在 Flutter Windows 桌面应用程序中使用 C++ DLL

Question

我有一些库可以与我用 C++ 打包到 DLL 中的 FTDI 芯片进行交互。 我想用 Flutter 创建一个前端并在 Windows 桌面应用程序中使用该库。 这些功能在 Flutter 中仍然是新功能，并且文档非常浅薄且特定于移动设备。

按照指南here，我使用 FFI 创建了一个插件：

import 'dart:ffi';
import 'dart:io';
import 'dart:async';

import 'package:flutter/services.dart';

final DynamicLibrary FT232H = DynamicLibrary.open("");

final int Function() initializeLibrary = FT232H
    .lookup<NativeFunction<Uint8 Function()>>("initialize_library")
    .asFunction();

final void Function() cleanupLibrary = FT232H
    .lookup<NativeFunction<Void Function()>>("cleanup_library")
    .asFunction();

final int Function() initializeI2C = FT232H
    .lookup<NativeFunction<Uint8 Function()>>("Initialize_I2C")
    .asFunction();

final int Function() closeI2C = FT232H
    .lookup<NativeFunction<Uint8 Function()>>("Close_I2C")
    .asFunction();

final int Function(
        Uint8 slaveAddress, Uint8 registerAddress, Uint32 data, Uint32 numBytes)
    i2cWriteBytes = FT232H
        .lookup<NativeFunction<Uint8 Function(Uint8, Uint8, Uint32, Uint32)>>(
            "I2C_write_bytes")
        .asFunction();

final int Function(Uint8 slaveAddress, Uint8 registerAddress,
        Uint8 bRegisterAddress, Pointer<Uint8> data, Uint32 numBytes)
    i2cReadBytes = FT232H
        .lookup<
            NativeFunction<
                Uint8 Function(Uint8, Uint8, Uint8, Pointer<Uint8>,
                    Uint32)>>("I2C_read_bytes")
        .asFunction();

class DllImport {
  static const MethodChannel _channel = const MethodChannel('dll_import');

  static Future<String> get platformVersion async {
    final String version = await _channel.invokeMethod('getPlatformVersion');
    return version;
  }
}

这是我在另一边的头文件：

#pragma once

/* Include D2XX header*/
#include "ftd2xx.h"

/* Include libMPSSE headers */
#include "libMPSSE_i2c.h"
#include "libMPSSE_spi.h"

#define FT232H_EXPORTS

#ifdef FT232H_EXPORTS
#define FT232H_API __declspec(dllexport)
#else
#define FT232H_API __declspec(dllimport)
#endif


extern "C" FT232H_API uint8 initialize_library();
extern "C" FT232H_API void cleanup_library();
extern "C" FT232H_API FT_STATUS Initialize_I2C();
extern "C" FT232H_API FT_STATUS Close_I2C();
extern "C" FT232H_API FT_STATUS I2C_write_bytes(uint8 slaveAddress, uint8 registerAddress, 
const uint8 * data, uint32 numBytes);
extern "C" FT232H_API FT_STATUS I2C_read_bytes(uint8 slaveAddress, uint8 registerAddress, 
uint8 bRegisterAddress, uint8 * data, uint32 numBytes);

在这里，我似乎对 Uint8 指针有一些问题，因为我从我的 Dart 代码中收到了这个错误：

The type 'Uint8 Function(Uint8, Uint8, Uint8, Pointer<Uint8>, Uint32)' must be a subtype of 'int 
Function(Uint8, Uint8, Uint8, Pointer<Uint8>, Uint32)' for 'asFunction'.
Try changing one or both of the type arguments.dart(must_be_a_subtype)

任何关于如何在颤振中实现这一点的指针将不胜感激！

谢谢。

Answer 1

我确实有一个解决方案，它适用于 Flutter-Desktop-Embedding 项目中提供的准系统代码，我假设您将其用于您的桌面应用程序。你在正确的轨道上，但只需要一些最终确定。

为了测试，我使用了这个简单的 c 代码和几个函数来测试传递指针、返回指针、填充内存、分配和释放。 这是我在我的 dll 中使用的 C 代码。

#define WIN32_LEAN_AND_MEAN
#include <Windows.h>
#include <stdint.h>
#include <stdlib.h>
#include <stdio.h>
__declspec(dllexport) uint8_t* createarray(int32_t size) {
    uint8_t* arr = malloc(size);
    return arr;
}
__declspec(dllexport) void populatearray(uint8_t* arr,uint32_t size){
    for (uint32_t index = 0; index < size; ++index) {
        arr[index] = index & 0xff;
    }
}
__declspec(dllexport) void destroyarray(uint8_t* arr) {
    free(arr);
}

createarray 分配给定大小的 uint8_t 指针并将其返回给调用者。

populatearray 接受 uint8_t 指针参数和大小并用索引填充它

destroyarray 只是释放分配的内存。

现在是样板颤振代码。

这是我在 Flutter-Desktop-Embedding 项目中为 main.dart 提供的默认代码，我从这里克隆了 https://github.com/google/flutter-desktop-embedding.git（我假设你已经完成了这一步）

// Copyright 2018 Google LLC
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
//      http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.
import 'dart:io' show Platform;
import 'dart:math' as math;

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

import 'package:menubar/menubar.dart';
import 'package:window_size/window_size.dart' as window_size;

import 'keyboard_test_page.dart';

void main() {
  // Try to resize and reposition the window to be half the width and height
  // of its screen, centered horizontally and shifted up from center.
  WidgetsFlutterBinding.ensureInitialized();
  window_size.getWindowInfo().then((window) {
    final screen = window.screen;
    if (screen != null) {
      final screenFrame = screen.visibleFrame;
      final width = math.max((screenFrame.width / 2).roundToDouble(), 800.0);
      final height = math.max((screenFrame.height / 2).roundToDouble(), 600.0);
      final left = ((screenFrame.width - width) / 2).roundToDouble();
      final top = ((screenFrame.height - height) / 3).roundToDouble();
      final frame = Rect.fromLTWH(left, top, width, height);
      window_size.setWindowFrame(frame);
      window_size.setWindowMinSize(Size(0.8 * width, 0.8 * height));
      window_size.setWindowMaxSize(Size(1.5 * width, 1.5 * height));
      window_size
          .setWindowTitle('Flutter Testbed on ${Platform.operatingSystem}');
    }
  });

  runApp(new MyApp());
}

/// Top level widget for the application.
class MyApp extends StatefulWidget {
  /// Constructs a new app with the given [key].
  const MyApp({Key? key}) : super(key: key);

  @override
  _AppState createState() => new _AppState();
}

class _AppState extends State<MyApp> {
  Color _primaryColor = Colors.blue;
  int _counter = 0;

  static _AppState? of(BuildContext context) =>
      context.findAncestorStateOfType<_AppState>();

  /// Sets the primary color of the app.
  void setPrimaryColor(Color color) {
    setState(() {
      _primaryColor = color;
    });
  }

  void incrementCounter() {
    _setCounter(_counter + 1);
  }

  void _decrementCounter() {
    _setCounter(_counter - 1);
  }

  void _setCounter(int value) {
    setState(() {
      _counter = value;
    });
  }

  /// Rebuilds the native menu bar based on the current state.
  void updateMenubar() {
    setApplicationMenu([
      Submenu(label: 'Color', children: [
        MenuItem(
            label: 'Reset',
            enabled: _primaryColor != Colors.blue,
            shortcut: LogicalKeySet(
                LogicalKeyboardKey.meta, LogicalKeyboardKey.backspace),
            onClicked: () {
              setPrimaryColor(Colors.blue);
            }),
        MenuDivider(),
        Submenu(label: 'Presets', children: [
          MenuItem(
              label: 'Red',
              enabled: _primaryColor != Colors.red,
              shortcut: LogicalKeySet(LogicalKeyboardKey.meta,
                  LogicalKeyboardKey.shift, LogicalKeyboardKey.keyR),
              onClicked: () {
                setPrimaryColor(Colors.red);
              }),
          MenuItem(
              label: 'Green',
              enabled: _primaryColor != Colors.green,
              shortcut: LogicalKeySet(LogicalKeyboardKey.meta,
                  LogicalKeyboardKey.alt, LogicalKeyboardKey.keyG),
              onClicked: () {
                setPrimaryColor(Colors.green);
              }),
          MenuItem(
              label: 'Purple',
              enabled: _primaryColor != Colors.deepPurple,
              shortcut: LogicalKeySet(LogicalKeyboardKey.meta,
                  LogicalKeyboardKey.control, LogicalKeyboardKey.keyP),
              onClicked: () {
                setPrimaryColor(Colors.deepPurple);
              }),
        ])
      ]),
      Submenu(label: 'Counter', children: [
        MenuItem(
            label: 'Reset',
            enabled: _counter != 0,
            shortcut: LogicalKeySet(
                LogicalKeyboardKey.meta, LogicalKeyboardKey.digit0),
            onClicked: () {
              _setCounter(0);
            }),
        MenuDivider(),
        MenuItem(
            label: 'Increment',
            shortcut: LogicalKeySet(LogicalKeyboardKey.f2),
            onClicked: incrementCounter),
        MenuItem(
            label: 'Decrement',
            enabled: _counter > 0,
            shortcut: LogicalKeySet(LogicalKeyboardKey.f1),
            onClicked: _decrementCounter),
      ]),
    ]);
  }

  @override
  Widget build(BuildContext context) {
    // Any time the state changes, the menu needs to be rebuilt.
    updateMenubar();

    return MaterialApp(
      title: 'Flutter Demo',
      theme: ThemeData(
        primarySwatch: Colors.blue,
        primaryColor: _primaryColor,
        accentColor: _primaryColor,
      ),
      darkTheme: ThemeData.dark(),
      home: _MyHomePage(title: 'Flutter Demo Home Page', counter: _counter),
    );
  }
}

class _MyHomePage extends StatelessWidget {
  const _MyHomePage({required this.title, this.counter = 0});

  final String title;
  final int counter;

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text(title),
      ),
      body: LayoutBuilder(
        builder: (context, viewportConstraints) {
          return SingleChildScrollView(
            child: ConstrainedBox(
              constraints:
                  BoxConstraints(minHeight: viewportConstraints.maxHeight),
              child: Center(
                child: Column(
                  mainAxisAlignment: MainAxisAlignment.center,
                  children: <Widget>[
                    const Text(
                      'You have pushed the button this many times:',
                    ),
                    new Text(
                      '$counter',
                      style: Theme.of(context).textTheme.headline4,
                    ),
                    TextInputTestWidget(),
                    new ElevatedButton(
                      child: new Text('Test raw keyboard events'),
                      onPressed: () {
                        Navigator.of(context).push(new MaterialPageRoute(
                            builder: (context) => KeyboardTestPage()));
                      },
                    ),
                    Padding(
                      padding: const EdgeInsets.all(8.0),
                      child: Container(
                        width: 380.0,
                        height: 100.0,
                        decoration: BoxDecoration(
                            border: Border.all(color: Colors.grey, width: 1.0)),
                        child: Scrollbar(
                          child: ListView.builder(
                            padding: EdgeInsets.all(8.0),
                            itemExtent: 20.0,
                            itemCount: 50,
                            itemBuilder: (context, index) {
                              return Text('entry $index');
                            },
                          ),
                        ),
                      ),
                    ),
                  ],
                ),
              ),
            ),
          );
        },
      ),
      floatingActionButton: FloatingActionButton(
        onPressed: _AppState.of(context)!.incrementCounter,
        tooltip: 'Increment',
        child: Icon(Icons.add),
      ),
    );
  }
}

/// A widget containing controls to test text input.
class TextInputTestWidget extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return Row(
      mainAxisAlignment: MainAxisAlignment.center,
      children: const <Widget>[
        SampleTextField(),
        SampleTextField(),
      ],
    );
  }
}

/// A text field with styling suitable for including in a TextInputTestWidget.
class SampleTextField extends StatelessWidget {
  /// Creates a new sample text field.
  const SampleTextField();

  @override
  Widget build(BuildContext context) {
    return Container(
      width: 200.0,
      padding: const EdgeInsets.all(10.0),
      child: TextField(
        decoration: InputDecoration(border: OutlineInputBorder()),
      ),
    );
  }
}

现在对于我们的部分代码，我们必须为要在 dll 中调用的每个函数创建一个函数指针，我们需要正确的函数名称和正确/正确的参数数量。

我们需要 dart:io 和 dart:ffi 包。 dart:io 已经在代码中了，只需要导入 dart:ffi import 'dart:ffi'; // For FFI

现在要创建需要加载的库的句柄，需要使用 dll 的名称调用 DynamicLibrary.open。 （dll需要放在dart应用的执行路径或者需要给出绝对路径，执行路径是build/windows/runner/Debug）

final DynamicLibrary nativePointerTestLib = DynamicLibrary.open("dynamicloadtest.dll");

我的句柄名为nativePointerTestLib，dll 的名称是“dynamicloadtest.dll”（是的，我应该使用更好的命名约定）

接下来，需要创建每个函数指针。我要调用 dll 中的三个函数：createarray、populatearray、destroyarray。

第一个取int的size参数->返回指向数组的指针（Pointer） 第二个将指针与 size -> void return 第三个只取指针 -> void return

final Pointer<Uint8> Function(int size) nativeCreateArray =
  nativePointerTestLib
    .lookup<NativeFunction<Pointer<Uint8> Function(Int32)>>("createarray")
    .asFunction();

final void Function(Pointer<Uint8> arr,int size) nativePopulateArray =
  nativePointerTestLib
    .lookup<NativeFunction<Void Function(Pointer<Uint8>, Int32)>>("populatearray")
    .asFunction();

final void Function(Pointer<Uint8> arr) nativeDestroyArray =
  nativePointerTestLib
    .lookup<NativeFunction<Void Function(Pointer<Uint8>)>>("destroyarray")
    .asFunction();

我将函数指针命名为nativeCreateArray、nativePopulateArray、nativeDestroyArray

最后，只需调用每个函数并测试它们是否有效。我刚刚在样板代码中选择了一个随机函数void _setCounter(int value)，它设置计数器值，然后显示。我只是要向该方法添加额外的代码来执行我们的函数调用以及打印结果以查看它是否有效。 老方法

void _setCounter(int value) {
    setState(() {
      _counter = value;
    });
  }

我们的函数调用的新方法

  void _setCounter(int value) {
    setState(() {

      Pointer<Uint8> parray = nativeCreateArray(5);
      nativePopulateArray(parray,5);
      //Now lets print
      print(parray);
      String str= "";
      for(int i = 0 ; i < 5; ++i){
        int val = parray.elementAt(i).value;
        str+=val.toString() +" ";
      }
      print(str);
      nativeDestroyArray(parray);
      _counter = value;
    });
  }

我调用了大小为 5 的 nativeCreate。dll 将为数组分配 5 个字节。

接下来我调用 populate，它将在数组的每个元素中插入索引 0 到 4。

然后我循环遍历数组，获取该数组索引处的每个元素，然后获取值。我将该值分配给一个字符串，最后打印并销毁该数组。

最后的代码全部放在一起：

// Copyright 2018 Google LLC
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
//      http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.
import 'dart:io' show Platform;
import 'dart:math' as math;

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

import 'package:menubar/menubar.dart';
import 'package:window_size/window_size.dart' as window_size;

import 'keyboard_test_page.dart';

import 'dart:ffi'; // For FFI

final DynamicLibrary nativePointerTestLib = DynamicLibrary.open("dynamicloadtest.dll");


final Pointer<Uint8> Function(int size) nativeCreateArray =
  nativePointerTestLib
    .lookup<NativeFunction<Pointer<Uint8> Function(Int32)>>("createarray")
    .asFunction();

final void Function(Pointer<Uint8> arr,int size) nativePopulateArray =
  nativePointerTestLib
    .lookup<NativeFunction<Void Function(Pointer<Uint8>, Int32)>>("populatearray")
    .asFunction();

final void Function(Pointer<Uint8> arr) nativeDestroyArray =
  nativePointerTestLib
    .lookup<NativeFunction<Void Function(Pointer<Uint8>)>>("destroyarray")
    .asFunction();


void main() {
  // Try to resize and reposition the window to be half the width and height
  // of its screen, centered horizontally and shifted up from center.
  WidgetsFlutterBinding.ensureInitialized();
  window_size.getWindowInfo().then((window) {
    final screen = window.screen;
    if (screen != null) {
      final screenFrame = screen.visibleFrame;
      final width = math.max((screenFrame.width / 2).roundToDouble(), 800.0);
      final height = math.max((screenFrame.height / 2).roundToDouble(), 600.0);
      final left = ((screenFrame.width - width) / 2).roundToDouble();
      final top = ((screenFrame.height - height) / 3).roundToDouble();
      final frame = Rect.fromLTWH(left, top, width, height);
      window_size.setWindowFrame(frame);
      window_size.setWindowMinSize(Size(0.8 * width, 0.8 * height));
      window_size.setWindowMaxSize(Size(1.5 * width, 1.5 * height));
      window_size
          .setWindowTitle('Flutter Testbed on ${Platform.operatingSystem}');
    }
  });

  runApp(new MyApp());
}

/// Top level widget for the application.
class MyApp extends StatefulWidget {
  /// Constructs a new app with the given [key].
  const MyApp({Key? key}) : super(key: key);

  @override
  _AppState createState() => new _AppState();
}

class _AppState extends State<MyApp> {
  Color _primaryColor = Colors.blue;
  int _counter = 0;

  static _AppState? of(BuildContext context) =>
      context.findAncestorStateOfType<_AppState>();

  /// Sets the primary color of the app.
  void setPrimaryColor(Color color) {
    setState(() {
      _primaryColor = color;
    });
  }

  void incrementCounter() {
    _setCounter(_counter + 1);
  }

  void _decrementCounter() {
    _setCounter(_counter - 1);
  }

  void _setCounter(int value) {
    setState(() {

      Pointer<Uint8> parray = nativeCreateArray(5);
      nativePopulateArray(parray,5);
      //Now lets print
      print(parray);
      String str= "";
      for(int i = 0 ; i < 5; ++i){
        int val = parray.elementAt(i).value;
        str+=val.toString() +" ";
      }
      print(str);
      nativeDestroyArray(parray);
      _counter = value;
    });
  }

  /// Rebuilds the native menu bar based on the current state.
  void updateMenubar() {
    setApplicationMenu([
      Submenu(label: 'Color', children: [
        MenuItem(
            label: 'Reset',
            enabled: _primaryColor != Colors.blue,
            shortcut: LogicalKeySet(
                LogicalKeyboardKey.meta, LogicalKeyboardKey.backspace),
            onClicked: () {
              setPrimaryColor(Colors.blue);
            }),
        MenuDivider(),
        Submenu(label: 'Presets', children: [
          MenuItem(
              label: 'Red',
              enabled: _primaryColor != Colors.red,
              shortcut: LogicalKeySet(LogicalKeyboardKey.meta,
                  LogicalKeyboardKey.shift, LogicalKeyboardKey.keyR),
              onClicked: () {
                setPrimaryColor(Colors.red);
              }),
          MenuItem(
              label: 'Green',
              enabled: _primaryColor != Colors.green,
              shortcut: LogicalKeySet(LogicalKeyboardKey.meta,
                  LogicalKeyboardKey.alt, LogicalKeyboardKey.keyG),
              onClicked: () {
                setPrimaryColor(Colors.green);
              }),
          MenuItem(
              label: 'Purple',
              enabled: _primaryColor != Colors.deepPurple,
              shortcut: LogicalKeySet(LogicalKeyboardKey.meta,
                  LogicalKeyboardKey.control, LogicalKeyboardKey.keyP),
              onClicked: () {
                setPrimaryColor(Colors.deepPurple);
              }),
        ])
      ]),
      Submenu(label: 'Counter', children: [
        MenuItem(
            label: 'Reset',
            enabled: _counter != 0,
            shortcut: LogicalKeySet(
                LogicalKeyboardKey.meta, LogicalKeyboardKey.digit0),
            onClicked: () {
              _setCounter(0);
            }),
        MenuDivider(),
        MenuItem(
            label: 'Increment',
            shortcut: LogicalKeySet(LogicalKeyboardKey.f2),
            onClicked: incrementCounter),
        MenuItem(
            label: 'Decrement',
            enabled: _counter > 0,
            shortcut: LogicalKeySet(LogicalKeyboardKey.f1),
            onClicked: _decrementCounter),
      ]),
    ]);
  }

  @override
  Widget build(BuildContext context) {
    // Any time the state changes, the menu needs to be rebuilt.
    updateMenubar();

    return MaterialApp(
      title: 'Flutter Demo',
      theme: ThemeData(
        primarySwatch: Colors.blue,
        primaryColor: _primaryColor,
        accentColor: _primaryColor,
      ),
      darkTheme: ThemeData.dark(),
      home: _MyHomePage(title: 'Flutter Demo Home Page', counter: _counter),
    );
  }
}

class _MyHomePage extends StatelessWidget {
  const _MyHomePage({required this.title, this.counter = 0});

  final String title;
  final int counter;

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text(title),
      ),
      body: LayoutBuilder(
        builder: (context, viewportConstraints) {
          return SingleChildScrollView(
            child: ConstrainedBox(
              constraints:
                  BoxConstraints(minHeight: viewportConstraints.maxHeight),
              child: Center(
                child: Column(
                  mainAxisAlignment: MainAxisAlignment.center,
                  children: <Widget>[
                    const Text(
                      'You have pushed the button this many times:',
                    ),
                    new Text(
                      '$counter',
                      style: Theme.of(context).textTheme.headline4,
                    ),
                    TextInputTestWidget(),
                    new ElevatedButton(
                      child: new Text('Test raw keyboard events'),
                      onPressed: () {
                        Navigator.of(context).push(new MaterialPageRoute(
                            builder: (context) => KeyboardTestPage()));
                      },
                    ),
                    Padding(
                      padding: const EdgeInsets.all(8.0),
                      child: Container(
                        width: 380.0,
                        height: 100.0,
                        decoration: BoxDecoration(
                            border: Border.all(color: Colors.grey, width: 1.0)),
                        child: Scrollbar(
                          child: ListView.builder(
                            padding: EdgeInsets.all(8.0),
                            itemExtent: 20.0,
                            itemCount: 50,
                            itemBuilder: (context, index) {
                              return Text('entry $index');
                            },
                          ),
                        ),
                      ),
                    ),
                  ],
                ),
              ),
            ),
          );
        },
      ),
      floatingActionButton: FloatingActionButton(
        onPressed: _AppState.of(context)!.incrementCounter,
        tooltip: 'Increment',
        child: Icon(Icons.add),
      ),
    );
  }
}

/// A widget containing controls to test text input.
class TextInputTestWidget extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return Row(
      mainAxisAlignment: MainAxisAlignment.center,
      children: const <Widget>[
        SampleTextField(),
        SampleTextField(),
      ],
    );
  }
}

/// A text field with styling suitable for including in a TextInputTestWidget.
class SampleTextField extends StatelessWidget {
  /// Creates a new sample text field.
  const SampleTextField();

  @override
  Widget build(BuildContext context) {
    return Container(
      width: 200.0,
      padding: const EdgeInsets.all(10.0),
      child: TextField(
        decoration: InputDecoration(border: OutlineInputBorder()),
      ),
    );
  }
}

运行示例应用程序后，点击增量按钮，然后将打印 0 1 2 3 4 以及指向控制台的指针地址。

如果这看起来很懒，我深表歉意，但我不是 Flutter 开发人员，实际上我在这方面的经验为零，今天是我接触它的第一天。但是弄清楚基本的代码和语法并不难。