Class FlutterJNI

  • public class FlutterJNI
    extends Object
    Interface between Flutter embedding's Java code and Flutter engine's C/C++ code.

    Flutter's engine is built with C/C++. The Android Flutter embedding is responsible for coordinating Android OS events and app user interactions with the C/C++ engine. Such coordination requires messaging from an Android app in Java code to the C/C++ engine code. This communication requires a JNI (Java Native Interface) API to cross the Java/native boundary.

    The entirety of Flutter's JNI API is codified in FlutterJNI. There are multiple reasons that all such calls are centralized in one class. First, JNI calls are inherently static and contain no Java implementation, therefore there is little reason to associate calls with different classes. Second, every JNI call must be registered in C/C++ code and this registration becomes more complicated with every additional Java class that contains JNI calls. Third, most Android developers are not familiar with native development or JNI intricacies, therefore it is in the interest of future maintenance to reduce the API surface that includes JNI declarations. Thus, all Flutter JNI calls are centralized in FlutterJNI.

    Despite the fact that individual JNI calls are inherently static, there is state that exists within FlutterJNI. Most calls within FlutterJNI correspond to a specific "platform view", of which there may be many. Therefore, each FlutterJNI instance holds onto a "native platform view ID" after attachToNative(), which is shared with the native C/C++ engine code. That ID is passed to every platform-view-specific native method. ID management is handled within FlutterJNI so that developers don't have to hold onto that ID.

    To connect part of an Android app to Flutter's C/C++ engine, instantiate a FlutterJNI and then attach it to the native side:

     // Instantiate FlutterJNI and attach to the native side.
     FlutterJNI flutterJNI = new FlutterJNI();
     // Use FlutterJNI as desired. flutterJNI.dispatchPointerDataPacket(...);
     // Destroy the connection to the native side and cleanup.

    To receive callbacks for certain events that occur on the native side, register listeners:

    1. addEngineLifecycleListener(FlutterEngine.EngineLifecycleListener)
    2. addIsDisplayingFlutterUiListener(FlutterUiDisplayListener)
    To facilitate platform messages between Java and Dart running in Flutter, register a handler:


    To invoke a native method that is not associated with a platform view, invoke it statically:

    bool enabled = FlutterJNI.getIsSoftwareRenderingEnabled();

    • Constructor Detail

      • FlutterJNI

        public FlutterJNI()
    • Method Detail

      • loadLibrary

        public void loadLibrary()
        Loads the C++ library.

        This must be called before any other native methods, and can be overridden by tests to avoid loading native libraries.

        This method should only be called once across all FlutterJNI instances.

      • prefetchDefaultFontManager

        public void prefetchDefaultFontManager()
        Prefetch the default font manager provided by SkFontMgr::RefDefault() which is a process-wide singleton owned by Skia. Note that, the first call to SkFontMgr::RefDefault() will take noticeable time, but later calls will return a reference to the preexisting font manager.

        This method should only be called once across all FlutterJNI instances.

      • init

        public void init​(@NonNull
                         Context context,
                         String[] args,
                         String bundlePath,
                         String appStoragePath,
                         String engineCachesPath,
                         long initTimeMillis)
        Perform one time initialization of the Dart VM and Flutter engine.

        This method must be called only once. Calling more than once will cause an exception.

        context - The application context.
        args - Arguments to the Dart VM/Flutter engine.
        bundlePath - For JIT runtimes, the path to the Dart kernel file for the application.
        appStoragePath - The path to the application data directory.
        engineCachesPath - The path to the application cache directory.
        initTimeMillis - The time, in milliseconds, taken for initialization.
      • getIsSoftwareRenderingEnabled

        public boolean getIsSoftwareRenderingEnabled()
        Checks launch settings for whether software rendering is requested.

        The value is the same per program.

      • setRefreshRateFPS

        public void setRefreshRateFPS​(float refreshRateFPS)
        Notifies the engine about the refresh rate of the display when the API level is below 30.

        For API 30 and above, this value is ignored.

        Calling this method multiple times will update the refresh rate for the next vsync period. However, callers should avoid calling Display.getRefreshRate() frequently, since it is expensive on some vendor implementations.

        refreshRateFPS - The refresh rate in nanoseconds.
      • updateRefreshRate

        public void updateRefreshRate()
      • setAsyncWaitForVsyncDelegate

        public void setAsyncWaitForVsyncDelegate​(@Nullable
                                                 FlutterJNI.AsyncWaitForVsyncDelegate delegate)
        The Android vsync waiter implementation in C++ needs to know when a vsync signal arrives, which is obtained via Java API. The delegate set here is called on the C++ side when the engine is ready to wait for the next vsync signal. The delegate is expected to add a postFrameCallback to the Choreographer, and call onVsync(long,long,long) to notify the engine.
        delegate - The delegate that will call the engine back on the next vsync signal.
      • onVsync

        public void onVsync​(long frameDelayNanos,
                            long refreshPeriodNanos,
                            long cookie)
        Notifies the engine that the Choreographer has signaled a vsync.
        frameDelayNanos - The time in nanoseconds when the frame started being rendered, subtracted from the System.nanoTime() timebase.
        refreshPeriodNanos - The display refresh period in nanoseconds.
        cookie - An opaque handle to the C++ VSyncWaiter object.
      • isCodePointEmoji

        public boolean isCodePointEmoji​(int codePoint)
      • isCodePointEmojiModifier

        public boolean isCodePointEmojiModifier​(int codePoint)
      • isCodePointEmojiModifierBase

        public boolean isCodePointEmojiModifierBase​(int codePoint)
      • isCodePointVariantSelector

        public boolean isCodePointVariantSelector​(int codePoint)
      • isCodePointRegionalIndicator

        public boolean isCodePointRegionalIndicator​(int codePoint)
      • isAttached

        public boolean isAttached()
        Returns true if this instance of FlutterJNI is connected to Flutter's native engine via a Java Native Interface (JNI).
      • attachToNative

        public void attachToNative()
        Attaches this FlutterJNI instance to Flutter's native engine, which allows for communication between Android code and Flutter's platform agnostic engine.

        This method must not be invoked if FlutterJNI is already attached to native.

      • performNativeAttach

        public long performNativeAttach​(@NonNull
                                        FlutterJNI flutterJNI)
      • spawn

        public FlutterJNI spawn​(@Nullable
                                String entrypointFunctionName,
                                String pathToEntrypointFunction,
                                String initialRoute,
                                List<String> entrypointArgs)
        Spawns a new FlutterJNI instance from the current instance.

        This creates another native shell from the current shell. This causes the 2 shells to re-use some of the shared resources, reducing the total memory consumption versus creating a new FlutterJNI by calling its standard constructor.

        This can only be called once the current FlutterJNI instance is attached by calling attachToNative().

        Static methods that should be only called once such as init(Context, String[], String, String, String, long) shouldn't be called again on the spawned FlutterJNI instance.

      • detachFromNativeAndReleaseResources

        public void detachFromNativeAndReleaseResources()
        Detaches this FlutterJNI instance from Flutter's native engine, which precludes any further communication between Android code and Flutter's platform agnostic engine.

        This method must not be invoked if FlutterJNI is not already attached to native.

        Invoking this method will result in the release of all native-side resources that were set up during attachToNative() or spawn(String, String, String, List), or accumulated thereafter.

        It is permissible to re-attach this instance to native after detaching it from native.

      • addIsDisplayingFlutterUiListener

        public void addIsDisplayingFlutterUiListener​(@NonNull
                                                     FlutterUiDisplayListener listener)
        Adds a FlutterUiDisplayListener, which receives a callback when Flutter's engine notifies FlutterJNI that Flutter is painting pixels to the Surface that was provided to Flutter.
      • nativeImageHeaderCallback

        public static void nativeImageHeaderCallback​(long imageGeneratorPointer,
                                                     int width,
                                                     int height)
      • decodeImage

        public static Bitmap decodeImage​(@NonNull
                                         ByteBuffer buffer,
                                         long imageGeneratorAddress)
        Called by native as a fallback method of image decoding. There are other ways to decode images on lower API levels, they involve copying the native data _and_ do not support any additional formats, whereas ImageDecoder supports HEIF images. Unlike most other methods called from native, this method is expected to be called on a worker thread, since it only uses thread safe methods and may take multiple frames to complete.
      • onFirstFrame

        public void onFirstFrame()
      • onSurfaceWindowChanged

        public void onSurfaceWindowChanged​(@NonNull
                                           Surface surface)
        In hybrid composition, call this method when the Surface has changed.

        In hybrid composition, the root surfaces changes from SurfaceHolder.getSurface() to ImageReader.getSurface() when a platform view is in the current frame.

      • setViewportMetrics

        public void setViewportMetrics​(float devicePixelRatio,
                                       int physicalWidth,
                                       int physicalHeight,
                                       int physicalPaddingTop,
                                       int physicalPaddingRight,
                                       int physicalPaddingBottom,
                                       int physicalPaddingLeft,
                                       int physicalViewInsetTop,
                                       int physicalViewInsetRight,
                                       int physicalViewInsetBottom,
                                       int physicalViewInsetLeft,
                                       int systemGestureInsetTop,
                                       int systemGestureInsetRight,
                                       int systemGestureInsetBottom,
                                       int systemGestureInsetLeft,
                                       int physicalTouchSlop,
                                       int[] displayFeaturesBounds,
                                       int[] displayFeaturesType,
                                       int[] displayFeaturesState)
        Call this method to notify Flutter of the current device viewport metrics that are applies to the Flutter UI that is being rendered.

        This method should be invoked with initial values upon attaching to native. Then, it should be invoked any time those metrics change while FlutterJNI is attached to native.

      • dispatchPointerDataPacket

        public void dispatchPointerDataPacket​(@NonNull
                                              ByteBuffer buffer,
                                              int position)
        Sends a packet of pointer data to Flutter's engine.
      • setPlatformViewsController

        public void setPlatformViewsController​(@NonNull
                                               PlatformViewsController platformViewsController)
      • dispatchSemanticsAction

        public void dispatchSemanticsAction​(int id,
                                            AccessibilityBridge.Action action)
        Sends a semantics action to Flutter's engine, without any additional arguments.
      • dispatchSemanticsAction

        public void dispatchSemanticsAction​(int id,
                                            AccessibilityBridge.Action action,
                                            Object args)
        Sends a semantics action to Flutter's engine, with additional arguments.
      • setSemanticsEnabled

        public void setSemanticsEnabled​(boolean enabled)
        Instructs Flutter to enable/disable its semantics tree, which is used by Flutter to support accessibility and related behaviors.
      • setAccessibilityFeatures

        public void setAccessibilityFeatures​(int flags)
      • registerTexture

        public void registerTexture​(long textureId,
                                    SurfaceTextureWrapper textureWrapper)
        Gives control of a SurfaceTexture to Flutter so that Flutter can display that texture within Flutter's UI.
      • markTextureFrameAvailable

        public void markTextureFrameAvailable​(long textureId)
        Call this method to inform Flutter that a texture previously registered with registerTexture(long, SurfaceTextureWrapper) has a new frame available.

        Invoking this method instructs Flutter to update its presentation of the given texture so that the new frame is displayed.

      • runBundleAndSnapshotFromLibrary

        public void runBundleAndSnapshotFromLibrary​(@NonNull
                                                    String bundlePath,
                                                    String entrypointFunctionName,
                                                    String pathToEntrypointFunction,
                                                    AssetManager assetManager,
                                                    List<String> entrypointArgs)
        Executes a Dart entrypoint.

        This can only be done once per JNI attachment because a Dart isolate can only be entered once.

      • setPlatformMessageHandler

        public void setPlatformMessageHandler​(@Nullable
                                              PlatformMessageHandler platformMessageHandler)
        Sets the handler for all platform messages that come from the attached platform view to Java.

        Communication between a specific Flutter context (Dart) and the host platform (Java) is accomplished by passing messages. Messages can be sent from Java to Dart with the corresponding FlutterJNI methods:

        FlutterJNI is also the recipient of all platform messages sent from its attached Flutter context. FlutterJNI does not know what to do with these messages, so a handler is exposed to allow these messages to be processed in whatever manner is desired:


        If a message is received but no PlatformMessageHandler is registered, that message will be dropped (ignored). Therefore, when using FlutterJNI to integrate a Flutter context in an app, a PlatformMessageHandler must be registered for 2-way Java/Dart communication to operate correctly. Moreover, the handler must be implemented such that fundamental platform messages are handled as expected. See FlutterNativeView for an example implementation.

      • cleanupMessageData

        public void cleanupMessageData​(long messageData)
        Destroys the resources provided sent to `handlePlatformMessage`.

        This can be called on any thread.

        messageData - the argument sent to handlePlatformMessage.
      • handlePlatformMessage

        public void handlePlatformMessage​(@NonNull
                                          String channel,
                                          ByteBuffer message,
                                          int replyId,
                                          long messageData)
      • dispatchEmptyPlatformMessage

        public void dispatchEmptyPlatformMessage​(@NonNull
                                                 String channel,
                                                 int responseId)
        Sends an empty reply (identified by responseId) from Android to Flutter over the given channel.
      • dispatchPlatformMessage

        public void dispatchPlatformMessage​(@NonNull
                                            String channel,
                                            ByteBuffer message,
                                            int position,
                                            int responseId)
        Sends a reply message from Android to Flutter over the given channel.
      • invokePlatformMessageEmptyResponseCallback

        public void invokePlatformMessageEmptyResponseCallback​(int responseId)
      • invokePlatformMessageResponseCallback

        public void invokePlatformMessageResponseCallback​(int responseId,
                                                          ByteBuffer message,
                                                          int position)
      • onDisplayOverlaySurface

        public void onDisplayOverlaySurface​(int id,
                                            int x,
                                            int y,
                                            int width,
                                            int height)
      • onBeginFrame

        public void onBeginFrame()
      • onEndFrame

        public void onEndFrame()
      • destroyOverlaySurfaces

        public void destroyOverlaySurfaces()
      • setLocalizationPlugin

        public void setLocalizationPlugin​(@Nullable
                                          io.flutter.plugin.localization.LocalizationPlugin localizationPlugin)
        Sets the localization plugin that is used in various localization methods.
      • computePlatformResolvedLocale

        public String[] computePlatformResolvedLocale​(@NonNull
                                                      String[] strings)
        Invoked by native to obtain the results of Android's locale resolution algorithm.
      • setDeferredComponentManager

        public void setDeferredComponentManager​(@Nullable
                                                io.flutter.embedding.engine.deferredcomponents.DeferredComponentManager deferredComponentManager)
        Sets the deferred component manager that is used to download and install split features.
      • requestDartDeferredLibrary

        public void requestDartDeferredLibrary​(int loadingUnitId)
        Called by dart to request that a Dart deferred library corresponding to loadingUnitId be downloaded (if necessary) and loaded into the dart vm.

        This method delegates the task to DeferredComponentManager, which handles the download and loading of the dart library and any assets.

        loadingUnitId - The loadingUnitId is assigned during compile time by gen_snapshot and is automatically retrieved when loadLibrary() is called on a dart deferred library.
      • loadDartDeferredLibrary

        public void loadDartDeferredLibrary​(int loadingUnitId,
                                            String[] searchPaths)
        Searches each of the provided paths for a valid Dart shared library .so file and resolves symbols to load into the dart VM.

        Successful loading of the dart library completes the future returned by loadLibrary() that triggered the install/load process.

        loadingUnitId - The loadingUnitId is assigned during compile time by gen_snapshot and is automatically retrieved when loadLibrary() is called on a dart deferred library. This is used to identify which Dart deferred library the resolved correspond to.
        searchPaths - An array of paths in which to look for valid dart shared libraries. This supports paths within zipped apks as long as the apks are not compressed using the `path/to/apk.apk!path/inside/apk/` format. Paths will be tried first to last and ends when a library is successfully found. When the found library is invalid, no additional paths will be attempted.
      • updateJavaAssetManager

        public void updateJavaAssetManager​(@NonNull
                                           AssetManager assetManager,
                                           String assetBundlePath)
        Adds the specified AssetManager as an APKAssetResolver in the Flutter Engine's AssetManager.

        This may be used to update the engine AssetManager when a new deferred component is installed and a new Android AssetManager is created with access to new assets.

        assetManager - An android AssetManager that is able to access the newly downloaded assets.
        assetBundlePath - The subdirectory that the flutter assets are stored in. The typical value is `flutter_assets`.
      • deferredComponentInstallFailure

        public void deferredComponentInstallFailure​(int loadingUnitId,
                                                    String error,
                                                    boolean isTransient)
        Indicates that a failure was encountered during the Android portion of downloading a dynamic feature module and loading a dart deferred library, which is typically done by DeferredComponentManager.

        This will inform dart that the future returned by loadLibrary() should complete with an error.

        loadingUnitId - The loadingUnitId that corresponds to the dart deferred library that failed to install.
        error - The error message to display.
        isTransient - When isTransient is false, new attempts to install will automatically result in same error in Dart before the request is passed to Android.
      • onDisplayPlatformView

        public void onDisplayPlatformView​(int viewId,
                                          int x,
                                          int y,
                                          int width,
                                          int height,
                                          int viewWidth,
                                          int viewHeight,
                                          FlutterMutatorsStack mutatorsStack)
      • getBitmap

        public Bitmap getBitmap()
      • notifyLowMemoryWarning

        public void notifyLowMemoryWarning()
        Notifies the Dart VM of a low memory event, or that the application is in a state such that now is an appropriate time to free resources, such as going to the background.

        This is distinct from sending a SystemChannel message about low memory, which only notifies the running Flutter application.