This project is a Flutter Plugin template.

It provides out-of-the box support for cross-compiling native Rust code for all available iOS and Android architectures and call it from plain Dart using Foreign Function Interface.

This template provides first class FFI support, the clean way.

• No Swift/Kotlin wrappers
• No message passing
• No async/await on Dart
• Write once, use everywhere
• No garbage collection
• Mostly automated development
• No need to export aar bundles or .framework's

Edit your code within rust/src/lib.rs and add any dependencies you need.

Make sure to annotate your exported functions with #[no_mangle] and pub extern so the function names can be matched from Dart.

Returning strings or structs may require using unsafe blocks. Returned strings or structs will need to be free'd from Dart.

• Make sure that the Android NDK is installed
  • You might also need LLVM from the SDK manager
• Ensure that the env variable $ANDROID_NDK_HOME points to the NDK base folder
  • It may look like /Users/brickpop/Library/Android/sdk/ndk-bundle on MacOS
  • And look like /home/brickpop/dev/android/ndk-bundle on Linux
• On the rust folder:
  • Run make to see the available actions
  • Run make init to install the Rust targets
  • Run make all to build the libraries and the .h file
• Update the name of your library in Cargo.toml
  • You'll need to update the symlinks to target the new file names. See iOS and Android below.

Generated artifacts:

• Android libraries
  • target/aarch64-linux-android/release/libgreet.so
  • target/armv7-linux-androideabi/release/libgreet.so
  • target/i686-linux-android/release/libgreet.so
• iOS library
  • target/universal/release/libgreet.a
• Bindings header
  • target/bindings.h

Ensure that ios/mylib.podspec includes the following directives:

...
   s.source           = { :path => '.' }
+  s.public_header_files = 'Classes**/*.h'
   s.source_files = 'Classes/**/*'
+  s.static_framework = true
+  s.vendored_libraries = "**/*.a"
   s.dependency 'Flutter'
   s.platform = :ios, '8.0'
...

On flutter/ios, place a symbolic link to the libgreet.a file

$ cd flutter/ios
$ ln -s ../rust/target/universal/release/libgreet.a .

Append the generated function signatures from rust/target/bindings.h into flutter/ios/Classes/MylibPlugin.h

$ cd flutter/ios
$ cat ../rust/target/bindings.h >> Classes/MylibPlugin.h

In our case, it will append char *rust_greeting(const char *to); and void rust_cstr_free(char *s);

NOTE: By default, XCode will skip bundling the libgreet.a library if it detects that it is not being used. To force its inclusion, add dummy invocations in SwiftMylibPlugin.swift that use every single native function that you use from Flutter:

...
  public func dummyMethodToEnforceBundling() {
    rust_greeting("...");
    compress_jpeg_file("...");
    compress_png_file("...");
    // ...
    // This code will force the bundler to use these functions, but will never be called
  }
}

If you won't be using Flutter channels, the rest of methods can be left empty.

Note: Support for avmv7, armv7s and i386 is deprecated. The targets can still be compiled with Rust 1.41 or earlier and by uncommenting the make init line on rust/makefile

Similarly as we did on iOS with libgreet.a, create symlinks pointing to the binary libraries on rust/target.

You should have the following structure on flutter/android for each architecture:

src
└── main
    └── jniLibs
        ├── arm64-v8a
        │   └── libgreet.so@ -> ../../../../../rust/target/aarch64-linux-android/release/libgreet.so
        ├── armeabi-v7a
        │   └── libgreet.so@ -> ../../../../../rust/target/armv7-linux-androideabi/release/libgreet.so
        ├── x86
        │   └── libgreet.so@ -> ../../../../../rust/target/i686-linux-android/release/libgreet.so
        └── x86_64
            └── libgreet.so@ -> ../../../../../rust/target/x86_64-linux-android/release/libgreet.so

As before, if you are not using Flutter channels, the methods within android/src/main/kotlin/org/mylib/mylib/MylibPlugin.kt can be left empty.

In /lib/mylib.dart, initialize the function bindings from Dart and implement any additional logic that you need.

Load the library:

final DynamicLibrary nativeExampleLib = Platform.isAndroid
    ? DynamicLibrary.open("libgreet.so")
    : DynamicLibrary.process();

Find the symbols we want to use, with the appropriate Dart signatures:

final Pointer<Utf8> Function(Pointer<Utf8>) rustGreeting = nativeExampleLib
    .lookup<NativeFunction<Pointer<Utf8> Function(Pointer<Utf8>)>>("rust_greeting")
    .asFunction();

final void Function(Pointer<Utf8>) freeGreeting = nativeExampleLib
    .lookup<NativeFunction<Void Function(Pointer<Utf8>)>>("rust_cstr_free")
    .asFunction();

Call them:

// Prepare the parameters
final name = "John Smith";
final Pointer<Utf8> namePtr = Utf8.toUtf8(name);
print("- Calling rust_greeting with argument:  $namePtr");

// Call rust_greeting
final Pointer<Utf8> resultPtr = rustGreeting(namePtr);
print("- Result pointer:  $resultPtr");

final String greetingStr = Utf8.fromUtf8(resultPtr);
print("- Response string:  $greetingStr");

When we are done using greetingStr, tell Rust to free it, since the Rust implementation kept it alive for us to use it.

freeGreeting(resultPtr);

• https://dart.dev/guides/libraries/c-interop
• https://flutter.dev/docs/development/platform-integration/c-interop
• https://github.com/dart-lang/samples/blob/master/ffi/structs/structs.dart
• https://mozilla.github.io/firefox-browser-architecture/experiments/2017-09-06-rust-on-ios.html
• https://mozilla.github.io/firefox-browser-architecture/experiments/2017-09-21-rust-on-android.html