJS Reactive Extensions">RxJS</a>
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is a third party library, endorsed by Angular, that implements the
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<a href="" target="_blank" title="Video: Rob Wormald on observables"><b>asynchronous observable</b></a> pattern.
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<a href="https://www.youtube.com/watch?v=VLGCCpOWFFw" target="_blank" title="Video: Rob Wormald on Observables"><b>asynchronous Observable</b></a> pattern.
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All of the Developer Guide samples have installed the RxJS npm package
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because observables are used widely in Angular applications.
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because Observables are used widely in Angular applications.
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_This_ app needs it when working with the HTTP client.
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But you must take a critical extra step to make RxJS observables usable:
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you must import the RxJS operators individually.
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But you must take a critical extra step to make RxJS Observables usable:
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_you must import the RxJS operators individually_.
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### Enable RxJS operators
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The RxJS library is large.
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@ -217,19 +239,21 @@ a#HeroService
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You should include only necessary features.
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Each code file should add the operators it needs by importing from an RxJS library.
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The `getHeroes` method needs the `map` and `catch` operators so it imports them like this.
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The `getHeroes()` method needs the `map()` and `catch()` operators so it imports them like this.
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+makeExample('server-communication/ts/src/app/toh/hero.service.ts', 'rxjs-imports', 'src/app/app.component.ts (import rxjs)')(format=".")
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.l-main-section
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a#extract-data
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:marked
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## Process the response object
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Remember that the `getHeroes()` method used an `!{_priv}extractData` helper method to map the `!{_priv}http.get` response object to heroes:
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Remember that the `getHeroes()` method used an `!{_priv}extractData()` helper method to map the `!{_priv}http.get` response object to heroes:
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+makeExample('server-communication/ts/src/app/toh/hero.service.ts', 'extract-data', 'src/app/toh/hero.service.ts (excerpt)')(format=".")
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:marked
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The `response` object doesn't hold the data in a form the app can use directly.
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You must parse the response data into a JSON object.
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a#parse-to-json
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:marked
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### Parse to JSON
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block parse-json
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:marked
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@ -256,6 +280,8 @@ block parse-json
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:marked
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Make no assumptions about the server API.
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Not all servers return an object with a `data` property.
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a#no-return-response-object
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:marked
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### Do not return the response object
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The `getHeroes()` method _could_ have returned the HTTP response but this wouldn't
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@ -268,9 +294,9 @@ block parse-json
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.callout.is-important
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header HTTP GET is delayed
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:marked
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The `!{_priv}http.get` does **not send the request just yet.** This observable is
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The `!{_priv}http.get` does **not send the request just yet.** This Observable is
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[*cold*](https://github.com/Reactive-Extensions/RxJS/blob/master/doc/gettingstarted/creating.md#cold-vs-hot-observables),
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which means that the request won't go out until something *subscribes* to the observable.
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which means that the request won't go out until something *subscribes* to the Observable.
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That *something* is the [HeroListComponent](#subscribe).
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a#error-handling
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@ -288,9 +314,9 @@ a#error-handling
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block error-handling
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:marked
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The `catch` operator passes the error object from `http` to the `handleError` method.
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The `catch()` operator passes the error object from `http` to the `handleError()` method.
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The `handleError` method transforms the error into a developer-friendly message,
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logs it to the console, and returns the message in a new, failed observable via `Observable.throw`.
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logs it to the console, and returns the message in a new, failed Observable via `Observable.throw`.
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a#subscribe
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a#hero-list-component
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@ -325,7 +351,7 @@ block hlc-error-handling
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:marked
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To implement it, you must know the server's API for creating heroes.
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[This sample's data server](#server) follows typical REST guidelines.
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[This sample's data server](#in-mem-web-api) follows typical REST guidelines.
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It expects a [`POST`](http://www.w3.org/Protocols/rfc2616/rfc2616-sec9.html#sec9.5) request
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at the same endpoint as `GET` heroes.
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It expects the new hero data to arrive in the body of the request,
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@ -339,12 +365,13 @@ code-example(format="." language="javascript").
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of the new hero including its generated id. The hero arrives tucked inside a response object
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with its own `data` property.
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Now that you know how the API works, implement `addHero()`as follows:
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Now that you know how the API works, implement `addHero()` as follows:
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+ifDocsFor('ts')
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+makeExample('server-communication/ts/src/app/toh/hero.service.ts', 'import-request-options', 'src/app/toh/hero.service.ts (additional imports)')(format=".")
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+makeExample('server-communication/ts/src/app/toh/hero.service.ts', 'addhero', 'src/app/toh/hero.service.ts (addHero)')(format=".")
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a#headers
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:marked
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### Headers
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@ -356,8 +383,9 @@ code-example(format="." language="javascript").
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object is a new instance of `RequestOptions`, a class that allows you to specify
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certain settings when instantiating a request. In this way, [headers](../api/http/index/Headers-class.html) is one of the [RequestOptions](../api/http/index/RequestOptions-class.html).
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In the `return` statement, `options` is the *third* argument of the `post` method, as shown above.
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In the `return` statement, `options` is the *third* argument of the `post()` method, as shown above.
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a#json-results
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:marked
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### JSON results
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@ -366,7 +394,7 @@ code-example(format="." language="javascript").
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block hero-list-comp-add-hero
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:marked
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Back in the `HeroListComponent`, *its* `addHero()` method subscribes to the observable returned by the *service's* `addHero()` method.
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Back in the `HeroListComponent`, *its* `addHero()` method subscribes to the Observable returned by the *service's* `addHero()` method.
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When the data arrive it pushes the new hero object into its `heroes` array for presentation to the user.
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+makeExample('server-communication/ts/src/app/toh/hero-list.component.ts', 'addHero', 'src/app/toh/hero-list.component.ts (addHero)')(format=".")
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@ -375,49 +403,54 @@ block hero-list-comp-add-hero
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:marked
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Although the Angular `http` client API returns an `Observable<Response>` you can turn it into a
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[`Promise<Response>`](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Promise).
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It's easy to do, and in simple cases, a promise-based version looks much
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like the observable-based version.
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It's easy to do, and in simple cases, a Promise-based version looks much
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like the Observable-based version.
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.l-sub-section
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:marked
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While promises may be more familiar, observables have many advantages.
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While Promises may be more familiar, Observables have many advantages.
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:marked
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Here is a comparison of the `HeroService` using promises versus observables,
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Here is a comparison of the `HeroService` using Promises versus Observables,
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highlighting just the parts that are different.
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+makeTabs(
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'server-communication/ts/src/app/toh/hero.service.promise.ts,server-communication/ts/src/app/toh/hero.service.ts',
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'methods, methods',
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'src/app/toh/hero.service.promise.ts (promise-based), src/app/toh/hero.service.ts (observable-based)')
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:marked
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You can follow the promise `then(this.extractData).catch(this.handleError)` pattern as in
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You can follow the Promise `then(this.extractData).catch(this.handleError)` pattern as in
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this example.
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Alternatively, you can call `toPromise(success, fail)`. The observable's `map` callback moves to the first *success* parameter and its `catch` callback to the second *fail* parameter in this pattern: `.toPromise(this.extractData, this.handleError)`.
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Alternatively, you can call `toPromise(success, fail)`. The Observable's `map` callback moves to the
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first *success* parameter and its `catch` callback to the second *fail* parameter
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in this pattern: `.toPromise(this.extractData, this.handleError)`.
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The `errorHandler` forwards an error message as a failed promise instead of a failed `observable`.
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The `errorHandler` forwards an error message as a failed `Promise` instead of a failed `Observable`.
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The diagnostic *log to console* is just one more `then` in the promise chain.
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The diagnostic *log to console* is just one more `then()` in the Promise chain.
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You have to adjust the calling component to expect a `Promise` instead of an `observable`:
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You have to adjust the calling component to expect a `Promise` instead of an `Observable`:
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+makeTabs(
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'server-communication/ts/src/app/toh/hero-list.component.promise.ts, server-communication/ts/src/app/toh/hero-list.component.ts',
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'methods, methods',
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'src/app/toh/hero-list.component.promise.ts (promise-based), src/app/toh/hero-list.component.ts (observable-based)')
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:marked
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The only obvious difference is that you call `then` on the returned promise instead of `subscribe`.
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The only obvious difference is that you call `then()` on the returned Promise instead of `subscribe`.
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Both methods take the same functional arguments.
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.l-sub-section
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:marked
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The less obvious but critical difference is that these two methods return very different results.
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The promise-based `then` returns another promise. You can keep chaining more `then` and `catch` calls, getting a new promise each time.
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The Promise-based `then()` returns another Promise. You can keep chaining
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more `then()` and `catch()` calls, getting a new promise each time.
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The `subscribe` method returns a `Subscription`. A `Subscription` is not another `Observable`.
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It's the end of the line for observables. You can't call `map` on it or call `subscribe` again.
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The `subscribe()` method returns a `Subscription`. A `Subscription` is not another `Observable`.
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It's the end of the line for Observables. You can't call `map()` on it or call `subscribe()` again.
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The `Subscription` object has a different purpose, signified by its primary method, `unsubscribe`.
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To understand the implications and consequences of subscriptions, watch [Ben Lesh's talk on observables](https://www.youtube.com/watch?v=3LKMwkuK0ZE) or his video course on [egghead.io](https://egghead.io/lessons/rxjs-rxjs-observables-vs-promises).
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To understand the implications and consequences of subscriptions,
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watch [Ben Lesh's talk on Observables](https://www.youtube.com/watch?v=3LKMwkuK0ZE)
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or his video course on [egghead.io](https://egghead.io/lessons/rxjs-rxjs-observables-vs-promises).
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h2#cors Cross-Origin Requests: Wikipedia example
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:marked
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@ -432,7 +465,7 @@ h2#cors Cross-Origin Requests: Wikipedia example
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:marked
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Modern browsers do allow `XHR` requests to servers from a different origin if the server supports the
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[CORS](https://en.wikipedia.org/wiki/Cross-origin_resource_sharing) protocol.
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If the server requires user credentials, you'll enable them in the [request headers](#headers).
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If the server requires user credentials, enable them in the [request headers](#headers).
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:marked
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Some servers do not support CORS but do support an older, read-only alternative called [JSONP](https://en.wikipedia.org/wiki/JSONP).
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@ -440,8 +473,9 @@ h2#cors Cross-Origin Requests: Wikipedia example
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.l-sub-section
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:marked
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This [Stack Overflow answer](http://stackoverflow.com/questions/2067472/what-is-jsonp-all-about/2067584#2067584) covers many details of JSONP.
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a#search-wikipedia
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:marked
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### Search wikipedia
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### Search Wikipedia
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Here is a simple search that shows suggestions from Wikipedia as the user
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types in a text box:
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@ -453,13 +487,13 @@ block wikipedia-jsonp+
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:marked
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Wikipedia offers a modern `CORS` API and a legacy `JSONP` search API. This example uses the latter.
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The Angular `Jsonp` service both extends the `!{_Http}` service for JSONP and restricts you to `GET` requests.
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All other HTTP methods throw an error because JSONP is a read-only facility.
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All other HTTP methods throw an error because `JSONP` is a read-only facility.
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As always, wrap the interaction with an Angular data access client service inside a dedicated service, here called `WikipediaService`.
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+makeExample('server-communication/ts/src/app/wiki/wikipedia.service.ts',null,'src/app/wiki/wikipedia.service.ts')
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:marked
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The constructor expects Angular to inject its `jsonp` service, which
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The constructor expects Angular to inject its `Jsonp` service, which
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is available because `JsonpModule` is in the root `@NgModule` `imports` array
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in `app.module.ts`.
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@ -486,7 +520,8 @@ block wikipedia-jsonp+
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This time you call `jsonp` with *two* arguments: the `wikiUrl` and an options object whose `search` property is the `params` object.
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+makeExample('server-communication/ts/src/app/wiki/wikipedia.service.ts','call-jsonp','src/app/wiki/wikipedia.service.ts (call jsonp)')(format=".")
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:marked
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`Jsonp` flattens the `params` object into the same query string you saw earlier, putting the request on the wire.
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`Jsonp` flattens the `params` object into the same query string you saw earlier, sending the request
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to the server.
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<a id="wikicomponent"></a>
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:marked
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@ -500,26 +535,27 @@ block wikipedia-jsonp+
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and calls a `search(term)` method after each `keyup` event.
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The component's `search(term)` method delegates to the `WikipediaService`, which returns an
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observable array of string results (`Observable<string[]>`).
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Instead of subscribing to the observable inside the component, as in the `HeroListComponent`,
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the app forwards the observable result to the template (via `items`) where the `async` pipe
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Observable !{_array} of string results (`Observable<string[]>`).
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Instead of subscribing to the Observable inside the component, as in the `HeroListComponent`,
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the app forwards the Observable result to the template (via `items`) where the `async` pipe
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in the `ngFor` handles the subscription. Read more about [async pipes](pipes.html#async-pipe)
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in the [Pipes](pipes.html) page.
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.l-sub-section
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:marked
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The [async pipe](pipes.html#async-pipe) is a good choice in read-only components where the component has no need to interact with the data.
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The [async pipe](pipes.html#async-pipe) is a good choice in read-only components
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where the component has no need to interact with the data.
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`HeroListComponent` can't use the pipe because `addHero()` pushes newly created heroes into the list.
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|
||||
a#wasteful-app
|
||||
:marked
|
||||
## A wasteful app
|
||||
|
||||
The wikipedia search makes too many calls to the server.
|
||||
It is inefficient, and potentially expensive on mobile devices with limited data plans.
|
||||
The 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
|
||||
Presently, the code calls the server after every keystroke.
|
||||
It should only make requests when the user *stops typing* .
|
||||
It should only make requests when the user *stops typing*.
|
||||
Here's how it will work after refactoring:
|
||||
figure.image-display
|
||||
img(src='/resources/images/devguide/server-communication/wiki-2.gif' alt="Wikipedia search app (v.2)" width="250")
|
||||
@ -544,13 +580,13 @@ block wikipedia-jsonp+
|
||||
no matter which response arrives first.
|
||||
|
||||
<a id="more-observables"></a>
|
||||
## More fun with observables
|
||||
## More fun with Observables
|
||||
|
||||
You could make changes to the `WikipediaService`, but for a better
|
||||
user experience, create a copy of the `WikiComponent` instead and make it smarter,
|
||||
with the help of some nifty observable operators.
|
||||
with the help of some nifty Observable operators.
|
||||
|
||||
Here's the `WikiSmartComponent`, shown next to the original `WikiComponent`
|
||||
Here's the `WikiSmartComponent`, shown next to the original `WikiComponent`:
|
||||
|
||||
+makeTabs(
|
||||
`server-communication/ts/src/app/wiki/wiki-smart.component.ts,
|
||||
@ -563,21 +599,24 @@ block wikipedia-jsonp+
|
||||
While the templates are virtually identical,
|
||||
there's a lot more RxJS in the "smart" version,
|
||||
starting with `debounceTime`, `distinctUntilChanged`, and `switchMap` operators,
|
||||
imported as [described above](#rxjs).
|
||||
imported as [described above](#rxjs-library).
|
||||
|
||||
a#create-stream
|
||||
:marked
|
||||
### Create a stream of search terms
|
||||
|
||||
The `WikiComponent` passes a new search term directly to the `WikipediaService` after every keystroke.
|
||||
|
||||
The `WikiSmartComponent` class turns the user's keystrokes into an observable _stream of search terms_
|
||||
The `WikiSmartComponent` class turns the user's keystrokes into an Observable _stream of search terms_
|
||||
with the help of a `Subject`, which you import from RxJS:
|
||||
+makeExample('server-communication/ts/src/app/wiki/wiki-smart.component.ts', 'import-subject')(format='.')
|
||||
:marked
|
||||
The component creates a `searchTermStream` as a `Subject` of type `string`.
|
||||
The `search` method adds each new search box value to that stream via the subject's `next` method.
|
||||
The `search()` method adds each new search box value to that stream via the subject's `next()` method.
|
||||
|
||||
+makeExample('server-communication/ts/src/app/wiki/wiki-smart.component.ts', 'subject')(format='.')
|
||||
|
||||
|
||||
a#listen-for-search-terms
|
||||
:marked
|
||||
### Listen for search terms
|
||||
|
||||
@ -595,11 +634,11 @@ block wikipedia-jsonp+
|
||||
calls the `WikipediaService` with a fresh, debounced search term and coordinates the stream(s) of service response.
|
||||
|
||||
The role of `switchMap` is particularly important.
|
||||
The `WikipediaService` returns a separate observable of string arrays (`Observable<string[]>`) for each search request.
|
||||
The `WikipediaService` returns a separate Observable of string arrays (`Observable<string[]>`) for each search request.
|
||||
The user could issue multiple requests before a slow server has had time to reply,
|
||||
which means a backlog of response observables could arrive at the client, at any moment, in any order.
|
||||
which means a backlog of response Observables could arrive at the client, at any moment, in any order.
|
||||
|
||||
The `switchMap` returns its own observable that _combines_ all `WikipediaService` response observables,
|
||||
The `switchMap` returns its own Observable that _combines_ all `WikipediaService` response Observables,
|
||||
re-arranges them in their original request order,
|
||||
and delivers to subscribers only the most recent search results.
|
||||
|
||||
@ -609,7 +648,7 @@ a#xsrf
|
||||
## Guarding against Cross-Site Request Forgery
|
||||
|
||||
In a cross-site request forgery (CSRF or XSRF), an attacker tricks the user into visiting
|
||||
a different web page with malignant code that secretly sends a malicious request to your application's web server,
|
||||
a different web page with malignant code that secretly sends a malicious request to your application's web server.
|
||||
|
||||
The server and client application must work together to thwart this attack.
|
||||
Angular's `Http` client does its part by applying a default `CookieXSRFStrategy` automatically to all requests.
|
||||
@ -631,7 +670,7 @@ a#override-default-request-options
|
||||
before the request is processed.
|
||||
The `HttpModule` provides these default options via the `RequestOptions` token.
|
||||
|
||||
You can override these defaults to suit your application needs.
|
||||
You can override these defaults to suit your application needs
|
||||
by creating a custom sub-class of `RequestOptions`
|
||||
that sets the default options for the application.
|
||||
|
||||
@ -646,7 +685,7 @@ a#override-default-request-options
|
||||
:marked
|
||||
Remember to include this provider during setup when unit testing the app's HTTP services.
|
||||
:marked
|
||||
After this change, the `header` option setting in `HeroService.addHero` is no longer necessary,
|
||||
After this change, the `header` option setting in `HeroService.addHero()` is no longer necessary,
|
||||
|
||||
+makeExample('server-communication/ts/src/app/toh/hero.service.ts', 'addhero', 'src/app/toh/hero.service.ts (addHero)')(format=".")
|
||||
:marked
|
||||
@ -709,7 +748,7 @@ block redirect-to-web-api
|
||||
:marked
|
||||
Finally, redirect client HTTP requests to the in-memory web API by
|
||||
adding the `InMemoryWebApiModule` to the `AppModule.imports` list.
|
||||
At the same time, call its `forRoot` configuration method with the `HeroData` class.
|
||||
At the same time, call its `forRoot()` configuration method with the `HeroData` class.
|
||||
+makeExample('server-communication/ts/src/app/app.module.ts', 'in-mem-web-api', 'src/app/app.module.ts')(format=".")
|
||||
:marked
|
||||
### How it works
|
||||
@ -722,7 +761,7 @@ block redirect-to-web-api
|
||||
At the same time, the `forRoot` method initializes the in-memory web API with the *seed data* from the mock hero dataset.
|
||||
.l-sub-section
|
||||
:marked
|
||||
The `forRoot` method name is a strong reminder that you should only call the `InMemoryWebApiModule` _once_,
|
||||
The `forRoot()` method name is a strong reminder that you should only call the `InMemoryWebApiModule` _once_,
|
||||
while setting the metadata for the root `AppModule`. Don't call it again.
|
||||
:marked
|
||||
Here is the final, revised version of <span ngio-ex>src/app/app.module.ts</span>, demonstrating these steps.
|
||||
|
||||
Loading…
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Reference in New Issue
Block a user