Flutter Desktop Windows：如何通过方法通道调用本机代码。（在 <windows.h> 头文件中进行 api 调用）

Question

标题说明了一切。flutter 的网站上几乎没有文档展示如何通过方法通道调用 Windows 原生代码中的方法。但是我在flutter的github上发现了一个问题，说这是可能的，但那里没有代码！

我在 github 上找不到任何也使用 Windows 本机代码的示例项目。

Answer 1

为此，您必须在your_flutter_project\windows\runner\flutter_window.cpp中创建一个方法通道。

1. 在文件中包含以下模块：

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

2.然后在onCreate()函数之前添加以下方法：

    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. 现在在插件注册后在 onCreate() 函数中调用此方法：

    // 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

• 最后只需在 dart 文件中创建一个方法通道并调用方法（在本例中为“测试”方法）：

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

这是完整编辑的 flutter_windows.cpp 文件：

#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);
}

Answer 2

高级平台通道文档尚未涵盖桌面，但此处和此处的 C++ API 标头具有解释其具体用途的声明注释。

从高层次来看，流程本质上与其他平台相同：从您有权访问的视图控制器中main可以获得引擎，从中您可以获得创建方法通道所需的信使。

对于调用方法的细节，单元测试是调用 API 不同方式的示例的来源。