Flutter Desktop Windows: How to call native code via Method Channel. (Make api calls in <windows.h> header file)

For that you have to create a method channel in your_flutter_project\windows\runner\flutter_window.cpp.

1. include below modules in the file:

    #include <flutter/binary_messenger.h>
    #include <flutter/standard_method_codec.h>
    #include <flutter/method_channel.h>
    #include <flutter/method_result_functions.h>

2. Then add below method before onCreate() function:

    void initMethodChannel(flutter::FlutterEngine* flutter_instance) {
    // name your channel
    const static std::string channel_name("test_channel");

    auto channel =
        std::make_unique<flutter::MethodChannel<>>(
            flutter_instance->messenger(), channel_name,
            &flutter::StandardMethodCodec::GetInstance());

    channel->SetMethodCallHandler(
        [](const flutter::MethodCall<>& call, 
    std::unique_ptr<flutter::MethodResult<>> result) {

            // cheack method name called from dart
            if (call.method_name().compare("test") == 0) {
            // do whate ever you want

            result->Success("pass result here");
            }
            else {
                result->NotImplemented();
            }
        });
     }

3. Now call this method in the onCreate() function after plugins registration:

    // other codes
    // the plugins registrations
    RegisterPlugins(flutter_controller_->engine());
    // initialize method channel here 
    initMethodChannel(flutter_controller_->engine());

    run_loop_->RegisterFlutterInstance(flutter_controller_->engine());
    // other codes

• Finally just create a method channel in your dart file and invoke method(in this example 'test' method):

  MethodChannel channel = MethodChannel('test_channel');
  var result = await channel.invokeMethod('test');
  

Here is the complete edited flutter_windows.cpp file:

#include "flutter_window.h"

#include <optional>
#include "flutter/generated_plugin_registrant.h"

#include <flutter/binary_messenger.h>
#include <flutter/standard_method_codec.h>
#include <flutter/method_channel.h>
#include <flutter/method_result_functions.h>

#include <iostream>
using namespace std;

FlutterWindow::FlutterWindow(RunLoop* run_loop,
                             const flutter::DartProject& project)
    : run_loop_(run_loop), project_(project) {}

FlutterWindow::~FlutterWindow() {}


void initMethodChannel(flutter::FlutterEngine* flutter_instance) {

    const static std::string channel_name("test_channel");

    auto channel =
        std::make_unique<flutter::MethodChannel<>>(
            flutter_instance->messenger(), channel_name,
            &flutter::StandardMethodCodec::GetInstance());

    channel->SetMethodCallHandler(
        [](const flutter::MethodCall<>& call, std::unique_ptr<flutter::MethodResult<>> result) {

            if (call.method_name().compare("test") == 0) {
               // do whate ever you want

                result->Success("pass result here");
            }
            else {
                result->NotImplemented();
            }
        });
}

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());
  // initialize method channel here **************************************************
  initMethodChannel(flutter_controller_->engine());

  run_loop_->RegisterFlutterInstance(flutter_controller_->engine());


  SetChildContent(flutter_controller_->view()->GetNativeWindow());
  return true;
}

void FlutterWindow::OnDestroy() {
  if (flutter_controller_) {
    run_loop_->UnregisterFlutterInstance(flutter_controller_->engine());
    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 opporutunity 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;
  }

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