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 or Kotlin wrappers
• No message channels
• No async calls
• 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 dealt with by Dart.

Returning strings or structs may require using unsafe code.

• Ensure the NDK is installed
• Adapt the paths from cargo-config.toml to your NDK installation and append them to your ~/.cargo/config file
• Set the name of your library in Cargo.toml
• On the rust folder, run make init and make all

Generated artifacts:

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

Ensure that rust/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 libexample.a file

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

Append the 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_greeting_free(char *s);

By default, XCode will skip bundling the libexample.a library if it detects that it is not being used. To force its inclusion, add a dummy method in SwiftMylibPlugin.m that uses at least one of the native functions:

...
  public func dummyMethodToEnforceBundling() {
    rust_greeting("");
  }
}

If you are not using Flutter channels, the rest of methods can be left empty.

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

Create the following structure on flutter/android for each architecture:

src
`-- main
    `-- jniLibs
        |-- arm64-v8a
        |   `-- libexample.so@ -> ../../../../../rust/target/aarch64-linux-android/release/libexample.so
        |-- armeabi-v7a
        |   `-- libexample.so@ -> ../../../../../rust/target/armv7-linux-androideabi/release/libexample.so
        `-- x86
            `-- libexample.so@ -> ../../../../../rust/target/i686-linux-android/release/libexample.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("libexample.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_greeting_free")
    .asFunction();

Call them:

// Prepare the parameters
final name = "John Smith";
final Pointer<Utf8> arg1 = Utf8.toUtf8(name);
print("- Calling rust_greeting with argument:  $arg1");

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

final String greetingStr = Utf8.fromUtf8(resultPointer);
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(resultPointer);

• https://flutter.dev/docs/development/platform-integration/c-interop
• 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
• https://github.com/dart-lang/samples/blob/master/ffi/structs/structs.dart