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libcurl for Flutter

Flutter plugin to use libcurl for HTTP calls in Flutter Android & iOS apps. HTTP stack built in to Dart as part of dart:io supports only HTTP 1.1. This plugin aims to bring upto date features in connectivity available in libcurl such as:

  • HTTP2 with Nghttp2
  • Automatic upgrade to HTTP2 from 1.1 with ALPN TLS extension
  • Brotli compression
  • Experimental HTTP3 and alt-svc support

Getting started

Add to project:

flutter pub add flutter_curl

Example usage:

import 'package:flutter_curl/flutter_curl.dart';
import 'package:path/path.dart' as path;
import 'package:path_provider/path_provider.dart' as paths;

// Initialize client
Client client = Client(
      verbose: true,
      interceptors: [
        // HTTPCaching(),
await client.init();

// Send request
final res = await c.send(Request(
      method: "GET",
      url: "",
      headers: {
          "user-agent": "myapp-v1.0/android9"
      // body
      // body: RequestBody.raw(utf8.encode("hello world")),
      // body: RequestBody.string("hello world"),
      // body: RequestBody.form({"age": "10", "hello": "world"}),
      // body: RequestBody.multipart([
      //    Multipart(name: "age", data: "24"),
      //    Multipart(name: "hello", data: "world"),
      //    Multipart.file(
      //      name: "fieldname",
      //      path: "/sdcard/todo.txt",
      //      filename: "filename.txt",
      //    ),
      //  ]),

// Read response
print("Status: ${res.statusCode}");
print("HTTP: ${res.httpVersion}");
res.headers.forEach((key, value) {
    print("$key: $value");

How it works

This plugin uses custom built libcurl libraries distributed via releases. If you would like to build these by yourself follow instructions in cURL project. The configuration used for above mentioned builds is:

  • Ngtcp2 for HTTP2
  • libbrotli
  • NDK min SDK: 21 (armv7a, arm64, x86_64), iOS min SDK: 8.0 (arm64, x86_64)
  • Android binaries are packaged as aar and iOS binary as framework

This will be downloaded once the project builds for Android or when pod install happens for iOS. In Android these binaries are dynamically linked and in iOS it is statically linked.

The cURL APIs are accessed directly in a different isolate in Flutter using the dart:ffi APIs(beta). Isolate is used to avoid unresponsiveness in the app as the cURL APIs are blocking.

Reducing APK size

Flutter's --target-platform argument only removes its own native binaries for unspecified architectures. This is not adding abiFilters to the project gradle. So this doesn't remove the binaries for unspecified architectures from the plugins like flutter_curl.

A workaround suggested in flutter_vpn project is to add this logic to the project gradle config manually like below:

android {
    buildTypes {
        release {
            ndk {
                if (!project.hasProperty('target-platform')) {
                    abiFilters 'arm64-v8a', 'armeabi-v7a', 'x86_64'
                } else {
                    def platforms ='target-platform').split(',')
                    def platformMap = [
                            'android-arm'  : 'armeabi-v7a',
                            'android-arm64': 'arm64-v8a',
                            'android-x86'  : 'x86',
                            'android-x64'  : 'x86_64',
                    abiFilters ={ e ->
                        platformMap.containsKey(e) ? platformMap[e] : e