How to create a Event Channel in Flutter Windows Desktop Application (Dart and C++) ? Dart and CPP

Question

I want create a event channel in my flutter windows application for asynchronous event streams between flutter and windows app.

where I want to get a continuous data to be listened, but i am not finding any example on internet on how to create a Event channel between flutter and native windows application.

Thanks in advance if any one can help

I have tried method channel which works but event channel i am not able to create

Expecting a sample working code of flutter_windows.cpp file to create a Event Channel

Answer 1

Now able to create the Event channel, it will be helpful for some one as me.

In Dart add below code to create a Event channel import 'dart:async';

import 'package:flutter/material.dart';
import 'package:flutter/services.dart';

class PlatformChannel extends StatefulWidget {
  const PlatformChannel({super.key});

  @override
  State<PlatformChannel> createState() => _PlatformChannelState();
}

class _PlatformChannelState extends State<PlatformChannel> {
  
  static const EventChannel eventChannel =
      EventChannel('samples.flutter.io/charging');

  String _chargingStatus = 'Battery status: unknown.';

  @override
  void initState() {
    super.initState();
    eventChannel.receiveBroadcastStream().listen(_onEvent,onError:_onError);
  }

  void _onEvent(Object? event) {
    setState(() {
      _chargingStatus =
          "Battery status: ${event == 'charging' ? '' : 'dis'}charging.";
    });
  }

  void _onError(Object error) {
    setState(() {
      _chargingStatus = 'Battery status: unknown.';
    });
  }

  @override
  Widget build(BuildContext context) {
    return Material(
      child:  Text(_chargingStatus),
    );
  }
}

void main() {
  runApp(const MaterialApp(home: PlatformChannel()));
}

In Windows directory add code to below files flutter_windows.cpp and flutter_windows.h

-> flutter_windows.cpp add the below code

#include "flutter_window.h"

#include <flutter/event_channel.h>
#include <flutter/event_sink.h>
#include <flutter/event_stream_handler_functions.h>
#include <flutter/method_channel.h>
#include <flutter/standard_method_codec.h>
#include <windows.h>

#include <memory>
#include <optional>

#include "flutter/generated_plugin_registrant.h"

static constexpr int kBatteryError = -1;
static constexpr int kNoBattery = -2;



FlutterWindow::FlutterWindow(const flutter::DartProject& project)
    : project_(project) {}

FlutterWindow::~FlutterWindow() {
  if (power_notification_handle_) {
    UnregisterPowerSettingNotification(power_notification_handle_);
  }
}

bool FlutterWindow::OnCreate() {
  if (!Win32Window::OnCreate()) {
    return false;
  }

  RECT frame = GetClientArea();

  // The size here must match the window dimensions to avoid unnecessary surface
  // creation / destruction in the startup path.
  flutter_controller_ = std::make_unique<flutter::FlutterViewController>(
      frame.right - frame.left, frame.bottom - frame.top, project_);
  // Ensure that basic setup of the controller was successful.
  if (!flutter_controller_->engine() || !flutter_controller_->view()) {
    return false;
  }
  RegisterPlugins(flutter_controller_->engine());

  /// This is the Code for Event channel creation

  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;
          }));

  SetChildContent(flutter_controller_->view()->GetNativeWindow());

  flutter_controller_->engine()->SetNextFrameCallback([&]() {
    this->Show();
  });

  // Flutter can complete the first frame before the "show window" callback is
  // registered. The following call ensures a frame is pending to ensure the
  // window is shown. It is a no-op if the first frame hasn't completed yet.
  flutter_controller_->ForceRedraw();

  return true;
}

void FlutterWindow::OnDestroy() {
  if (flutter_controller_) {
    flutter_controller_ = nullptr;
  }

  Win32Window::OnDestroy();
}

LRESULT
FlutterWindow::MessageHandler(HWND hwnd, UINT const message,
                              WPARAM const wparam,
                              LPARAM const lparam) noexcept {
  // Give Flutter, including plugins, an opportunity to handle window messages.
  if (flutter_controller_) {
    std::optional<LRESULT> result =
        flutter_controller_->HandleTopLevelWindowProc(hwnd, message, wparam,
                                                      lparam);
    if (result) {
      return *result;
    }
  }

  switch (message) {
    case WM_FONTCHANGE:
      flutter_controller_->engine()->ReloadSystemFonts();
      break;
    case WM_POWERBROADCAST:
      SendBatteryStateEvent();
      break;
  }

  return Win32Window::MessageHandler(hwnd, message, wparam, lparam);
}

void FlutterWindow::OnStreamListen(
    std::unique_ptr<flutter::EventSink<>>&& events) {
    event_sink_ = std::move(events);
 
}

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

void FlutterWindow::SendBatteryStateEvent() {
  SYSTEM_POWER_STATUS status;
  if (GetSystemPowerStatus(&status) == 0 || status.ACLineStatus == 255) {
    event_sink_->Error("UNAVAILABLE", "Charging status unavailable");
  } else {
    event_sink_->Success(flutter::EncodableValue(
        status.ACLineStatus == 1 ? "charging" : "discharging"));
  }
}

-> flutter_windows.h add the below code to header file

#ifndef RUNNER_FLUTTER_WINDOW_H_
#define RUNNER_FLUTTER_WINDOW_H_

#include <flutter/dart_project.h>
#include <flutter/event_sink.h>
#include <flutter/flutter_view_controller.h>
#include <winuser.h>

#include <memory>

#include "win32_window.h"

// A window that does nothing but host a Flutter view.
class FlutterWindow : public Win32Window {
 public:
  // Creates a new FlutterWindow hosting a Flutter view running |project|.
  explicit FlutterWindow(const flutter::DartProject& project);
  virtual ~FlutterWindow();

 protected:
  // Win32Window:
  bool OnCreate() override;
  void OnDestroy() override;
  LRESULT MessageHandler(HWND window, UINT const message, WPARAM const wparam,
                         LPARAM const lparam) noexcept override;

 private:
  // 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 SendBatteryStateEvent();

  // The project to run.
  flutter::DartProject project_;

  // The Flutter instance hosted by this window.
  std::unique_ptr<flutter::FlutterViewController> flutter_controller_;

  std::unique_ptr<flutter::EventSink<>> event_sink_;
  HPOWERNOTIFY power_notification_handle_ = nullptr;
};

#endif  // RUNNER_FLUTTER_WINDOW_H_