目录

发送网络数据

目录

Sending data to the internet is necessary for most apps. The http package has got that covered, too.

This recipe uses the following steps:

  1. Add the http package.
  2. Send data to a server using the http package.
  3. Convert the response into a custom Dart object.
  4. Get a title from user input.
  5. Display the response on screen.

1. Add the http package

#

To add the http package as a dependency, run flutter pub add:

flutter pub add http

Import the http package.

dart
import 'package:http/http.dart' as http;

如果你要部署 Android，请编辑 AndroidManifest.xml 文件，添加 Internet 权限。

xml
<!-- Required to fetch data from the internet. -->
<uses-permission android:name="android.permission.INTERNET" />

同样，如果你要部署 macOS，请编辑 macos/Runner/DebugProfile.entitlements 和 macos/Runner/Release.entitlements 文件，添加 network client 权限。

xml
<!-- Required to fetch data from the internet. -->
<key>com.apple.security.network.client</key>
<true/>

2. Sending data to server

#

This recipe covers how to create an Album by sending an album title to the JSONPlaceholder using the http.post() method.

Import dart:convert for access to jsonEncode to encode the data:

dart
import 'dart:convert';

Use the http.post() method to send the encoded data:

dart
Future<http.Response> createAlbum(String title) {
  return http.post(
    Uri.parse('https://jsonplaceholder.typicode.com/albums'),
    headers: <String, String>{
      'Content-Type': 'application/json; charset=UTF-8',
    },
    body: jsonEncode(<String, String>{
      'title': title,
    }),
  );
}

The http.post() method returns a Future that contains a Response.

  • Future is a core Dart class for working with asynchronous operations. A Future object represents a potential value or error that will be available at some time in the future.
  • The http.Response class contains the data received from a successful http call.
  • The createAlbum() method takes an argument title that is sent to the server to create an Album.

3. Convert the http.Response to a custom Dart object

#

While it's easy to make a network request, working with a raw Future<http.Response> isn't very convenient. To make your life easier, convert the http.Response into a Dart object.

Create an Album class

#

First, create an Album class that contains the data from the network request. It includes a factory constructor that creates an Album from JSON.

Converting JSON with pattern matching is only one option. For more information, see the full article on JSON and serialization.

dart
class Album {
  final int id;
  final String title;

  const Album({required this.id, required this.title});

  factory Album.fromJson(Map<String, dynamic> json) {
    return switch (json) {
      {
        'id': int id,
        'title': String title,
      } =>
        Album(
          id: id,
          title: title,
        ),
      _ => throw const FormatException('Failed to load album.'),
    };
  }
}

Convert the http.Response to an Album

#

Use the following steps to update the createAlbum() function to return a Future<Album>:

  1. Convert the response body into a JSON Map with the dart:convert package.
  2. If the server returns a CREATED response with a status code of 201, then convert the JSON Map into an Album using the fromJson() factory method.
  3. If the server doesn't return a CREATED response with a status code of 201, then throw an exception. (Even in the case of a "404 Not Found" server response, throw an exception. Do not return null. This is important when examining the data in snapshot, as shown below.)
dart
Future<Album> createAlbum(String title) async {
  final response = await http.post(
    Uri.parse('https://jsonplaceholder.typicode.com/albums'),
    headers: <String, String>{
      'Content-Type': 'application/json; charset=UTF-8',
    },
    body: jsonEncode(<String, String>{
      'title': title,
    }),
  );

  if (response.statusCode == 201) {
    // If the server did return a 201 CREATED response,
    // then parse the JSON.
    return Album.fromJson(jsonDecode(response.body) as Map<String, dynamic>);
  } else {
    // If the server did not return a 201 CREATED response,
    // then throw an exception.
    throw Exception('Failed to create album.');
  }
}

Hooray! Now you've got a function that sends the title to a server to create an album.

4. Get a title from user input

#

Next, create a TextField to enter a title and a ElevatedButton to send data to server. Also define a TextEditingController to read the user input from a TextField.

When the ElevatedButton is pressed, the _futureAlbum is set to the value returned by createAlbum() method.

dart
Column(
  mainAxisAlignment: MainAxisAlignment.center,
  children: <Widget>[
    TextField(
      controller: _controller,
      decoration: const InputDecoration(hintText: 'Enter Title'),
    ),
    ElevatedButton(
      onPressed: () {
        setState(() {
          _futureAlbum = createAlbum(_controller.text);
        });
      },
      child: const Text('Create Data'),
    ),
  ],
)

On pressing the Create Data button, make the network request, which sends the data in the TextField to the server as a POST request. The Future, _futureAlbum, is used in the next step.

5. Display the response on screen

#

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:

  1. The Future you want to work with. In this case, the future returned from the createAlbum() function.
  2. A builder function that tells Flutter what to render, depending on the state of the Future: loading, success, or error.

Note that snapshot.hasData only returns true when the snapshot contains a non-null data value. This is why the createAlbum() function should throw an exception even in the case of a "404 Not Found" server response. If createAlbum() returns null, then CircularProgressIndicator displays indefinitely.

dart
FutureBuilder<Album>(
  future: _futureAlbum,
  builder: (context, snapshot) {
    if (snapshot.hasData) {
      return Text(snapshot.data!.title);
    } else if (snapshot.hasError) {
      return Text('${snapshot.error}');
    }

    return const CircularProgressIndicator();
  },
)

Complete example

#
dart
import 'dart:async';
import 'dart:convert';

import 'package:flutter/material.dart';
import 'package:http/http.dart' as http;

Future<Album> createAlbum(String title) async {
  final response = await http.post(
    Uri.parse('https://jsonplaceholder.typicode.com/albums'),
    headers: <String, String>{
      'Content-Type': 'application/json; charset=UTF-8',
    },
    body: jsonEncode(<String, String>{
      'title': title,
    }),
  );

  if (response.statusCode == 201) {
    // If the server did return a 201 CREATED response,
    // then parse the JSON.
    return Album.fromJson(jsonDecode(response.body) as Map<String, dynamic>);
  } else {
    // If the server did not return a 201 CREATED response,
    // then throw an exception.
    throw Exception('Failed to create album.');
  }
}

class Album {
  final int id;
  final String title;

  const Album({required this.id, required this.title});

  factory Album.fromJson(Map<String, dynamic> json) {
    return switch (json) {
      {
        'id': int id,
        'title': String title,
      } =>
        Album(
          id: id,
          title: title,
        ),
      _ => throw const FormatException('Failed to load album.'),
    };
  }
}

void main() {
  runApp(const MyApp());
}

class MyApp extends StatefulWidget {
  const MyApp({super.key});

  @override
  State<MyApp> createState() {
    return _MyAppState();
  }
}

class _MyAppState extends State<MyApp> {
  final TextEditingController _controller = TextEditingController();
  Future<Album>? _futureAlbum;

  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Create Data Example',
      theme: ThemeData(
        colorScheme: ColorScheme.fromSeed(seedColor: Colors.deepPurple),
      ),
      home: Scaffold(
        appBar: AppBar(
          title: const Text('Create Data Example'),
        ),
        body: Container(
          alignment: Alignment.center,
          padding: const EdgeInsets.all(8),
          child: (_futureAlbum == null) ? buildColumn() : buildFutureBuilder(),
        ),
      ),
    );
  }

  Column buildColumn() {
    return Column(
      mainAxisAlignment: MainAxisAlignment.center,
      children: <Widget>[
        TextField(
          controller: _controller,
          decoration: const InputDecoration(hintText: 'Enter Title'),
        ),
        ElevatedButton(
          onPressed: () {
            setState(() {
              _futureAlbum = createAlbum(_controller.text);
            });
          },
          child: const Text('Create Data'),
        ),
      ],
    );
  }

  FutureBuilder<Album> buildFutureBuilder() {
    return FutureBuilder<Album>(
      future: _futureAlbum,
      builder: (context, snapshot) {
        if (snapshot.hasData) {
          return Text(snapshot.data!.title);
        } else if (snapshot.hasError) {
          return Text('${snapshot.error}');
        }

        return const CircularProgressIndicator();
      },
    );
  }
}

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