Flutter Rust FFI Template

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

Getting started

Write your native code

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.

Compile the library

• 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/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

Reference the shared objects

iOS

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 libexample.a file

$ cd flutter/ios
$ ln -s ../rust/target/universal/release/libexample.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 libexample.a library if it detects that it is not being used. To force its inclusion, add a dummy method in SwiftMylibPlugin.swift that uses at least one of the native functions:

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

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

Android

Similarly as we did on iOS with libexample.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
        │   └── 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.

Declare the bindings in Dart

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_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);

More information

• 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