手把手教你写 Dart ffi

本文以step by step的方式说明了Dart ffi的使用，适合新手学习。

什么是ffi

创建sample工程

flutter --version
Flutter 3.3.0 • channel stable • https://github.com/flutter/flutter.gitFramework • revision ffccd96b62 (6 weeks ago) • 2022-08-29 17:28:57 -0700Engine • revision 5e9e0e0aa8Tools • Dart 2.18.0 • DevTools 2.15.0

flutter create --template=plugin_ffi --platforms=macos plugin_ffi_sample

cd plugin_ffi_sample/exampleflutter run

plugin_ffi_sample 工程详解

dependencies:  flutter:    sdk: flutter
  plugin_ffi_sample:    # When depending on this package from a real application you should use:    #   plugin_ffi_sample: ^x.y.z    # See https://dart.dev/tools/pub/dependencies#version-constraints    # The example app is bundled with the plugin so we use a path dependency on    # the parent directory to use the current plugin's version.    path: ../

import 'package:plugin_ffi_sample/plugin_ffi_sample.dart' as plugin_ffi_sample;//...
class _MyAppState extends State<MyApp> {  late int sumResult;  late Future<int> sumAsyncResult;
  @override  void initState() {    super.initState();    sumResult = plugin_ffi_sample.sum(1, 2); //    sumAsyncResult = plugin_ffi_sample.sumAsync(3, 4);//  }//...

节选部分代码如下：

import 'plugin_ffi_sample_bindings_generated.dart';
/// A very short-lived native function.////// For very short-lived functions, it is fine to call them on the main isolate./// They will block the Dart execution while running the native function, so/// only do this for native functions which are guaranteed to be short-lived.int sum(int a, int b) => _bindings.sum(a, b);
//...const String _libName = 'plugin_ffi_sample';
/// The dynamic library in which the symbols for [PluginFfiSampleBindings] can be found.final DynamicLibrary _dylib = () {  if (Platform.isMacOS || Platform.isIOS) {    return DynamicLibrary.open('$_libName.framework/$_libName');  }  if (Platform.isAndroid || Platform.isLinux) {    return DynamicLibrary.open('lib$_libName.so');  }  if (Platform.isWindows) {    return DynamicLibrary.open('$_libName.dll');  }  throw UnsupportedError('Unknown platform: ${Platform.operatingSystem}');}();
/// The bindings to the native functions in [_dylib].final PluginFfiSampleBindings _bindings = PluginFfiSampleBindings(_dylib);

// AUTO GENERATED FILE, DO NOT EDIT.//// Generated by `package:ffigen`.import 'dart:ffi' as ffi;
/// Bindings for `src/plugin_ffi_sample.h`.////// Regenerate bindings with `flutter pub run ffigen --config ffigen.yaml`.///class PluginFfiSampleBindings {  /// Holds the symbol lookup function.  final ffi.Pointer<T> Function<T extends ffi.NativeType>(String symbolName)      _lookup;
  /// The symbols are looked up in [dynamicLibrary].  PluginFfiSampleBindings(ffi.DynamicLibrary dynamicLibrary)      : _lookup = dynamicLibrary.lookup;
  /// The symbols are looked up with [lookup].  PluginFfiSampleBindings.fromLookup(      ffi.Pointer<T> Function<T extends ffi.NativeType>(String symbolName)          lookup)      : _lookup = lookup;
  /// A very short-lived native function.  ///  /// For very short-lived functions, it is fine to call them on the main isolate.  /// They will block the Dart execution while running the native function, so  /// only do this for native functions which are guaranteed to be short-lived.  int sum(    int a,    int b,  ) {    return _sum(      a,      b,    );  }

flutter pub run ffigen --config ffigen.yaml

brew install llvm

# Run with `flutter pub run ffigen --config ffigen.yaml`.name: PluginFfiSampleBindingsdescription: |  Bindings for `src/plugin_ffi_sample.h`.
  Regenerate bindings with `flutter pub run ffigen --config ffigen.yaml`.output: 'lib/plugin_ffi_sample_bindings_generated.dart'headers:  entry-points:    - 'src/plugin_ffi_sample.h'  include-directives:    - 'src/plugin_ffi_sample.h'

#if _WIN32#define FFI_PLUGIN_EXPORT __declspec(dllexport)#else#define FFI_PLUGIN_EXPORT#endif
// A very short-lived native function.//// For very short-lived functions, it is fine to call them on the main isolate.// They will block the Dart execution while running the native function, so// only do this for native functions which are guaranteed to be short-lived.FFI_PLUGIN_EXPORT intptr_t sum(intptr_t a, intptr_t b);
// A longer lived native function, which occupies the thread calling it.//// Do not call these kind of native functions in the main isolate. They will// block Dart execution. This will cause dropped frames in Flutter applications.// Instead, call these native functions on a separate isolate.FFI_PLUGIN_EXPORT intptr_t sum_long_running(intptr_t a, intptr_t b);

C/C++如何调用Dart

typedef void (*pong) (void);
FFI_PLUGIN_EXPORT void ping(pong callback);

FFI_PLUGIN_EXPORT void ping(pong callback) {  printf("ping\n");  callback();}

flutter pub run ffigen --config ffigen.yaml

// C/C++要回调的必须是Top level的方法void pong() {  print("pong");}
void ping() {  _bindings.ping(Pointer.fromFunction(pong));}

  @override  void initState() {    super.initState();    plugin_ffi_sample.ping(); //ping    sumResult = plugin_ffi_sample.sum(1, 2);    sumAsyncResult = plugin_ffi_sample.sumAsync(3, 4);  }

Building macOS application...                                           pingflutter: pongSyncing files to device macOS...                                   108ms

C/C++异步线程调用Dart

#include <thread>
#ifdef __cplusplusextern "C" {#include "plugin_ffi_sample.h"#endif
// A very short-lived native function.//// For very short-lived functions, it is fine to call them on the main isolate.// They will block the Dart execution while running the native function, so// only do this for native functions which are guaranteed to be short-lived.FFI_PLUGIN_EXPORT intptr_t sum(intptr_t a, intptr_t b) { return a + b; }
// A longer-lived native function, which occupies the thread calling it.//// Do not call these kind of native functions in the main isolate. They will// block Dart execution. This will cause dropped frames in Flutter applications.// Instead, call these native functions on a separate isolate.FFI_PLUGIN_EXPORT intptr_t sum_long_running(intptr_t a, intptr_t b) {  // Simulate work.#if _WIN32  Sleep(5000);#else  usleep(5000 * 1000);#endif  return a + b;}
void entry_point(pong call) {    printf("entry_point\n");    call();}
FFI_PLUGIN_EXPORT void ping(pong callback) {    printf("ping\n");
    pong p = callback;    std::thread* t = new std::thread(entry_point, p);    }
#ifdef __cplusplus}#endif

macos/Classes/plugin_ffi_sample.c
// Relative import to be able to reuse the C sources.// See the comment in ../{projectName}}.podspec for more information.#include "../../src/plugin_ffi_sample.h"

# The Flutter tooling requires that developers have CMake 3.10 or later# installed. You should not increase this version, as doing so will cause# the plugin to fail to compile for some customers of the plugin.cmake_minimum_required(VERSION 3.10)
project(plugin_ffi_sample_library VERSION 0.0.1 LANGUAGES C CXX)set (CMAKE_CXX_STANDARD 11)
add_library(plugin_ffi_sample SHARED  "plugin_ffi_sample.cc")
set_target_properties(plugin_ffi_sample PROPERTIES  PUBLIC_HEADER plugin_ffi_sample.h  OUTPUT_NAME "plugin_ffi_sample")
target_compile_definitions(plugin_ffi_sample PUBLIC DART_SHARED_LIB)

cmake .make

  s.vendored_libraries = 'libplugin_ffi_sample.dylib'

ln -s ../src/libplugin_ffi_sample.dylib

 // C/C++要回调的必须是Top level的方法 void pong() {-  print("pong");+  print("pong ${sum(3, 4)}"); }

../../third_party/dart/runtime/vm/runtime_entry.cc: 3766: error: Cannot invoke native callback outside an isolate.

drwxr-xr-x   9 anql  staff     288  7 13 06:23 .drwxr-xr-x  10 anql  staff     320  8 15 20:00 ..-rw-r--r--   1 anql  staff  146111  7 13 06:33 dart_api.h-rw-r--r--   1 anql  staff    2240  7 13 06:33 dart_api_dl.c-rw-r--r--   1 anql  staff    7881  7 13 06:33 dart_api_dl.h-rw-r--r--   1 anql  staff    6775  7 13 06:33 dart_native_api.h-rw-r--r--   1 anql  staff   18150  7 13 06:33 dart_tools_api.h-rw-r--r--   1 anql  staff     620  7 13 06:33 dart_version.hdrwxr-xr-x   3 anql  staff      96  7 13 06:23 internal

/** \mainpage Dynamically Linked Dart API** This exposes a subset of symbols from dart_api.h and dart_native_api.h* available in every Dart embedder through dynamic linking.** All symbols are postfixed with _DL to indicate that they are dynamically* linked and to prevent conflicts with the original symbol.** Link `dart_api_dl.c` file into your library and invoke* `Dart_InitializeApiDL` with `NativeApi.initializeApiDLData`.*/

DART_EXPORT intptr_t Dart_InitializeApiDL(void* data);
//...
/* Dart_Port */                                                              \F(Dart_Post, bool, (Dart_Port_DL port_id, Dart_Handle object))               \F(Dart_NewSendPort, Dart_Handle, (Dart_Port_DL port_id))                     \F(Dart_SendPortGetId, Dart_Handle,                                           \(Dart_Handle port, Dart_Port_DL * port_id))

1. 将flutter sdk路径/flutter/bin/cache/dart-sdk/include拷贝到plugin_ffi_sample/src目录；
2. src/CMakeLists.txt修改如下，这里主要是让“include/dart_api_dl.c”文件参与编译，以及指定include目录：

cmake_minimum_required(VERSION 3.10)
project(plugin_ffi_sample_library VERSION 0.0.1 LANGUAGES C CXX)set (CMAKE_CXX_STANDARD 11)
add_library(plugin_ffi_sample SHARED  "include/dart_api_dl.c"  "plugin_ffi_sample.cc")
include_directories(include)
set_target_properties(plugin_ffi_sample PROPERTIES  PUBLIC_HEADER plugin_ffi_sample.h  OUTPUT_NAME "plugin_ffi_sample")
target_compile_definitions(plugin_ffi_sample PUBLIC DART_SHARED_LIB)

3. src/plugin_ffi_sample.h 变更如下，不再让函数指针作为ping方法的参数，而是让Main isolate的SendPort作为参数：

+#include <dart_api_dl.h>
-FFI_PLUGIN_EXPORT void ping(pong callback);+FFI_PLUGIN_EXPORT void ping(Dart_Port_DL main_isolate_send_port);++FFI_PLUGIN_EXPORT intptr_t ffi_Dart_InitializeApiDL(void* data);

4. src/plugin_ffi_sample.cc 这里的实现会在C++的子线程中发送消息给Dart，需要注意的是，因为涉及不同的技术栈，所以这里的消息类型也是非常有限的。不过你可以设定一些协议进行通信。

-void entry_point(pong call) {+void entry_point(Dart_Port_DL main_isolate_send_port) {     printf("entry_point\n");-    call();++    Dart_CObject dart_object;+    dart_object.type = Dart_CObject_kString;+    dart_object.value.as_string = "pong";++    const bool result = Dart_PostCObject_DL(main_isolate_send_port, &dart_object);+    if (!result) {+        printf("C   :  Posting message to port failed.\n");+    } } -FFI_PLUGIN_EXPORT void ping(pong callback) {+FFI_PLUGIN_EXPORT void ping(Dart_Port_DL main_isolate_send_port) {     printf("ping\n"); -    pong p = callback;-    std::thread t(entry_point, p);    +    std::thread* t = new std::thread(entry_point, main_isolate_send_port);     } +FFI_PLUGIN_EXPORT intptr_t ffi_Dart_InitializeApiDL(void* data) {+    return Dart_InitializeApiDL(data); +}

5. plugin_ffi_sample/macos/Classes/plugin_ffi_sample.c 这里不再需要了，因为我们通过CMake进行编译：

-#include "../../src/plugin_ffi_sample.h"+// #include "../../src/plugin_ffi_sample.h"

6. plugin_ffi_sample/lib/plugin_ffi_sample.dart 这里主要是通过ReceivePort添加监听：

// C/C++要回调的必须是Top level的方法-void pong() {  -  print("pong ${sum(3, 4)}");  +void pong(dynamic msg) {  +  print("pong $msg");}
  void ping() {  -  _bindings.ping(Pointer.fromFunction(pong));  +  final receivePort = ReceivePort()  +    ..listen((message) {  +      pong(message);  +    });  +  _bindings.ping(receivePort.sendPort.nativePort);}  +  +int initializeApiDL() =>  +    _bindings.ffi_Dart_InitializeApiDL(NativeApi.initializeApiDLData);

7. plugin_ffi_sample/example/lib/main.dart 务必记得先初始化动态链接的api：

  @override  void initState() {    super.initState();+    plugin_ffi_sample.initializeApiDL();    plugin_ffi_sample.ping(); //ping    sumResult = plugin_ffi_sample.sum(1, 2);    sumAsyncResult = plugin_ffi_sample.sumAsync(3, 4);

结语

1. 在Dart中调用 String的 toNativeUtf8方法，务必记得传入Allocator对象，ffi方法执行完成后要释放内存;

2. NativeFinalizer 对象可以绑定Dart与C++对象，Dart对象被gc时可以回调指定的方法从而回收对象。

[1]https://en.wikipedia.org/wiki/Foreign_function_interface

[2]https://docs.flutter.dev/development/packages-and-plugins/developing-packages

[3]https://docs.flutter.dev/development/packages-and-plugins/developing-packages

[4]https://pub.dev/documentation/ffigen/latest/

[5]https://pub.dev/packages/ffigen

[6https://dart.dev/guides/language/concurrency]