Best way to set default header for all request in flutter http request

This can be easily made with the Dio package.

https://pub.dartlang.org/packages/dio

Update

Based on the new Dio API:

var dio = Dio();dio.interceptors.add(InterceptorsWrapper(onRequest: (RequestOptions options) async {  var customHeaders = {    'content-type': 'application/json'    // other headers  };  options.headers.addAll(customHeaders);  return options;}));Response response = await dio.get("url");print(response.data.toString());

Refer the documentation for more details.

import 'package:http/http.dart' as http;class MyClient extends http.BaseClient{  http.Client _httpClient = new http.Client();  MyClient(Map defaultHeaders);  @override  Future<http.StreamedResponse> send(http.BaseRequest request) {    request.headers.addAll(defaultHeaders);    return _httpClient.send(request);  }}

Register custom HttpClientFactory

Dart allows to register a factory for creating an HttpClient

class MyHttpOverrides extends HttpOverrides {  @override  HttpClient createHttpClient(SecurityContext context) {    return new MyHttpClient(super.createHttpClient(context));  }}void main() {  HttpOverrides.global = new MyHttpOverrides();  runApp(MyApp());}

Implement custom HttpClient

Implementing such a custom HttpClient seems overly complicated, but perhaps there is a better way.

The class implements HttpClient from dart:io and all its abstract classes.

The methods get(...) and getUrl(...) are customized to add a custom header using _updateHeaders(...). You would need to extend that to all other methods like head, headUrl, open, post, ...

import 'dart:io';class MyHttpClient implements HttpClient {  HttpClient _realClient;  MyHttpClient(this._realClient);  @override  bool get autoUncompress => _realClient.autoUncompress;  @override  set autoUncompress(bool value) => _realClient.autoUncompress = value;  @override  Duration get connectionTimeout => _realClient.connectionTimeout;  @override  set connectionTimeout(Duration value) =>      _realClient.connectionTimeout = value;  @override  Duration get idleTimeout => _realClient.idleTimeout;  @override  set idleTimeout(Duration value) => _realClient.idleTimeout = value;  @override  int get maxConnectionsPerHost => _realClient.maxConnectionsPerHost;  @override  set maxConnectionsPerHost(int value) =>      _realClient.maxConnectionsPerHost = value;  @override  String get userAgent => _realClient.userAgent;  @override  set userAgent(String value) => _realClient.userAgent = value;  @override  void addCredentials(          Uri url, String realm, HttpClientCredentials credentials) =>      _realClient.addCredentials(url, realm, credentials);  @override  void addProxyCredentials(String host, int port, String realm,          HttpClientCredentials credentials) =>      _realClient.addProxyCredentials(host, port, realm, credentials);  @override  void set authenticate(          Future<bool> Function(Uri url, String scheme, String realm) f) =>      _realClient.authenticate = f;  @override  void set authenticateProxy(          Future<bool> Function(                  String host, int port, String scheme, String realm)              f) =>      _realClient.authenticateProxy = f;  @override  void set badCertificateCallback(          bool Function(X509Certificate cert, String host, int port)              callback) =>      _realClient.badCertificateCallback = callback;  @override  void close({bool force = false}) => _realClient.close(force: force);  @override  Future<HttpClientRequest> delete(String host, int port, String path) =>      _realClient.delete(host, port, path);  @override  Future<HttpClientRequest> deleteUrl(Uri url) => _realClient.deleteUrl(url);  @override  void set findProxy(String Function(Uri url) f) => _realClient.findProxy = f;  @override  Future<HttpClientRequest> get(String host, int port, String path) =>      _updateHeaders(_realClient.get(host, port, path));  Future<HttpClientRequest> _updateHeaders(      Future<HttpClientRequest> httpClientRequest) async {    return (await httpClientRequest)..headers.add('foo', 'bar');  }  @override  Future<HttpClientRequest> getUrl(Uri url) =>      _updateHeaders(_realClient.getUrl(url.replace(path: url.path)));  @override  Future<HttpClientRequest> head(String host, int port, String path) =>      _realClient.head(host, port, path);  @override  Future<HttpClientRequest> headUrl(Uri url) => _realClient.headUrl(url);  @override  Future<HttpClientRequest> open(          String method, String host, int port, String path) =>      _realClient.open(method, host, port, path);  @override  Future<HttpClientRequest> openUrl(String method, Uri url) =>      _realClient.openUrl(method, url);  @override  Future<HttpClientRequest> patch(String host, int port, String path) =>      _realClient.patch(host, port, path);  @override  Future<HttpClientRequest> patchUrl(Uri url) => _realClient.patchUrl(url);  @override  Future<HttpClientRequest> post(String host, int port, String path) =>      _realClient.post(host, port, path);  @override  Future<HttpClientRequest> postUrl(Uri url) => _realClient.postUrl(url);  @override  Future<HttpClientRequest> put(String host, int port, String path) =>      _realClient.put(host, port, path);  @override  Future<HttpClientRequest> putUrl(Uri url) => _realClient.putUrl(url);}