Binding to native Android code using dart:ffi

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 Android 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 both Android and iOS. 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 Android, a dynamically linked library is distributed as a set of .so (ELF) files, one for each architecture.

A dynamically linked library can be loaded into Dart via 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=android,ios --template=plugin native_add
$ cd native_add

Step 2: Add C/C++ sources

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

You can add Android-specific sources to the android folder and modify CMakeLists.txt appropriately. Also, Gradle allows you to point to the ios folder, if that helps, but it’s not required to use the same sources for both iOS and Android;

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.

On Android, you need to create a CMakeLists.txt file to define how the sources should be compiled and point Gradle to it. From the root of your project directory, use the following instructions

cat > android/CMakeLists.txt << EOF
cmake_minimum_required(VERSION 3.4.1)  # for example

add_library( native_add

             # Sets the library as a shared library.
             SHARED

             # Provides a relative path to your source file(s).
             ../ios/Classes/native_add.cpp )
EOF

Finally, add an externalNativeBuild section to android/build.gradle. For example:

android {
  // ...
  externalNativeBuild {
    // Encapsulates your CMake build configurations.
    cmake {
      // Provides a relative path to your CMake build script.
      path "CMakeLists.txt"
    }
  }
  // ...
}

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 is not important.

First, you must create a DynamicLibrary handle to the native code. The following example shows how to create a handle for an iOS app OR an Android app:

import 'dart:ffi'; // For FFI
import 'dart:io'; // For Platform.isX

final DynamicLibrary nativeAddLib = Platform.isAndroid
    ? DynamicLibrary.open('libnative_add.so')
    : DynamicLibrary.process();

Note that on Android the native library is named in CMakeLists.txt (see above), but on iOS it takes the plugin’s name.

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

Platform library

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

  1. Find the desired library in the Android NDK Native APIs list in the Android docs. This lists stable native APIs.
  2. Load the library using DynamicLibrary.open. For example, to load OpenGL ES (v3):
    DynamicLibrary.open('libGLES_v3.so');
    

You might need to update the Android manifest file of the app or plugin if indicated by the documentation.

First-party library

The process for including native code in source code or binary form is the same for an app or plugin.

Open-source third-party

Follow the Add C and C++ code to your project instructions in the Android docs to add native code and support for the native code toolchain (either CMake or ndk-build).

Closed-source third-party library

To create a Flutter plugin that includes Dart source code, but distribute the C/C++ library in binary form, use the following instructions:

  1. Open the android/build.gradle file for your project.
  2. Add the AAR artifact as a dependency. Don’t include the artifact in your Flutter package. Instead, it should be downloaded from a repository, such as JCenter.

Android APK size (shared object compression)

Android guidelines in general recommend distributing native shared objects uncompressed because that actually saves on device space. Shared objects can be directly loaded from the APK instead of unpacking them on device into a temporary location and then loading. APKs are additionally packed in transit—that’s why you should be looking at download size.

Flutter APKs by default don’t follow these guidelines and compress libflutter.so and libapp.so—this leads to smaller APK size but larger on device size.

Shared objects from third parties can change this default setting with android:extractNativeLibs="true" in their AndroidManifest.xml and stop the compression of libflutter.so, libapp.so, and any user-added shared objects. To re-enable compression, override the setting in your_app_name/android/app/src/main/AndroidManifest.xml in the following way.

@@ -1,5 +1,6 @@
 <manifest xmlns:android="http://schemas.android.com/apk/res/android"
-    package="com.example.your_app_name">
+    xmlns:tools="http://schemas.android.com/tools"
+    package="com.example.your_app_name" >
     <!-- io.flutter.app.FlutterApplication is an android.app.Application that
          calls FlutterMain.startInitialization(this); in its onCreate method.
          In most cases you can leave this as-is, but you if you want to provide
          additional functionality it is fine to subclass or reimplement
          FlutterApplication and put your custom class here. -->
@@ -8,7 +9,9 @@
     <application
         android:name="io.flutter.app.FlutterApplication"
         android:label="your_app_name"
-        android:icon="@mipmap/ic_launcher">
+        android:icon="@mipmap/ic_launcher"
+        android:extractNativeLibs="true"
+        tools:replace="android:extractNativeLibs">