flutter dart listen cpp 部分

示例：

// EventStream handlers:

  void OnStreamListen(std::unique_ptr<flutter::EventSink<>>&& events);

  void OnStreamCancel();

  // Sends a state event to |event_sink_| with the current charging status.

  void SendNotifyEvent(const char* event);

  std::unique_ptr<flutter::EventSink<>> event_sink_;

cPP：

flutter::EventChannel<> charging_channel(

      flutter_controller_->engine()->messenger(), "samples.flutter.io/charging",

      &flutter::StandardMethodCodec::GetInstance());

  charging_channel.SetStreamHandler(

      std::make_unique<flutter::StreamHandlerFunctions<>>(

          [this](auto arguments, auto events) {

            this->OnStreamListen(std::move(events));

            return nullptr;

          },

          [this](auto arguments) {

            this->OnStreamCancel();

            return nullptr;

          }));

void FlutterWindow::OnStreamListen(

    std::unique_ptr<flutter::EventSink<>>&& events) {

    event_sink_ = std::move(events);

}

void FlutterWindow::OnStreamCancel() { event_sink_ = nullptr; }

void FlutterWindow::SendNotifyEvent(const char* event) {

  event_sink_->Success(flutter::EncodableValue(event));

}