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userId: userId,
id: id,
title: title,
),
_ => throw const FormatException('Failed to load album.'),
};
}
}
<code_end>
<topic_end>
<topic_start>
convert the http.Response to an album
now, use the following steps to update the fetchAlbum()
function to return a Future<Album>:
<code_start>
Future<Album> fetchAlbum() async {
final response = await http
.get(uri.parse('https://jsonplaceholder.typicode.com/albums/1'));
if (response.statuscode == 200) {
// if the server did return a 200 OK response,
// then parse the JSON.
return Album.fromJson(jsonDecode(response.body) as Map<String, dynamic>);
} else {
// if the server did not return a 200 OK response,
// then throw an exception.
throw Exception('Failed to load album');
}
}
<code_end>
hooray!
now you’ve got a function that fetches an album from the internet.
<topic_end>
<topic_start>
4. fetch the data
call the fetchAlbum() method in either the
initState() or didChangeDependencies()
methods.
the initState() method is called exactly once and then never again.
if you want to have the option of reloading the API in response to an
InheritedWidget changing, put the call into the
didChangeDependencies() method.
see state for more details.
<code_start>
class _MyAppState extends State<MyApp> {
late Future<Album> futureAlbum;
@override
void initState() {
super.initState();
futureAlbum = fetchAlbum();
}
// ···
}
<code_end>
this future is used in the next step.
<topic_end>
<topic_start>
5. display the data
to display the data on screen, use the
FutureBuilder widget.
the FutureBuilder widget comes with flutter and
makes it easy to work with asynchronous data sources.
you must provide two parameters:
note that snapshot.hasData only returns true
when the snapshot contains a non-null data value.
because fetchAlbum can only return non-null values,
the function should throw an exception
even in the case of a “404 not found” server response.
throwing an exception sets the snapshot.hasError to true
which can be used to display an error message.
otherwise, the spinner will be displayed.
<code_start>
FutureBuilder<Album>(
future: futureAlbum,
builder: (context, snapshot) {
if (snapshot.hasdata) {
return text(snapshot.data!.title);
} else if (snapshot.haserror) {
return text('${snapshot.error}');
}
// by default, show a loading spinner.
return const CircularProgressIndicator();
},
)
<code_end>
<topic_end>
<topic_start>
why is fetchAlbum() called in initState()?
although it’s convenient,
it’s not recommended to put an API call in a build() method.
flutter calls the build() method every time it needs
to change anything in the view,
and this happens surprisingly often.
the fetchAlbum() method, if placed inside build(), is repeatedly
called on each rebuild causing the app to slow down.
storing the fetchAlbum() result in a state variable ensures that
the future is executed only once and then cached for subsequent
rebuilds.
<topic_end>
<topic_start>
testing