顫振/飛鏢：如何使用 Dart FFI 的異步回調？

Question

我的應用程序的后端是用 C++ 編寫的，前端是用 Dart/flutter 編寫的。 每當數據准備好時，我希望后端通知前端。 這需要在 Dart 和 C++ 之間實現異步回調方案。

環境

$ 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 團隊的 github 示例。

為了使示例代碼適應 flutter 項目，我執行了以下操作

• 創建 FFI 插件項目
• 用樣品function替換main function
• 刪除不重要的依賴項，例如expect
• 使用引用的 C++ 代碼創建原生庫，例如dart-sdk-master/runtime/bin/ffi_test/ffi_test_functions.cc

問題

但是，我無法運行示例。

Dart版

我遇到的第一個問題是直接運行示例時不支持 dart 版本。 我在最新的穩定頻道，也嘗試過開發頻道。 錯誤說

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

本機編譯

示例中缺少有關本機端實現的信息。 所以我留下了很多編譯器錯誤。

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

問題

無法從 Dart 團隊獲得答案，我有幾個問題

• 我應該從源代碼構建 Dart 運行時以使異步回調工作嗎？ 上面的錯誤似乎表明我缺少運行時支持。 這是否意味着官方 dart/flutter 版本中不包含 FFI 異步回調？
• 某處是否有准備運行的 ffi 異步回調示例或文檔？

恐怕有了這么少的信息，很難通過跟蹤和錯誤來弄清楚。

Answer 1

查看我的新存儲庫可能會有所幫助-我設法讓 JUCE (C++) 后端運行自己的線程，該線程可以調用 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. 它編譯，但不會鏈接。

鑒於 Dart 是單線程的，並且異步回調技術涉及向主 Dart 線程發出信號，以指示是時候調用 C 以允許回調在單個主線程上占據太多優勢，比如說，從 Dart（可能來自計時器）輪詢以查看響應是否准備好。 在 60 fps 時，知道響應在 16 毫秒的幀間間隔中途准備就緒有什么意義？ 無論如何，這種變化要等到下一次油漆才會體現出來。 當然，輪詢感覺不是很有效，但每 16 毫秒一次並不那么昂貴。