A simple mobile app example (iOS, Android) that uses a Flutter (Dart) UI and JUCE (C++) as backend.

• Created a JUCE GUI project JucyFluttering with Android and iOS exporters (well it does not really show a GUI, but has a message loop running, to provide timers, event handling etc.)

• Created a flutter plugin project (subfolder fluttering) for platforms android and iOs, using Java for android, and ObjectiveC for iOS

  • flutter create --template=plugin --platforms=android,ios -a java -i objc fluttering
  • the .dart source file are placed in fluttering/lib

JUCE is already does a good job at setting up all the platform-dependend boilerplate code to instantiate and run iOs and Android apps. Some adjustments had to be done to do the Flutter instantiation part too, as documented here:

• Adding Flutter to Android
• Adding Flutter to iOS

Made custom activity and application Java class files, placed in Sources/Android.

• The Actvity extends the FlutteringActivity class
• The Application initialises JUCE, but also and instantiates, pre-warms and caches the Flutter Engine, and sets the initial Flutter navigation route to '/'

Some manual configuration for the Projucers Android exporter was necessary:

• Java Source code folders:

  Source/Android
  

• Custom Android Activity:

  eu.selfhost.audiooffler.jucyfluttering.FlutteringActivity
  

• Custom Android Application:

  eu.selfhost.audiooffler.jucyfluttering.FlutteringApplication
  

• Module Dependencies:

  implementation project(':flutter')
  

• Extra module's build.gradle content:

  defaultConfig {
      ndk {
          // filter for architectures supported by Flutter
          abiFilters 'armeabi-v7a', 'arm64-v8a', 'x86', 'x86_64'
      }
  }
  
  compileOptions {
      // Flutter Android engine uses Java 8 features
      sourceCompatibility 1.8
      targetCompatibility 1.8
  }
  

• Custom settings.gradle content:

  setBinding(new Binding([gradle: this]))
  evaluate(new File(
      settingsDir.parentFile.parentFile,
      'fluttering/.android/include_flutter.groovy'
  ))
  

• Android Theme:

  @android:style/Theme.NoTitleBar
  

Projucer can't create / manipulate a gradle.properties file, but this is needed for Flutter

• The file with the following content was manually placed at Builds/Android folder of this repository, it won't get overwritten by the Projucre, just do not delete it manually:

  android.useAndroidX=true
  android.enableJetifier=true
  

Made a custom App Delegate class, placed at Sources/iOS. This is an UIApplicationDelegate for running JUCE and Flutter in an iOS app. It's .mm File also has the START_JUCE_APPLICATION_WITH_CUSTOM_DELEGATE call to automatically start the JUCEApplication with this Delegate, as documented in JUCE/modules/juce_events/messages/juce_Initialisation.h. The delegate takes care of:

• Juce Initialisation and handling
• Flutter Engine Initalisation and handling
• UIWindow (showing the Flutter View)
• FlutterViewController
• FlutterPluginAppLifeCycleDelegate

CocoaPods is needed for FLutter integration.

• If do not have it installed yet (check with which pod), do so with sudo gem install cocoapod
• A Podfile was made manually and placed in this repository (Builds/iOS/Podfile), containing the following. The Projucer won't overwite it, just do not delete it manually.

    platform :ios, '9.0'
  
    flutter_application_path = '../../fluttering'
    load File.join(flutter_application_path, '.ios', 'Flutter', 'podhelper.rb')
  
    target 'Jucy Fluttering - App' do
    
    install_all_flutter_pods(flutter_application_path)
  
    end
  

In Projucer, some iOS export settings had to be configured:

• To make it work with CocoaPods (as found in adamski's thread in the JUCE forum), for all build target configuations (Release/Debug), set Binary Location:

  $(BUILD_DIR)/$(CONFIGURATION)$(EFFECTIVE_PLATFORM_NAME)
  

• Custom PList

  <plist>
      <dict>
          <key>UIBackgroundModes</key>
          <array>
              <string>fetch</string>
              <string>remote-notification</string>
          </array>
      </dict>
  </plist>
  

No extra steps needed building. The project can be opened with Android Studio from the Projucer or by opening the Builds/Android project folder in Android Studio. If dart support is enabled for the project in Android Studio, it even attaches the flutter debugger. Else, while running / debugging, one may call flutter attach from terminal (first go to cd ./fluttering).

After saving / exporting with Projucer, the XCode Project can't simply be build as usual for JUCE projects. Two more steps are necassary:

• from terminal run pod install (first go to cd ./Builds/iOS)
• then open the generated .xcworkspace project and build that
  • the .xcodeproj would fail, due to the missing Flutter dependencies

While running / debugging from xCode, one may call flutter attach from terminal (first go to cd ./fluttering).

Check out the Flutter/dart:ffi documentation for the basic setup of accessing nativ C++ code from Flutter/dart.

Some examples can be found in Sources/JucyFlutteringInterop.h and the dart counterparts in fluttering/lib/juce_fluttering_interop.dart.

• Externalized JUCE/C++ functions can be called from Fluttter/Dart via dart:ffi NativeFunctions, since the dart isolate (thread) will execute it.
• Some boilercode / wrapping is still needed. See fluttering/lib/juce_fluttering_interop.dart.

• For getting juce::Strings, JUCE has to return UTF-8 char* pointers
• If the String is local it has to be copied to memory or it will get lost on return.
• This can be done using toRawUTF8, malloc and strcpy, see the example in EXTERN_C const char *getAppName() in Source/JucyFlutteringInterop.h, using the helper function const char *copyStringToUTF8(String juceString) from the same file
• The dart caller then can convert this to a dart String, using fromUtf8 and call free on the returned Pointer<Utf8>, utilizing import package:ffi/ffi.dart. See tthe example in fluttering/lib/juce_fluttering_interop.dart

Problem:

• Access to classes and values is only allowed from the same isolate!
• JUCE Message Loop (e.g. juce::Timer) or AudioCallback Threads (e.g. juce::AudioProcessor) won't run in the same isolate as Flutter/Dart UI.
• Therefore, just registering callback dart:ffi function pointers in your JUCE/C++ code will not be sufficient. (Except if the callback was called from some C++ function that was started from dart).

Solution:

• Different isolates can communicate by sending values through ports (see ReceivePort, SendPort).
• To access the dart native API from C++, files from GitHub: dart-lang/sdk/runtime/include where included in the JUCE project Source/DartApiDL (the folder was added to the header search paths for all Build/Debug targets). see https://github.com/mraleph/go_dart_ffi_example
• Init by calling the NativeAPI from dart (that will be the receiver) and tell JUCE/C++ the port.
• From then on JUCE/C++ may send messages containing objects with int, double or UTF-8 char* string pointers to the dart receiver port.