http/2 over http tunnel proxy for Dart

Flutter 项目里用到了 http2 包 (https://pub.dev/packages/http2) 。但官方似乎没有支持 http proxy ，转念一想，似乎也对，proxy 这层不属于 http2 协议该负责实现的事。

研究了一下 http/1.1 tunnel proxy 的协议（RFC 2817）发现其实挺简单的。

这里直接放出示例了，仅供参考哦（需要登录的 proxy 就留给你自己实现了）。

import 'dart:async';
import 'dart:convert';
import 'dart:io';
import 'package:http2/http2.dart';

void main() async {
  var uri = Uri.parse('https://cn.bing.com/search?q=http2');
  var transport = await connectTunnel('127.0.0.1', 8888, uri.host);
  var headers = [
    Header.ascii(':method', 'GET'),
    Header.ascii(':path', uri.hasQuery ? uri.path + '?' + uri.query : uri.path),
    Header.ascii(':scheme', uri.scheme),
    Header.ascii(':authority', uri.host),
  ];
  var stream = transport.makeRequest(headers, endStream: true);
  var bodyStream = StreamController<List<int>>();
  utf8.decoder.bind(bodyStream.stream).listen((body) {
    print(body);
  });
  await for (var message in stream.incomingMessages) {
    if (message is HeadersStreamMessage) {
      for (var header in message.headers) {
        var name = utf8.decode(header.name);
        var value = utf8.decode(header.value);
        print('Header: $name: $value');
      }
    } else if (message is DataStreamMessage) {
      bodyStream.add(message.bytes);
      if (message.endStream) {
        bodyStream.close();
      }
    }
  }
  if (!bodyStream.isClosed) {
    await bodyStream.close();
  }
  await transport.finish();
}

Future<ClientTransportConnection> connectTunnel(String proxyHost, int proxyPort, String targetHost,
    [int targetPort = 443]) async {
  var proxy = await Socket.connect(proxyHost, proxyPort, timeout: const Duration(seconds: 1));
  const CRLF = "\r\n";
  proxy.write("CONNECT $targetHost:$targetPort HTTP/1.1"); // request line
  proxy.write(CRLF);
  proxy.write("Host: $targetHost:$targetPort"); // header
  proxy.write(CRLF);
  proxy.write(CRLF);
  var completer = Completer<bool>.sync();
  var sub = proxy.listen((event) {
    var response = ascii.decode(event);
    var lines = response.split(CRLF);
    // status line
    var statusLine = lines.first;
    if (statusLine.startsWith("HTTP/1.1 200")) {
      completer.complete();
    } else {
      completer.completeError(statusLine);
    }
  }, onError: completer.completeError);
  await completer.future; // established
  await sub.pause();

  var socket = await SecureSocket.secure(proxy, host: targetHost, supportedProtocols: const ["h2"]);
  return ClientTransportConnection.viaSocket(socket);
}