Flutter初始化分析（二）

接着上篇分析，这篇从FlutterActivity创建开始切入。

public class MainActivity extends FlutterActivity {


    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        GeneratedPluginRegistrant.registerWith(this);
    }

}

首先可以看出，默认创建的MainActivity 没有我们常见的setContentView()，那应该是在FlutterActivity里面设置的。

public class FlutterActivity extends Activity implements Provider, PluginRegistry, ViewFactory {
    private static final String TAG = "FlutterActivity";
    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 FlutterView getFlutterView() {
        return this.viewProvider.getFlutterView();
    }

    public FlutterView createFlutterView(Context context) {
        return null;
    }

    public FlutterNativeView createFlutterNativeView() {
        return null;
    }

    public boolean retainFlutterNativeView() {
        return false;
    }

    public final boolean hasPlugin(String key) {
        return this.pluginRegistry.hasPlugin(key);
    }

    public final <T> T valuePublishedByPlugin(String pluginKey) {
        return this.pluginRegistry.valuePublishedByPlugin(pluginKey);
    }

    public final Registrar registrarFor(String pluginKey) {
        return this.pluginRegistry.registrarFor(pluginKey);
    }

    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        this.eventDelegate.onCreate(savedInstanceState);
    }

    protected void onStart() {
        super.onStart();
        this.eventDelegate.onStart();
    }

    protected void onResume() {
        super.onResume();
        this.eventDelegate.onResume();
    }

    protected void onDestroy() {
        this.eventDelegate.onDestroy();
        super.onDestroy();
    }

    public void onBackPressed() {
        if(!this.eventDelegate.onBackPressed()) {
            super.onBackPressed();
        }

    }

    protected void onStop() {
        this.eventDelegate.onStop();
        super.onStop();
    }

    protected void onPause() {
        super.onPause();
        this.eventDelegate.onPause();
    }

    protected void onPostResume() {
        super.onPostResume();
        this.eventDelegate.onPostResume();
    }

    public void onRequestPermissionsResult(int requestCode, @NonNull String[] permissions, @NonNull int[] grantResults) {
        this.eventDelegate.onRequestPermissionsResult(requestCode, permissions, grantResults);
    }

    protected void onActivityResult(int requestCode, int resultCode, Intent data) {
        if(!this.eventDelegate.onActivityResult(requestCode, resultCode, data)) {
            super.onActivityResult(requestCode, resultCode, data);
        }

    }

    protected void onNewIntent(Intent intent) {
        this.eventDelegate.onNewIntent(intent);
    }

    public void onUserLeaveHint() {
        this.eventDelegate.onUserLeaveHint();
    }

    public void onTrimMemory(int level) {
        this.eventDelegate.onTrimMemory(level);
    }

    public void onLowMemory() {
        this.eventDelegate.onLowMemory();
    }

    public void onConfigurationChanged(Configuration newConfig) {
        super.onConfigurationChanged(newConfig);
        this.eventDelegate.onConfigurationChanged(newConfig);
    }
}

可以看出FlutterActivity也没有setContentView()，并且创建FlutterView也是为null，那么核心逻辑应该在FlutterActivityDelegate。

FlutterActivityDelegate.class

public void onCreate(Bundle savedInstanceState) {
        if(VERSION.SDK_INT >= 21) {
            Window window = this.activity.getWindow();
            window.addFlags(-2147483648);
            window.setStatusBarColor(1073741824);
            window.getDecorView().setSystemUiVisibility(1280);
        }

        String[] args = getArgsFromIntent(this.activity.getIntent());
        FlutterMain.ensureInitializationComplete(this.activity.getApplicationContext(), args);
        this.flutterView = this.viewFactory.createFlutterView(this.activity);
        if(this.flutterView == null) {
            FlutterNativeView nativeView = this.viewFactory.createFlutterNativeView();
            this.flutterView = new FlutterView(this.activity, (AttributeSet)null, nativeView);
            this.flutterView.setLayoutParams(matchParent);
            this.activity.setContentView(this.flutterView);
            this.launchView = this.createLaunchView();
            if(this.launchView != null) {
                this.addLaunchView();
            }
        }

        if(!this.loadIntent(this.activity.getIntent())) {
            String appBundlePath = FlutterMain.findAppBundlePath(this.activity.getApplicationContext());
            if(appBundlePath != null) {
                this.runBundle(appBundlePath);
            }

        }
    }

1. 有个关键的方法FlutterMain.ensureInitializationComplete()，下面分析。
2. 接着看创建FlutterView。this.viewFactory.createFlutterNativeView()最终来到FlutterActivity.createFlutterNativeView()，结果返回的为null，所以FlutterNativeView的创建是在FlutterView中，下面分析。
3. 创建了flutterView后，用了setContentView(this.flutterView)，所以整个activity的布局就是一个surfaceView
4. 运行的方法this.runBundle(appBundlePath);，最后分析。

FlutterMain.ensureInitializationComplete()

FlutterMain.class

    public static void ensureInitializationComplete(Context applicationContext, String[] args) {
        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 {
                sResourceExtractor.waitForCompletion();
                List<String> shellArgs = new ArrayList();
                shellArgs.add("--icu-symbol-prefix=_binary_icudtl_dat");
                if(args != null) {
                    Collections.addAll(shellArgs, args);
                }

                if(sIsPrecompiledAsSharedLibrary) {
                    shellArgs.add("--aot-shared-library-path=" + new File(PathUtils.getDataDirectory(applicationContext), sAotSharedLibraryPath));
                } else {
                    if(sIsPrecompiledAsBlobs) {
                        shellArgs.add("--aot-snapshot-path=" + PathUtils.getDataDirectory(applicationContext));
                    } else {
                        shellArgs.add("--cache-dir-path=" + PathUtils.getCacheDirectory(applicationContext));
                        shellArgs.add("--aot-snapshot-path=" + PathUtils.getDataDirectory(applicationContext) + "/" + sFlutterAssetsDir);
                    }

                    shellArgs.add("--vm-snapshot-data=" + sAotVmSnapshotData);
                    shellArgs.add("--vm-snapshot-instr=" + sAotVmSnapshotInstr);
                    shellArgs.add("--isolate-snapshot-data=" + sAotIsolateSnapshotData);
                    shellArgs.add("--isolate-snapshot-instr=" + sAotIsolateSnapshotInstr);
                }

                if(sSettings.getLogTag() != null) {
                    shellArgs.add("--log-tag=" + sSettings.getLogTag());
                }

                String appBundlePath = findAppBundlePath(applicationContext);
                String appStoragePath = PathUtils.getFilesDir(applicationContext);
                String engineCachesPath = PathUtils.getCacheDirectory(applicationContext);
                nativeInit(applicationContext, (String[])shellArgs.toArray(new String[0]), appBundlePath, appStoragePath, engineCachesPath);
                sInitialized = true;
            } catch (Exception var6) {
                Log.e("FlutterMain", "Flutter initialization failed.", var6);
                throw new RuntimeException(var6);
            }
        }
    }

方法作用在创建FlutterView之前先确保FlutterMain初始化完成，就是FlutterMain.startInitialization()要完成，关于此方法分析见上文。
关键的检查方法是sResourceExtractor.waitForCompletion();，在资源初始化完成后进行nativeInit，此时调用了C++层的方法init，开始初始化。

FlutterView

接着if(this.flutterView == null) 的判断，这里需要创建FlutterView。
FlutterView不是直接继承View，而是SurfaceView，相关知识可以自行搜索。
android SurfaceView 详解

FlutterView.class

    public FlutterView(Context context, AttributeSet attrs, FlutterNativeView nativeView) {
        super(context, attrs);
        this.nextTextureId = new AtomicLong(0L);
        this.mIsSoftwareRenderingEnabled = false;
        this.mAccessibilityEnabled = false;
        this.mTouchExplorationEnabled = false;
        this.mAccessibilityFeatureFlags = 0;
        Activity activity = (Activity)this.getContext();
        if(nativeView == null) {
            this.mNativeView = new FlutterNativeView(activity.getApplicationContext());
        } else {
            this.mNativeView = nativeView;
        }

        this.dartExecutor = new DartExecutor(this.mNativeView.getFlutterJNI());
        this.mNativeView.getFlutterJNI();
        this.mIsSoftwareRenderingEnabled = FlutterJNI.nativeGetIsSoftwareRenderingEnabled();
        this.mAnimationScaleObserver = new FlutterView.AnimationScaleObserver(new Handler());
        this.mMetrics = new FlutterView.ViewportMetrics();
        this.mMetrics.devicePixelRatio = context.getResources().getDisplayMetrics().density;
        this.setFocusable(true);
        this.setFocusableInTouchMode(true);
        this.mNativeView.attachViewAndActivity(this, activity);
        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);
        this.mAccessibilityManager = (AccessibilityManager)this.getContext().getSystemService("accessibility");
        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.systemChannel = new SystemChannel(this.dartExecutor);
        this.settingsChannel = new SettingsChannel(this.dartExecutor);
        this.mFlutterLocalizationChannel = new MethodChannel(this, "flutter/localization", JSONMethodCodec.INSTANCE);
        PlatformPlugin platformPlugin = new PlatformPlugin(activity);
        MethodChannel flutterPlatformChannel = new MethodChannel(this, "flutter/platform", JSONMethodCodec.INSTANCE);
        flutterPlatformChannel.setMethodCallHandler(platformPlugin);
        this.addActivityLifecycleListener(platformPlugin);
        this.mImm = (InputMethodManager)this.getContext().getSystemService("input_method");
        this.mTextInputPlugin = new TextInputPlugin(this);
        this.androidKeyProcessor = new AndroidKeyProcessor(this.keyEventChannel);
        this.setLocales(this.getResources().getConfiguration());
        this.sendUserPlatformSettingsToDart();
    }

在onCreate()比较关键的有

1. 创建FlutterNativeView
2. 初始化DartExecutor，JNI的创建在下面FlutterNativeView提到。
3. 设置surfaceView的回调，通过FlutterJNI使得与Java层和dart层的状态同步。
4. 创建系统的native插件，关于插件的分析会另起一文。

FlutterNativeView

FlutterNativeView不是View，它是与C++层进行消息传递的媒介，实现了BinaryMessenger，二进制消息传递方法。

public interface BinaryMessenger {
    void send(String var1, ByteBuffer var2);

    void send(String var1, ByteBuffer var2, BinaryMessenger.BinaryReply var3);

    void setMessageHandler(String var1, BinaryMessenger.BinaryMessageHandler var2);

    public interface BinaryReply {
        void reply(ByteBuffer var1);
    }

    public interface BinaryMessageHandler {
        void onMessage(ByteBuffer var1, BinaryMessenger.BinaryReply var2);
    }
}

由类源码可知，定义消息发送、处理消息、设置消息处理器、响应消息的接口或者方法。在FlutterNativeView实现。

FlutterNativeView.class

 public FlutterNativeView(Context context) {
        this(context, false);
    }

    public FlutterNativeView(Context context, boolean isBackgroundView) {
        this.mNextReplyId = 1;
        this.mPendingReplies = new HashMap();
        this.mContext = context;
        this.mPluginRegistry = new FlutterPluginRegistry(this, context);
        this.mFlutterJNI = new FlutterJNI();
        this.mFlutterJNI.setRenderSurface(new FlutterNativeView.RenderSurfaceImpl());
        this.mFlutterJNI.setPlatformMessageHandler(new FlutterNativeView.PlatformMessageHandlerImpl());
        this.mFlutterJNI.addEngineLifecycleListener(new FlutterNativeView.EngineLifecycleListenerImpl());
        this.attach(this, isBackgroundView);
        this.assertAttached();
        this.mMessageHandlers = new HashMap();
    }

可知

1. 创建了插件的注册者
2. 创建了FlutterJNI用于与C++层进行交互
3. 关键方法attach(this, isBackgroundView);

FlutterNativeView.attach()

FlutterNativeView.class

    private void attach(FlutterNativeView view, boolean isBackgroundView) {
        this.mFlutterJNI.attachToNative(isBackgroundView);
    }

FlutterJNI.class

    @UiThread
    public void attachToNative(boolean isBackgroundView) {
        this.ensureNotAttachedToNative();
        this.nativePlatformViewId = Long.valueOf(this.nativeAttach(this, isBackgroundView));
    }

    private native long nativeAttach(FlutterJNI var1, boolean var2);

最终会调用C++层的AttachJNI()

static jlong AttachJNI(JNIEnv* env,
                       jclass clazz,
                       jobject flutterJNI,
                       jboolean is_background_view) {
  fml::jni::JavaObjectWeakGlobalRef java_object(env, flutterJNI);
  auto shell_holder = std::make_unique<AndroidShellHolder>(
      FlutterMain::Get().GetSettings(), java_object, is_background_view);
  if (shell_holder->IsValid()) {
    return reinterpret_cast<jlong>(shell_holder.release());
  } else {
    return 0;
  }
}

attach成功后会返回nativePlatformViewId，用于标志attach过，供一些方法可用于检查。

    private void ensureNotAttachedToNative() {
        if(this.nativePlatformViewId != null) {
            throw new RuntimeException("Cannot execute operation because FlutterJNI is attached to native.");
        }
    }

    private void ensureAttachedToNative() {
        if(this.nativePlatformViewId == null) {
            throw new RuntimeException("Cannot execute operation because FlutterJNI is not attached to native.");
        }
    }

至此，Java层初始化核心逻辑大致说完，还剩下一个运行的方法。

FlutterActivityDelegate.runBundle(appBundlePath)

FlutterActivityDelegate.class

    public void onCreate(Bundle savedInstanceState) {
        if(VERSION.SDK_INT >= 21) {
            Window window = this.activity.getWindow();
            window.addFlags(-2147483648);
            window.setStatusBarColor(1073741824);
            window.getDecorView().setSystemUiVisibility(1280);
        }

        String[] args = getArgsFromIntent(this.activity.getIntent());
        FlutterMain.ensureInitializationComplete(this.activity.getApplicationContext(), args);
        this.flutterView = this.viewFactory.createFlutterView(this.activity);
        if(this.flutterView == null) {
            FlutterNativeView nativeView = this.viewFactory.createFlutterNativeView();
            this.flutterView = new FlutterView(this.activity, (AttributeSet)null, nativeView);
            this.flutterView.setLayoutParams(matchParent);
            this.activity.setContentView(this.flutterView);
            this.launchView = this.createLaunchView();
            if(this.launchView != null) {
                this.addLaunchView();
            }
        }

        if(!this.loadIntent(this.activity.getIntent())) {
            String appBundlePath = FlutterMain.findAppBundlePath(this.activity.getApplicationContext());
            if(appBundlePath != null) {
                this.runBundle(appBundlePath);
            }

        }
    }

FlutterActivityDelegate.class

private void runBundle(String appBundlePath) {
        if(!this.flutterView.getFlutterNativeView().isApplicationRunning()) {
            FlutterRunArguments args = new FlutterRunArguments();
            ArrayList<String> bundlePaths = new ArrayList();
            ResourceUpdater resourceUpdater = FlutterMain.getResourceUpdater();
            if(resourceUpdater != null) {
                File patchFile = resourceUpdater.getInstalledPatch();
                JSONObject manifest = resourceUpdater.readManifest(patchFile);
                if(resourceUpdater.validateManifest(manifest)) {
                    bundlePaths.add(patchFile.getPath());
                }
            }

            bundlePaths.add(appBundlePath);
            args.bundlePaths = (String[])bundlePaths.toArray(new String[0]);
            args.entrypoint = "main";
            this.flutterView.runFromBundle(args);
        }

    }

FlutterView.class

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

FlutterNativeView.class

    public void runFromBundle(FlutterRunArguments args) {
        boolean hasBundlePaths = args.bundlePaths != null && args.bundlePaths.length != 0;
        if(args.bundlePath == null && !hasBundlePaths) {
            throw new AssertionError("Either bundlePath or bundlePaths must be specified");
        } else if((args.bundlePath != null || args.defaultPath != null) && hasBundlePaths) {
            throw new AssertionError("Can't specify both bundlePath and bundlePaths");
        } else if(args.entrypoint == null) {
            throw new AssertionError("An entrypoint must be specified");
        } else {
            if(hasBundlePaths) {
                this.runFromBundleInternal(args.bundlePaths, args.entrypoint, args.libraryPath);
            } else {
                this.runFromBundleInternal(new String[]{args.bundlePath, args.defaultPath}, args.entrypoint, args.libraryPath);
            }

        }
    }

FlutterNativeView.class

    private void runFromBundleInternal(String[] bundlePaths, String entrypoint, String libraryPath) {
        this.assertAttached();
        if(this.applicationIsRunning) {
            throw new AssertionError("This Flutter engine instance is already running an application");
        } else {
            this.mFlutterJNI.runBundleAndSnapshotFromLibrary(bundlePaths, entrypoint, libraryPath, this.mContext.getResources().getAssets());
            this.applicationIsRunning = true;
        }
    }

最后还是调用FlutterJNI.runBundleAndSnapshotFromLibrary()，并设置启动标志为true。

FlutterJNI.class

    @UiThread
    public void runBundleAndSnapshotFromLibrary(@NonNull String[] prioritizedBundlePaths, @Nullable String entrypointFunctionName, @Nullable String pathToEntrypointFunction, @NonNull AssetManager assetManager) {
        this.ensureAttachedToNative();
        this.nativeRunBundleAndSnapshotFromLibrary(this.nativePlatformViewId.longValue(), prioritizedBundlePaths, entrypointFunctionName, pathToEntrypointFunction, assetManager);
    }

    private native void nativeRunBundleAndSnapshotFromLibrary(long var1, @NonNull String[] var3, @Nullable String var4, @Nullable String var5, @NonNull AssetManager var6);

platform_view_android_jni.cc

static void RunBundleAndSnapshotFromLibrary(JNIEnv* env,
                                            jobject jcaller,
                                            jlong shell_holder,
                                            jobjectArray jbundlepaths,
                                            jstring jEntrypoint,
                                            jstring jLibraryUrl,
                                            jobject jAssetManager) {
  auto asset_manager = std::make_shared<blink::AssetManager>();
  for (const auto& bundlepath :
       fml::jni::StringArrayToVector(env, jbundlepaths)) {
    if (bundlepath.empty()) {
      continue;
    }

    // If we got a bundle path, attempt to use that as a directory asset
    // bundle or a zip asset bundle.
    const auto file_ext_index = bundlepath.rfind(".");
    if (bundlepath.substr(file_ext_index) == ".zip") {
      asset_manager->PushBack(std::make_unique<blink::ZipAssetStore>(
          bundlepath, "assets/flutter_assets"));

    } else {
      asset_manager->PushBack(
          std::make_unique<blink::DirectoryAssetBundle>(fml::OpenDirectory(
              bundlepath.c_str(), false, fml::FilePermission::kRead)));

      // Use the last path component of the bundle path to determine the
      // directory in the APK assets.
      const auto last_slash_index = bundlepath.rfind("/", bundlepath.size());
      if (last_slash_index != std::string::npos) {
        auto apk_asset_dir = bundlepath.substr(
            last_slash_index + 1, bundlepath.size() - last_slash_index);

        asset_manager->PushBack(std::make_unique<blink::APKAssetProvider>(
            env,                       // jni environment
            jAssetManager,             // asset manager
            std::move(apk_asset_dir))  // apk asset dir
        );
      }
    }
  }

  auto isolate_configuration = CreateIsolateConfiguration(*asset_manager);
  if (!isolate_configuration) {
    FML_DLOG(ERROR)
        << "Isolate configuration could not be determined for engine launch.";
    return;
  }

  RunConfiguration config(std::move(isolate_configuration),
                          std::move(asset_manager));

  {
    auto entrypoint = fml::jni::JavaStringToString(env, jEntrypoint);
    auto libraryUrl = fml::jni::JavaStringToString(env, jLibraryUrl);

    if ((entrypoint.size() > 0) && (libraryUrl.size() > 0)) {
      config.SetEntrypointAndLibrary(std::move(entrypoint),
                                     std::move(libraryUrl));
    } else if (entrypoint.size() > 0) {
      config.SetEntrypoint(std::move(entrypoint));
    }
  }

  ANDROID_SHELL_HOLDER->Launch(std::move(config));
}

可见最后走到C++层的Launch()，后面开始启动的逻辑，这里不再展开了。

总结

1. 在FlutterApplication中调用FlutterMain.startInitialization()开始初始化，初始化一些配置和加载so库，在C++层动态注册供Java使用的方法，同时也获取Java的类和方法。
2. FlutterActivity创建FlutterActivityDelegate用与处理所有逻辑。
3. FlutterNativeView创建FlutterJNI进行attach C++层、创建插件注册者进行插件消息的管理等。
4. FlutterView创建FlutterNativeView、创建PlatformPlugin管理平台插件等。
5. FlutterActivityDelegate.onCreate()中进行C++层的init、创建FlutterView、setContentView()、最后调用runBundle()开始启动App。