HTTP Client 4.0
HTTP is the primary protocol for browser/server communication.
WebSocket protocol is another important communication technology;
it isn’t covered in this page.
The Dart http library simplifies app programming with the XHR and JSONP APIs.
This page describes server communication with the help of the following demos:
AppComponent orchestrates these demos:
Providing HTTP services
First, configure the app to use server communication facilities.
BrowserClient client communicates with the server using a familiar HTTP request/response protocol.
BrowserClient client is one of a family of services in the Dart http library.
Before you can use the
BrowserClient client, you need to register it as a service provider with the dependency injection system.
Read about providers in the Dependency Injection page.
Register providers using the
The Tour of Heroes HTTP client demo
The first demo is a mini-version of the tutorial’s “Tour of Heroes” (ToH) app.
This version gets some heroes from the server, displays them in a list, lets the user add new heroes, and saves them to the server.
The app uses the Dart
BrowserClient client to communicate via XMLHttpRequest (XHR).
It works like this:
This demo has a single component, the
HeroListComponent. Here’s its template:
It presents the list of heroes with an
Below the list is an input box and an Add Hero button where you can enter the names of new heroes
and add them to the database.
A template reference variable,
newHeroName, accesses the
value of the input box in the
(click) event binding.
When the user clicks the button, that value is passed to the component’s
addHero method and then
the event binding clears it to make it ready for a new hero name.
Below the button is an area for an error message.
The HeroListComponent class
Here’s the component class:
Angular injects a
HeroService into the constructor
and the component calls that service to fetch and save data.
The component does not talk directly to the Dart
The component doesn’t know or care how it gets the data.
It delegates to the
This is a golden rule: always delegate data access to a supporting service class.
Although at runtime the component requests heroes immediately after creation,
you don’t call the service’s
get method in the component’s constructor.
Instead, call it inside the
ngOnInit lifecycle hook
and rely on Angular to call
ngOnInit when it instantiates this component.
This is a best practice. Components are easier to test and debug when their constructors are simple, and all real work (especially calling a remote server) is handled in a separate method.
The hero service
create() asynchronous methods return the Future
values of the current hero list and the newly added hero,
respectively. The hero list component
addHero() methods specify
the actions to be taken when the asynchronous method calls succeed or fail.
With a basic understanding of the component, you’re ready to look inside the
Fetch data with http.get()
In many of the previous samples the app faked the interaction with the server by returning mock heroes in a service like this one:
You can revise that
HeroService to get the heroes from the server using the Dart
BrowserClient client service:
Notice that the Dart
BrowserClient client service is
injected into the
Look closely at how to call
You pass the resource URL to
get and it calls the server which returns heroes.
Process the response object
Remember that the
getHeroes() method used an
_extractData() helper method to map the
_http.get response object to heroes:
response object doesn’t hold the data in a form the app can use directly.
You must parse the response data into a JSON object.
Parse to JSON
The response data are in JSON string form.
You must parse that string into objects, which you do by calling
JSON.decode() method from the
Don’t expect the decoded JSON to be the heroes list directly.
This server always wraps JSON results in an object with a
property. You have to unwrap it to get the heroes.
This is conventional web API behavior, driven by
Make no assumptions about the server API.
Not all servers return an object with a
Do not return the response object
getHeroes() method could have returned the HTTP response but this wouldn’t
follow best practices.
The point of a data service is to hide the server interaction details from consumers.
The component that calls the
HeroService only wants heroes and is kept separate
from getting them, the code dealing with where they come from, and the response object.
Always handle errors
An important part of dealing with I/O is anticipating errors by preparing to catch them and do something with them. One way to handle errors is to pass an error message back to the component for presentation to the user, but only if it says something that the user can understand and act upon.
This simple app conveys that idea, albeit imperfectly, in the way it handles a
HeroListComponent error handling
Back in the
HeroListComponent, you wrapped the call to
_heroService.getHeroes() in a
try clause. When an exception is
errorMessage variable — which you’ve bound conditionally in the
template — gets assigned to.
Want to see it fail? In the
HeroService, reset the api endpoint to a bad value. Afterward, remember to restore it.
Send data to the server
So far you’ve seen how to retrieve data from a remote location using an HTTP service. Now you’ll add the ability to create new heroes and save them in the backend.
You’ll write a method for the
HeroListComponent to call, a
create() method, that takes
just the name of a new hero. It begins like this:
To implement it, you must know the server’s API for creating heroes.
This sample’s data server follows typical REST guidelines.
It expects a
at the same endpoint as
It expects the new hero data to arrive in the body of the request,
structured like a
Hero entity but without the
The body of the request should look like this:
The server generates the
id and returns the entire
of the new hero including its generated id. The hero arrives tucked inside a response object
with its own
Now that you know how the API works, implement
create() as follows:
headers object, the
Content-Type specifies that the body represents JSON.
getHeroes(), use the
_extractData() helper to extract the data
from the response.
Back in the
addHero() method waits for the service’s
create() method to create a hero. When
create() is finished,
addHero() puts the new hero in the
heroes list for presentation to the user.
Cross-Origin Requests: Wikipedia example
You just learned how to make
XMLHttpRequests using the Dart
This is the most common approach to server communication, but it doesn’t work in all scenarios.
For security reasons, web browsers block
XHR calls to a remote server whose origin is different from the origin of the web page.
The origin is the combination of URI scheme, hostname, and port number.
This is called the same-origin policy.
Some servers do not support CORS but do support an older, read-only alternative called JSONP. Wikipedia is one such server.
This Stack Overflow answer covers many details of JSONP.
Here is a simple search that shows suggestions from Wikipedia as the user types in a text box:
Wikipedia offers a modern
CORS API and a legacy
JSONP search API.
The remaining content of this section is coming soon.
In the meantime, consult the
to see how to access Wikipedia via its
See the full source code in the