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Flutter Android 初始化流程

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Flutter UI容器

承载Flutter的Android容器

Flutter容器包含有FlutterActivity和FlutterFragmentActivity。他们分别实现了Provider, PluginRegistry, ViewFactory，二者的方法主要通过FlutterActivityDelegate 代理出去

FlutterActivityDelegateFlutterActivityEventsProviderPluginRegistryFlutterActivityActivityViewFactoryFlutterFragmentActivityFragmentActivity 
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public class FlutterActivity extends Activity implements Provider, PluginRegistry, ViewFactory{
    private final FlutterActivityDelegate delegate = new FlutterActivityDelegate(this, this);
    private final FlutterActivityEvents eventDelegate;
    private final Provider viewProvider;
    private final PluginRegistry pluginRegistry;
    public FlutterActivity() {
        this.eventDelegate = this.delegate;
        this.viewProvider = this.delegate;
        this.pluginRegistry = this.delegate;
    }
}
public class FlutterFragmentActivity extends FragmentActivity implements Provider, PluginRegistry, ViewFactory {
    private final FlutterActivityDelegate delegate = new FlutterActivityDelegate(this, this);
    private final FlutterActivityEvents eventDelegate;
    private final Provider viewProvider;
    private final PluginRegistry pluginRegistry;

    public FlutterFragmentActivity() {
        this.eventDelegate = this.delegate;
        this.viewProvider = this.delegate;
        this.pluginRegistry = this.delegate;
    }
}

其中 Provider 负责提供FlutterView

ProviderFlutterView getFlutterView()

PluginRegistry负责插件管理

PluginRegistryPluginRegistry.Registrar registrarFor(String var1);boolean hasPlugin(String var1);<T> T valuePublishedByPlugin(String var1);

ViewFactory 负责管理FlutterView和FlutterNative

ViewFactoryFlutterView createFlutterView(Context var1);FlutterNativeView createFlutterNativeView();boolean retainFlutterNativeView();
Flutter容器代理类 FlutterActivityDelegate
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public final class FlutterActivityDelegate implements FlutterActivityEvents, Provider, PluginRegistry {
    public void onCreate(Bundle savedInstanceState) {
        // 设置全屏
        if (VERSION.SDK_INT >= 21) {
            Window window = this.activity.getWindow();
            window.addFlags(-2147483648);
            window.setStatusBarColor(1073741824);
            window.getDecorView().setSystemUiVisibility(1280);
        }
        // 获取activity入参
        String[] args = getArgsFromIntent(this.activity.getIntent());
        // 确认Flutter Engine初始化完成，详细见后面
        FlutterMain.ensureInitializationComplete(this.activity.getApplicationContext(), args);

        // 创建FlutterView
  
        // 这里实际上调用的是FlutterActivity或FlutterFragmentActivity中的createFlutterView，默认返回null
        this.flutterView = this.viewFactory.createFlutterView(this.activity); 
        if (this.flutterView == null) {
            FlutterNativeView nativeView = this.viewFactory.createFlutterNativeView();

            // 新建FlutterView
            this.flutterView = new FlutterView(this.activity, (AttributeSet)null, nativeView);
            this.flutterView.setLayoutParams(matchParent);
            // activity 的ContentView设置为flutterView
            this.activity.setContentView(this.flutterView);
            // 创建并添加launchView
            this.launchView = this.createLaunchView();
            if (this.launchView != null) {
                this.addLaunchView();
            }
        }

        // 运行Flutter appBundle，也就是加载Flutter代码
        if (!this.loadIntent(this.activity.getIntent())) {
            String appBundlePath = FlutterMain.findAppBundlePath();
            if (appBundlePath != null) {
                this.runBundle(appBundlePath);
            }

        }
    }
}
FlutterView

FlutterView 是Flutter在Native上实际的绘制视图。

FlutterViewSurfaceViewBinaryMessengerTextureRegistry

BinaryMessenger 是Flutter与Native同学的通道，后面会详细讲到
TextureRegistry 用于管理SurfaceTexture

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public class FlutterView extends SurfaceView implements BinaryMessenger, TextureRegistry {

    public FlutterView(Context context, AttributeSet attrs, FlutterNativeView nativeView) {

    
        // 创建FlutterNativeView
        if (nativeView == null) {
            this.mNativeView = new FlutterNativeView(activity.getApplicationContext());
        } else {
            this.mNativeView = nativeView;
        }

        // 设置dartExecutor
        this.dartExecutor = this.mNativeView.getDartExecutor();
        // 设置flutterRenderer
        this.flutterRenderer = new FlutterRenderer(this.mNativeView.getFlutterJNI());
    
        // 设置渲染和视图大小信息
        this.mIsSoftwareRenderingEnabled = FlutterJNI.nativeGetIsSoftwareRenderingEnabled();
        this.mMetrics = new FlutterView.ViewportMetrics();
        this.mMetrics.devicePixelRatio = context.getResources().getDisplayMetrics().density;
        this.setFocusable(true);
        this.setFocusableInTouchMode(true);

        // attachViewAndActivity
        this.mNativeView.attachViewAndActivity(this, activity);

        // 设置SurfaceCallback
        this.mSurfaceCallback = new Callback() {
            public void surfaceCreated(SurfaceHolder holder) {
                FlutterView.this.assertAttached();
                FlutterView.this.mNativeView.getFlutterJNI().onSurfaceCreated(holder.getSurface());
            }

            public void surfaceChanged(SurfaceHolder holder, int format, int width, int height) {
                FlutterView.this.assertAttached();
                FlutterView.this.mNativeView.getFlutterJNI().onSurfaceChanged(width, height);
            }

            public void surfaceDestroyed(SurfaceHolder holder) {
                FlutterView.this.assertAttached();
                FlutterView.this.mNativeView.getFlutterJNI().onSurfaceDestroyed();
            }
        };
        this.getHolder().addCallback(this.mSurfaceCallback);

        // 设置内置Channel
        this.mActivityLifecycleListeners = new ArrayList();
        this.mFirstFrameListeners = new ArrayList();
        this.navigationChannel = new NavigationChannel(this.dartExecutor);
        this.keyEventChannel = new KeyEventChannel(this.dartExecutor);
        this.lifecycleChannel = new LifecycleChannel(this.dartExecutor);
        this.localizationChannel = new LocalizationChannel(this.dartExecutor);
        this.platformChannel = new PlatformChannel(this.dartExecutor);
        this.systemChannel = new SystemChannel(this.dartExecutor);
        this.settingsChannel = new SettingsChannel(this.dartExecutor);


        final PlatformPlugin platformPlugin = new PlatformPlugin(activity, this.platformChannel);
        this.addActivityLifecycleListener(new ActivityLifecycleListener() {
            public void onPostResume() {
                platformPlugin.updateSystemUiOverlays();
            }
        });
        this.mImm = (InputMethodManager)this.getContext().getSystemService("input_method");
        PlatformViewsController platformViewsController = this.mNativeView.getPluginRegistry().getPlatformViewsController();
        this.mTextInputPlugin = new TextInputPlugin(this, this.dartExecutor, platformViewsController);
        this.androidKeyProcessor = new AndroidKeyProcessor(this.keyEventChannel, this.mTextInputPlugin);
        this.androidTouchProcessor = new AndroidTouchProcessor(this.flutterRenderer);
        this.mNativeView.getPluginRegistry().getPlatformViewsController().attachTextInputPlugin(this.mTextInputPlugin);
        this.sendLocalesToDart(this.getResources().getConfiguration());
        this.sendUserPlatformSettingsToDart();

    }
}

在FlutterActivityDelegate 中我们知道，加载dart执行的是FlutterView的runFromBundle。

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 public void runFromBundle(FlutterRunArguments args) {
    this.assertAttached();
    this.preRun();
    this.mNativeView.runFromBundle(args);
    this.postRun();
}

实际上调用的是mNativeView.runFromBundle(args)

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public class FlutterNativeView implements BinaryMessenger {
    public void runFromBundle(FlutterRunArguments args) {
        if (args.entrypoint == null) {
            throw new AssertionError("An entrypoint must be specified");
        } else {
            this.assertAttached();
            if (this.applicationIsRunning) {
                throw new AssertionError("This Flutter engine instance is already running an application");
            } else {
                this.mFlutterJNI.runBundleAndSnapshotFromLibrary(args.bundlePath, args.entrypoint, args.libraryPath, this.mContext.getResources().getAssets());
                this.applicationIsRunning = true;
            }
        }
    }
}

可以看到，最后调用的是mFlutterJNI。

FlutterNativeView与FlutterJNI
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public class FlutterNativeView implements BinaryMessenger {
     public FlutterNativeView(@NonNull Context context, boolean isBackgroundView) {
        this.mContext = context;
        // 创建FlutterPluginRegistry，插件注册器
        this.mPluginRegistry = new FlutterPluginRegistry(this, context);
        // 创建FlutterJNI
        this.mFlutterJNI = new FlutterJNI();
        // 将
        this.mFlutterJNI.setRenderSurface(new FlutterNativeView.RenderSurfaceImpl());
        // 创建DartExecutor
        this.dartExecutor = new DartExecutor(this.mFlutterJNI, context.getAssets());
        // addEngineLifecycleListener
        this.mFlutterJNI.addEngineLifecycleListener(new FlutterNativeView.EngineLifecycleListenerImpl());
        this.attach(this, isBackgroundView);
        this.assertAttached();
    }
    // 加载Flutter Bundle
    public void runFromBundle(FlutterRunArguments args) {
        if (args.entrypoint == null) {
            throw new AssertionError("An entrypoint must be specified");
        } else {
            this.assertAttached();
            if (this.applicationIsRunning) {
                throw new AssertionError("This Flutter engine instance is already running an application");
            } else {
                this.mFlutterJNI.runBundleAndSnapshotFromLibrary(args.bundlePath, args.entrypoint, args.libraryPath, this.mContext.getResources().getAssets());
                this.applicationIsRunning = true;
            }
        }
    }
}
小结
FlutterActivityFlutterActivityDelegateFlutterActivity 是主要承载页面，具体的实现由FlutterActivityDelegate代理FlutterActivityDelegate 创建FlutterView，并加载Flutter bundleFlutterViewFlutterView 继承于SurfaceView，为实际渲染的native viewFlutterView初始化时会设置UI相关的信息以及内置的Channel，FlutterView与FlutterNativeView相互绑定FlutterNativeViewFlutterNativeView继承于BinaryMessenger，并持有FlutterJNI，而FlutterJNI是Android与FlutterEngine交互的桥梁FlutterJNI大部分是jni的native方法，与Flutter Engine交互

Flutter Engine

Flutter 初始化

在使用Flutter前需要初始化Flutter

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FlutterMain.startInitialization(applicationContext);
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public class FlutterMain {
    public static void startInitialization(@NonNull Context applicationContext) {
        if (!isRunningInRobolectricTest) {
            startInitialization(applicationContext, new FlutterMain.Settings());
        }
    }
    public static void startInitialization(@NonNull Context applicationContext, @NonNull FlutterMain.Settings settings) {
        if (!isRunningInRobolectricTest) {
            // 初始化必须在主线程
            if (Looper.myLooper() != Looper.getMainLooper()) {
                throw new IllegalStateException("startInitialization must be called on the main thread");
            } else if (sSettings == null) {
                sSettings = settings;
                long initStartTimestampMillis = SystemClock.uptimeMillis();
                // 初始化配置，主要是读取metadata中自定义的libapp.so、flutter_assets、vm_snapshot_data和isolate_snapshot_data对于的路径
                // 一般可以不是自行设置
                initConfig(applicationContext);
                // 初始化Flutter资源，从flutter_assets读取并加载资源
                // 包含vm_snapshot_data，isolate_snapshot_data、kernel_blob.bin
                initResources(applicationContext);
                // 记载flutter.so
                System.loadLibrary("flutter");
                // 初始化Vsync工具
                VsyncWaiter.getInstance((WindowManager)applicationContext.getSystemService("window")).init();
                // 记录初始化耗时
                long initTimeMillis = SystemClock.uptimeMillis() - initStartTimestampMillis;
                FlutterJNI.nativeRecordStartTimestamp(initTimeMillis);
            }
        }
    }
}

android assets目录下flutter_assets

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assets
└── flutter_assets
    ├── AssetManifest.json
    ├── FontManifest.json
    ├── LICENSE
    ├── fonts
    │   └── MaterialIcons-Regular.ttf
    ├── isolate_snapshot_data
    ├── kernel_blob.bin
    ├── packages
    │   └── cupertino_icons
    │       └── assets
    │           └── CupertinoIcons.ttf
    └── vm_snapshot_data

在FlutterActivityDelegate中，onCreate时，

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public final class FlutterActivityDelegate{
     public void onCreate(Bundle savedInstanceState) {
         FlutterMain.ensureInitializationComplete(this.activity.getApplicationContext(), args);
     }
}
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public class FlutterMain {
    public static void ensureInitializationComplete(@NonNull Context applicationContext, @Nullable String[] args) {
        if (!isRunningInRobolectricTest) {
            // 确认在主线程，且sSettings不为空，sSettings在startInitialization中设置
            if (Looper.myLooper() != Looper.getMainLooper()) {
                throw new IllegalStateException("ensureInitializationComplete must be called on the main thread");
            } else if (sSettings == null) {
                throw new IllegalStateException("ensureInitializationComplete must be called after startInitialization");
            } else if (!sInitialized) {
                try {

                    // 等待资源加载完成，在startInitialization中设置
                    if (sResourceExtractor != null) {
                        sResourceExtractor.waitForCompletion();
                    }

                    // 设置flutter参数
                    // icu-symbol-prefix, icu-native-lib-path,snapshot-asset-path,
                    // vm-snapshot-data,isolate-snapshot-data,cache-dir-path,log-tag
                    List<String> shellArgs = new ArrayList();
                    shellArgs.add("--icu-symbol-prefix=_binary_icudtl_dat");
                    ApplicationInfo applicationInfo = getApplicationInfo(applicationContext);
                    shellArgs.add("--icu-native-lib-path=" + applicationInfo.nativeLibraryDir + File.separator + "libflutter.so");
                    if (args != null) {
                        Collections.addAll(shellArgs, args);
                    }

                    String kernelPath = null;
                    String appStoragePath = PathUtils.getDataDirectory(applicationContext) + File.separator + sFlutterAssetsDir;
                    kernelPath = appStoragePath + File.separator + "kernel_blob.bin";
                    shellArgs.add("--snapshot-asset-path=" + appStoragePath);
                    shellArgs.add("--vm-snapshot-data=" + sVmSnapshotData);
                    shellArgs.add("--isolate-snapshot-data=" + sIsolateSnapshotData);
                    shellArgs.add("--cache-dir-path=" + PathUtils.getCacheDirectory(applicationContext));
                    if (sSettings.getLogTag() != null) {
                        shellArgs.add("--log-tag=" + sSettings.getLogTag());
                    }

                    appStoragePath = PathUtils.getFilesDir(applicationContext);
                    String engineCachesPath = PathUtils.getCacheDirectory(applicationContext);

                    // FlutterJNI 初始化Flutter Engine
                    FlutterJNI.nativeInit(applicationContext, (String[])shellArgs.toArray(new String[0]), kernelPath, appStoragePath, engineCachesPath);
                    sInitialized = true;
                } catch (Exception var7) {
                    Log.e("FlutterMain", "Flutter initialization failed.", var7);
                    throw new RuntimeException(var7);
                }
            }
        }
    }
}

可以看到，所有与Flutter Engine相关的操作都是用过FlutterJNI来交互的

小结

Flutter Engine初始化主要是加载flutter.so，设置flutter_assets，并通过FlutterJNI设置到Flutter Engine中。

关于FlutterJNI相关的内容，再后面关于Flutter Engine定制中会详细介绍到。