颤振/飞镖：如何使用 Dart FFI 的异步回调？

Question

我的应用程序的后端是用 C++ 编写的，前端是用 Dart/flutter 编写的。 每当数据准备好时，我希望后端通知前端。 这需要在 Dart 和 C++ 之间实现异步回调方案。

环境

$ flutter doctor -v
[✓] Flutter (Channel stable, 1.20.1, on Mac OS X 10.15.5 19F101, locale
    en-CN)
    • Flutter version 1.20.1 at /Applications/Android/flutter
    • Framework revision 2ae34518b8 (2 days ago), 2020-08-05 19:53:19 -0700
    • Engine revision c8e3b94853
    • Dart version 2.9.0
    • Pub download mirror https://pub.flutter-io.cn
    • Flutter download mirror https://storage.flutter-io.cn

 
[✓] Android toolchain - develop for Android devices (Android SDK version
    30.0.1)
    • Android SDK at /Applications/Android/sdk
    • Platform android-30, build-tools 30.0.1
    • ANDROID_HOME = /Applications/Android/sdk
    • ANDROID_SDK_ROOT = /Applications/Android/sdk
    • Java binary at: /Applications/Android
      Studio.app/Contents/jre/jdk/Contents/Home/bin/java
    • Java version OpenJDK Runtime Environment (build
      1.8.0_242-release-1644-b3-6222593)
    • All Android licenses accepted.

[✓] Xcode - develop for iOS and macOS (Xcode 11.6)
    • Xcode at /Applications/Xcode.app/Contents/Developer
    • Xcode 11.6, Build version 11E708
    • CocoaPods version 1.8.4

[✓] Android Studio (version 4.0)
    • Android Studio at /Applications/Android Studio.app/Contents
    • Flutter plugin version 48.0.2
    • Dart plugin version 193.7361
    • Java version OpenJDK Runtime Environment (build
      1.8.0_242-release-1644-b3-6222593)

[✓] VS Code (version 1.47.3)
    • VS Code at /Applications/Visual Studio Code.app/Contents
    • Flutter extension version 3.8.1

 
[!] Connected device                          
    ! No devices available

我做了什么

我查看了 Dart 团队的 github 示例。

为了使示例代码适应 flutter 项目，我执行了以下操作

• 创建 FFI 插件项目
• 用样品function替换main function
• 删除不重要的依赖项，例如expect
• 使用引用的 C++ 代码创建原生库，例如dart-sdk-master/runtime/bin/ffi_test/ffi_test_functions.cc

问题

但是，我无法运行示例。

Dart版

我遇到的第一个问题是直接运行示例时不支持 dart 版本。 我在最新的稳定频道，也尝试过开发频道。 错误说

Error: The specified language version is too high. The highest supported language version is 2.9.

本机编译

示例中缺少有关本机端实现的信息。 所以我留下了很多编译器错误。

Launching lib/main.dart on ONEPLUS A6000 in debug mode...
Running Gradle task 'assembleDebug'...

FAILURE: Build failed with an exception.

* What went wrong:
Execution failed for task ':app:externalNativeBuildDebug'.
> Build command failed.
  Error while executing process /Applications/Android/sdk/cmake/3.6.4111459/bin/cmake with arguments {--build /path/to/ffiasync/example/android/app/.cxx/cmake/debug/armeabi-v7a --target ffiasync}
  [1/2] Building CXX object CMakeFiles/ffiasync.dir/path/to/ffiasync/lib/ffi_test_functions_vmspecific.cc.o
  [2/2] Linking CXX shared library /path/to/ffiasync/example/build/app/intermediates/cmake/debug/obj/armeabi-v7a/libffiasync.so
  FAILED: : && /Applications/Android/sdk/ndk/21.3.6528147/toolchains/llvm/prebuilt/darwin-x86_64/bin/clang++  --target=armv7-none-linux-androideabi16 --gcc-toolchain=/Applications/Android/sdk/ndk/21.3.6528147/toolchains/llvm/prebuilt/darwin-x86_64 --sysroot=/Applications/Android/sdk/ndk/21.3.6528147/toolchains/llvm/prebuilt/darwin-x86_64/sysroot -fPIC -g -DANDROID -fdata-sections -ffunction-sections -funwind-tables -fstack-protector-strong -no-canonical-prefixes -D_FORTIFY_SOURCE=2 -march=armv7-a -mthumb -Wformat -Werror=format-security   -O0 -fno-limit-debug-info  -Wl,--exclude-libs,libgcc.a -Wl,--exclude-libs,libgcc_real.a -Wl,--exclude-libs,libatomic.a -static-libstdc++ -Wl,--build-id -Wl,--fatal-warnings -Wl,--exclude-libs,libunwind.a -Wl,--no-undefined -Qunused-arguments -shared -Wl,-soname,libffiasync.so -o /path/to/ffiasync/example/build/app/intermediates/cmake/debug/obj/armeabi-v7a/libffiasync.so CMakeFiles/ffiasync.dir/path/to/ffiasync/lib/ffiasync.cpp.o CMakeFiles/ffiasync.dir/path/to/ffiasync/lib/ffi_test_functions_vmspecific.cc.o  -latomic -lm && :
  /path/to/ffiasync/lib/ffi_test_functions_vmspecific.cc:54: error: undefined reference to 'Dart_ExecuteInternalCommand'
  /path/to/ffiasync/lib/ffi_test_functions_vmspecific.cc:60: error: undefined reference to 'Dart_ExecuteInternalCommand'
  /path/to/ffiasync/lib/ffi_test_functions_vmspecific.cc:66: error: undefined reference to 'Dart_ExecuteInternalCommand'
  /path/to/ffiasync/lib/ffi_test_functions_vmspecific.cc:74: error: undefined reference to 'Dart_ExecuteInternalCommand'
  /path/to/ffiasync/lib/ffi_test_functions_vmspecific.cc:143: error: undefined reference to 'Dart_CurrentIsolate'
  /path/to/ffiasync/lib/ffi_test_functions_vmspecific.cc:183: error: undefined reference to 'ClobberAndCall'
  /path/to/ffiasync/lib/ffi_test_functions_vmspecific.cc:277: error: undefined reference to 'Dart_InitializeApiDL(void*)'
  /path/to/ffiasync/lib/ffi_test_functions_vmspecific.cc:288: error: undefined reference to 'Dart_DumpNativeStackTrace'
  /path/to/ffiasync/lib/ffi_test_functions_vmspecific.cc:289: error: undefined reference to 'Dart_PrepareToAbort'
  /path/to/ffiasync/lib/ffi_test_functions_vmspecific.cc:0: error: undefined reference to 'Dart_PostCObject_DL'
  /path/to/ffiasync/lib/ffi_test_functions_vmspecific.cc:0: error: undefined reference to 'Dart_NewNativePort_DL'
  /path/to/ffiasync/lib/ffi_test_functions_vmspecific.cc:0: error: undefined reference to 'Dart_PostCObject_DL'
  /path/to/ffiasync/lib/ffi_test_functions_vmspecific.cc:0: error: undefined reference to 'Dart_CloseNativePort_DL'
  /path/to/ffiasync/lib/ffi_test_functions_vmspecific.cc:0: error: undefined reference to 'Dart_PostCObject_DL'
  /path/to/ffiasync/lib/ffi_test_functions_vmspecific.cc:791: error: undefined reference to 'Dart_NewPersistentHandle'
  /path/to/ffiasync/lib/ffi_test_functions_vmspecific.cc:793: error: undefined reference to 'Dart_HandleFromPersistent'
  /path/to/ffiasync/lib/ffi_test_functions_vmspecific.cc:794: error: undefined reference to 'Dart_DeletePersistentHandle'
  /path/to/ffiasync/lib/ffi_test_functions_vmspecific.cc:795: error: undefined reference to 'Dart_IsError'
  /path/to/ffiasync/lib/ffi_test_functions_vmspecific.cc:796: error: undefined reference to 'Dart_PropagateError'
  /path/to/ffiasync/lib/ffi_test_functions_vmspecific.cc:800: error: undefined reference to 'Dart_IsNull'
  /path/to/ffiasync/lib/ffi_test_functions_vmspecific.cc:802: error: undefined reference to 'Dart_NewWeakPersistentHandle'
  /path/to/ffiasync/lib/ffi_test_functions_vmspecific.cc:804: error: undefined reference to 'Dart_HandleFromWeakPersistent'
  /path/to/ffiasync/lib/ffi_test_functions_vmspecific.cc:809: error: undefined reference to 'Dart_DeleteWeakPersistentHandle'
  /path/to/ffiasync/lib/ffi_test_functions_vmspecific.cc:826: error: undefined reference to 'Dart_IsError'
  /path/to/ffiasync/lib/ffi_test_functions_vmspecific.cc:829: error: undefined reference to 'Dart_PropagateError'
  /path/to/ffiasync/lib/ffi_test_functions_vmspecific.cc:846: error: undefined reference to 'Dart_NewStringFromCString'
  /path/to/ffiasync/lib/ffi_test_functions_vmspecific.cc:847: error: undefined reference to 'Dart_IsError'
  /path/to/ffiasync/lib/ffi_test_functions_vmspecific.cc:848: error: undefined reference to 'Dart_PropagateError'
  /path/to/ffiasync/lib/ffi_test_functions_vmspecific.cc:850: error: undefined reference to 'Dart_NewInteger'
  /path/to/ffiasync/lib/ffi_test_functions_vmspecific.cc:851: error: undefined reference to 'Dart_IsError'
  /path/to/ffiasync/lib/ffi_test_functions_vmspecific.cc:852: error: undefined reference to 'Dart_PropagateError'
  /path/to/ffiasync/lib/ffi_test_functions_vmspecific.cc:855: error: undefined reference to 'Dart_Invoke'
  /path/to/ffiasync/lib/ffi_test_functions_vmspecific.cc:874: error: undefined reference to 'Dart_NewStringFromCString'
  /path/to/ffiasync/lib/ffi_test_functions_vmspecific.cc:879: error: undefined reference to 'Dart_GetField'
  /path/to/ffiasync/lib/ffi_test_functions_vmspecific.cc:885: error: undefined reference to 'Dart_IntegerToInt64'
  /path/to/ffiasync/lib/ffi_test_functions_vmspecific.cc:895: error: undefined reference to 'Dart_EnterScope'
  /path/to/ffiasync/lib/ffi_test_functions_vmspecific.cc:900: error: undefined reference to 'Dart_ExitScope'
  /path/to/ffiasync/lib/ffi_test_functions_vmspecific.cc:905: error: undefined reference to 'Dart_True'
  /path/to/ffiasync/lib/ffi_test_functions_vmspecific.cc:0: error: undefined reference to 'Dart_NewPersistentHandle_DL'
  /path/to/ffiasync/lib/ffi_test_functions_vmspecific.cc:0: error: undefined reference to 'Dart_HandleFromPersistent_DL'
  /path/to/ffiasync/lib/ffi_test_functions_vmspecific.cc:0: error: undefined reference to 'Dart_SetPersistentHandle_DL'
  /path/to/ffiasync/lib/ffi_test_functions_vmspecific.cc:0: error: undefined reference to 'Dart_DeletePersistentHandle_DL'
  /path/to/ffiasync/lib/ffi_test_functions_vmspecific.cc:0: error: undefined reference to 'Dart_NewWeakPersistentHandle_DL'
  /path/to/ffiasync/lib/ffi_test_functions_vmspecific.cc:0: error: undefined reference to 'Dart_HandleFromWeakPersistent_DL'
  /path/to/ffiasync/lib/ffi_test_functions_vmspecific.cc:0: error: undefined reference to 'Dart_DeleteWeakPersistentHandle_DL'
  /path/to/ffiasync/lib/ffi_test_functions_vmspecific.cc:0: error: undefined reference to 'Dart_NewPersistentHandle_DL'
  /path/to/ffiasync/lib/ffi_test_functions_vmspecific.cc:0: error: undefined reference to 'Dart_HandleFromPersistent_DL'
  /path/to/ffiasync/lib/ffi_test_functions_vmspecific.cc:0: error: undefined reference to 'Dart_DeletePersistentHandle_DL'
  clang++: error: linker command failed with exit code 1 (use -v to see invocation)
  ninja: build stopped: subcommand failed.
  


* Try:
Run with --stacktrace option to get the stack trace. Run with --info or --debug option to get more log output. Run with --scan to get full insights.

* Get more help at https://help.gradle.org

BUILD FAILED in 12s
Exception: Gradle task assembleDebug failed with exit code 1

问题

无法从 Dart 团队获得答案，我有几个问题

• 我应该从源代码构建 Dart 运行时以使异步回调工作吗？ 上面的错误似乎表明我缺少运行时支持。 这是否意味着官方 dart/flutter 版本中不包含 FFI 异步回调？
• 某处是否有准备运行的 ffi 异步回调示例或文档？

恐怕有了这么少的信息，很难通过跟踪和错误来弄清楚。

Answer 1

查看我的新存储库可能会有所帮助-我设法让 JUCE (C++) 后端运行自己的线程，该线程可以调用 Flutter UI： JucyFluttering

Answer 2

Agreed, it's not clear how you link against the Dart SDK executable(?)/shared library(?) while building your Android shared library or your iOS code. 它编译，但不会链接。

鉴于 Dart 是单线程的，并且异步回调技术涉及向主 Dart 线程发出信号，以指示是时候调用 C 以允许回调在单个主线程上占据太多优势，比如说，从 Dart（可能来自计时器）轮询以查看响应是否准备好。 在 60 fps 时，知道响应在 16 毫秒的帧间间隔中途准备就绪有什么意义？ 无论如何，这种变化要等到下一次油漆才会体现出来。 当然，轮询感觉不是很有效，但每 16 毫秒一次并不那么昂贵。