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Binding to native macOS code using dart:ffi

Contents

Flutter mobile and desktop apps can use the dart:ffi library to call native C APIs. FFI stands for foreign function interface. Other terms for similar functionality include native interface and language bindings.

Before your library or program can use the FFI library to bind to native code, you must ensure that the native code is loaded and its symbols are visible to Dart. This page focuses on compiling, packaging, and loading macOS native code within a Flutter plugin or app.

This tutorial demonstrates how to bundle C/C++ sources in a Flutter plugin and bind to them using the Dart FFI library on macOS. In this walkthrough, you’ll create a C function that implements 32-bit addition and then exposes it through a Dart plugin named “native_add”.

Dynamic vs static linking

A native library can be linked into an app either dynamically or statically. A statically linked library is embedded into the app’s executable image, and is loaded when the app starts.

Symbols from a statically linked library can be loaded using DynamicLibrary.executable or DynamicLibrary.process.

A dynamically linked library, by contrast, is distributed in a separate file or folder within the app, and loaded on-demand. On macOS, the dynamically linked library is distributed as a .framework folder.

A dynamically linked library can be loaded into Dart using DynamicLibrary.open.

API documentation is available from the Dart dev channel: Dart API reference documentation.

Step 1: Create a plugin

If you already have a plugin, skip this step.

To create a plugin called “native_add”, do the following:

$ flutter create --platforms=macos --template=plugin native_add
$ cd native_add

Step 2: Add C/C++ sources

You need to inform the macOS build system about the native code so the code can be compiled and linked appropriately into the final application.

Add the sources to the macos folder, because CocoaPods doesn’t allow including sources above the podspec file.

The FFI library can only bind against C symbols, so in C++ these symbols must be marked extern C. You should also add attributes to indicate that the symbols are referenced from Dart, to prevent the linker from discarding the symbols during link-time optimization.

For example, to create a C++ file named macos/Classes/native_add.cpp, use the following instructions. (Note that the template has already created this file for you.) Start from the root directory of your project:

cat > macos/Classes/native_add.cpp << EOF
#include <stdint.h>

extern "C" __attribute__((visibility("default"))) __attribute__((used))
int32_t native_add(int32_t x, int32_t y) {
    return x + y;
}
EOF

On macOS, you need to tell Xcode to statically link the file:

  1. In Xcode, open Runner.xcworkspace.
  2. Add the C/C++/Objective-C/Swift source files to the Xcode project.

Step 3: Load the code using the FFI library

In this example, you can add the following code to lib/native_add.dart. However the location of the Dart binding code isn’t important.

First, you must create a DynamicLibrary handle to the native code.

import 'dart:ffi'; // For FFI

final DynamicLibrary nativeAddLib = DynamicLibrary.process();

With a handle to the enclosing library, you can resolve the native_add symbol:

final int Function(int x, int y) nativeAdd = nativeAddLib
    .lookup<NativeFunction<Int32 Function(Int32, Int32)>>('native_add')
    .asFunction();

Finally, you can call it. To demonstrate this within the auto-generated “example” app (example/lib/main.dart):

// Inside of _MyAppState.build:
        body: Center(
          child: Text('1 + 2 == ${nativeAdd(1, 2)}'),
        ),

Other use cases

iOS and macOS

Dynamically linked libraries are automatically loaded by the dynamic linker when the app starts. Their constituent symbols can be resolved using DynamicLibrary.process. You can also get a handle to the library with DynamicLibrary.open to restrict the scope of symbol resolution, but it’s unclear how Apple’s review process handles this.

Symbols statically linked into the application binary can be resolved using DynamicLibrary.executable or DynamicLibrary.process.

Platform library

To link against a platform library, use the following instructions:

  1. In Xcode, open Runner.xcworkspace.
  2. Select the target platform.
  3. Click + in the Linked Frameworks and Libraries section.
  4. Select the system library to link against.

First-party library

A first-party native library can be included either as source or as a (signed) .framework file. It’s probably possible to include statically linked archives as well, but it requires testing.

Source code

To link directly to source code, use the following instructions:

  1. In Xcode, open Runner.xcworkspace.
  2. Add the C/C++/Objective-C/Swift source files to the Xcode project.

  3. Add the following prefix to the exported symbol declarations to ensure they are visible to Dart:

    C/C++/Objective-C

    extern "C" /* <= C++ only */ __attribute__((visibility("default"))) __attribute__((used))

    Swift

    @_cdecl("myFunctionName")

Compiled (dynamic) library

To link to a compiled dynamic library, use the following instructions:

  1. If a properly signed Framework file is present, open Runner.xcworkspace.
  2. Add the framework file to the Embedded Binaries section.
  3. Also add it to the Linked Frameworks & Libraries section of the target in Xcode.

Compiled (dynamic) library (macOS)

To add a closed source library to a Flutter macOS Desktop app, use the following instructions:

  1. Follow the instructions for Flutter desktop to create a Flutter desktop app.
  2. Open the yourapp/macos/Runner.xcworkspace in Xcode.
    1. Drag your precompiled library (libyourlibrary.dylib) into Runner/Frameworks.
    2. Click Runner and go to the Build Phases tab.
      1. Drag libyourlibrary.dylib into the Copy Bundle Resources list.
      2. Under Embed Libraries, check Code Sign on Copy.
      3. Under Link Binary With Libraries, set status to Optional. (We use dynamic linking, no need to statically link.)
    3. Click Runner and go to the General tab.
      1. Drag libyourlibrary.dylib into the Frameworks, Libraries and Embedded Content list.
      2. Select Embed & Sign.
    4. Click Runner and go to the Build Settings tab.
      1. In the Search Paths section configure the Library Search Paths to include the path where libyourlibrary.dylib is located.
  3. Edit lib/main.dart.
    1. Use DynamicLibrary.open('libyourlibrary.dylib') to dynamically link to the symbols.
    2. Call your native function somewhere in a widget.
  4. Run flutter run and check that your native function gets called.
  5. Run flutter build macos to build a self-contained release version of your app.