webrtc中的AsyncResolver使用

连接远程服务时，如果没有直接提供ip，只有域名，就需要使用gethostbyname或者getaddrinfo来解析出ip地址，这一过程需要dns访问，是阻塞的。在webrtc的例子peer_connection_client.cc代码中，给出了使用rtc::AsyncResolver获取ip地址的方法，值得效仿。

void PeerConnectionClient::Connect(const std::string& server,
                                   int port,
                                   const std::string& client_name) {
  RTC_DCHECK(!server.empty());
  RTC_DCHECK(!client_name.empty());

  if (state_ != NOT_CONNECTED) {
    RTC_LOG(WARNING)
        << "The client must not be connected before you can call Connect()";
    callback_->OnServerConnectionFailure();
    return;
  }

  if (server.empty() || client_name.empty()) {
    callback_->OnServerConnectionFailure();
    return;
  }

  if (port <= 0)
    port = kDefaultServerPort;

  server_address_.SetIP(server);
  server_address_.SetPort(port);
  client_name_ = client_name;

  if (server_address_.IsUnresolvedIP()) {
    state_ = RESOLVING;
    resolver_ = new rtc::AsyncResolver();
    resolver_->SignalDone.connect(this, &PeerConnectionClient::OnResolveResult);
    resolver_->Start(server_address_);
  } else {
    DoConnect();
  }
}

void PeerConnectionClient::OnResolveResult(
    rtc::AsyncResolverInterface* resolver) {
  if (resolver_->GetError() != 0) {
    callback_->OnServerConnectionFailure();
    resolver_->Destroy(false);
    resolver_ = NULL;
    state_ = NOT_CONNECTED;
  } else {
    server_address_ = resolver_->address();
    DoConnect();
  }
}

它实现了异步的调用方式，感兴趣的话可以进一步看看rtc::AsyncResolver实现的源码。