前面给大家分析过 ReactNative For Android (RN4A) 的通信机制，这次我们从源码出发，分析下RN4A的启动过程。启动过程基于通信机制，涉及通信机制原理大家可以查看前一篇文章，本篇不赘述。

上面是2016 React.js Conf FB 工程师分享的RN启动时序图，整个过程比较清晰，先启动终端运行时，随后由终端上下文去启动JS的运行时，进而布局，最后再由终端进行渲染，最后将View添加到RootView上。那接下来，我们先理解几个概念，方便后续我们对整个启动过程的理解。

模块：

模块即暴露给调用方的API集合，在RN4A存在两种模块。

一种是Native层暴露给Js层的API集合模块，即NativeModule，如ToastModule,DialogModule,或是创建View的UIManagerModule。业务方可以通过实现NativeModule自定义模块，通过重写getName将模块名暴露给Js层，通过注解的方式将API暴露给Js层调用。

另一种是Js层暴露给Java层的API集合模块，即JavascriptModule，如DeviceEventEmitter,AppRegistry等。业务方可以通过继承JavaScriptModule接口自定义接口模块，申明与Js层相应的方法即可。

无论是NativeModule还是JavascriptModule，在Js层存在与之相互映射同名的Module，Js层通过require引用Module。

模块注册表：

各模块信息统一收集到模块注册表。同样，在RN4A中存在两种模块注册表，一是由集合所有Java层模块接口信息的NativeModuleRegistry，另一种是集合所有Js层模块接口信息的JavascriptModuleRegistry。在启动RN4A后，终端将注册表信息存入与前端互通的全局变量 __fbBatchedBridgeConfig 中，使得Js层与Java层存在同样的模块注册表。

正如上面FB攻城狮提出的时序图，从终端启动，入口是ReactRootView.startReactApplication，在构造JavaScriptExecutor&JSBundleLoader后，进而通过ReactContextInitAsycnTask去创建ReactContext，这部分主要创建了NativeModules,JavaScriptModule及其对的注册表，负责Js与Java通信的高层接口CatalystInstance等。在创建完ReactContext后，通过CatalystInstance获取AppRegistry并调用其runApplication启动Js Application。整体流程如下：

接下来进入正题，从源码来分析RN4A的启动（为阅读方便，源码适当裁剪）

ReactInstanceManager createReactContextInBackground，通过AysncTask初始化ReactNative上下文。mJSModuleName是与前端约定好所要启动的JS Application Name。mLauncahOptions是终端启动前端Application可选的传入的参数。

/**
 * ReactRootView.java
 */
public void startReactApplication(
    ReactInstanceManager reactInstanceManager,
    String moduleName,
    @Nullable Bundle launchOptions) {
  UiThreadUtil.assertOnUiThread();

  mReactInstanceManager = reactInstanceManager;
  mJSModuleName = moduleName;
  mLaunchOptions = launchOptions;

  if (!mReactInstanceManager.hasStartedCreatingInitialContext()) {
    mReactInstanceManager.createReactContextInBackground();
  }

  if (mWasMeasured && mIsAttachedToWindow) {
    mReactInstanceManager.attachMeasuredRootView(this);
    mIsAttachedToInstance = true;
    getViewTreeObserver().addOnGlobalLayoutListener(getKeyboardListener());
  } else {
    mAttachScheduled = true;
  }
}

createReactContextInBackground最终调用到recreateReactContextInBackgroundFromBundleFile。这里会创建两个Key Obj : JSCJavaScriptExecutor&JSBundleLoader。

JSCJavaScriptExecutor继承自JavaScriptExecutor，在JSCJavaScriptExecutor.class加载会加载ReactNative的SO，并且，在初始JSCJavaScriptExecutor时会调用initialze去初始C++层ReactNative与JSC的通信框架等。

JSBundleLoader缓存了JsBundle的信息，封装了上层加载JsBundle相关接口，CatalystInstance通过其间接调用ReactBridge去加载文件。

 /**
 * ReactInstanceManagerImpl.java
 */
private void recreateReactContextInBackgroundFromBundleFile() {
  recreateReactContextInBackground(
      new JSCJavaScriptExecutor.Factory(),
      JSBundleLoader.createFileLoader(mApplicationContext, mJSBundleFile));
}

创建完JSCJavaScriptExecutor&JSBundleLoader后，execute ReactContextInitAsyncTask继续初始化ReactContext。

/**
 * ReactInstanceManagerImpl.java
 */
private void recreateReactContextInBackground(
    JavaScriptExecutor.Factory jsExecutorFactory,
    JSBundleLoader jsBundleLoader) {

  ReactContextInitParams initParams =
      new ReactContextInitParams(jsExecutorFactory, jsBundleLoader);
  if (!mIsContextInitAsyncTaskRunning) {

    ReactContextInitAsyncTask initTask = new ReactContextInitAsyncTask();
    initTask.execute(initParams);
    mIsContextInitAsyncTaskRunning = true;
  } else {
    mPendingReactContextInitParams = initParams;
  }
}

ReactContextInitAsyncTask为创建ReactContext的核心类，在执行初始化前会销毁先前的上下文，保证只存在一个上下文。随后，调用createReactContext进一步创建ReactContext。在创建完React Context后会调用setUpReactContext，进而通知DevSupportManager更新上下文，更新生命周期，将ReactRootView做为Root View传递给UIManagerModule，调用AppRegistry的runApplication去启动Js Application等。

/**
 * ReactInstanceManagerImpl$ReactContextInitAsynTask.java
 */
private final class ReactContextInitAsyncTask extends
    AsyncTask> {
  @Override
  protected void onPreExecute() {
    if (mCurrentReactContext != null) {
      tearDownReactContext(mCurrentReactContext);
      mCurrentReactContext = null;
    }
  }

  @Override
  protected Result doInBackground(ReactContextInitParams... params) {
    Assertions.assertCondition(params != null && params.length > 0 && params[0] != null);
    try {
      JavaScriptExecutor jsExecutor = params[0].getJsExecutorFactory().create();
      return Result.of(createReactContext(jsExecutor, params[0].getJsBundleLoader()));
    } catch (Exception e) {
      // Pass exception to onPostExecute() so it can be handled on the main thread
      return Result.of(e);
    }
  }

  @Override
  protected void onPostExecute(Result result) {
    try {
      setupReactContext(result.get());
    } catch (Exception e) {
      mDevSupportManager.handleException(e);
    } finally {
      mIsContextInitAsyncTaskRunning = false;
    }

    // Handle enqueued request to re-initialize react context.
    if (mPendingReactContextInitParams != null) {
      recreateReactContextInBackground(
          mPendingReactContextInitParams.getJsExecutorFactory(),
          mPendingReactContextInitParams.getJsBundleLoader());
      mPendingReactContextInitParams = null;
    }
  }
}

在CreateReactContext中，主要有以下5个key path:

1.通过Builder构建上文概念讲过的NativeModuleRegistry及JavaScriptModuleConfig；

2.创建ReactApplicationContext。ReactApplicationContext继承自ContextWrapper，主要缓存了Application Context，Activity Context，ReactNative处理消息的三个thread(下篇讲述)，还有就是全局控制JS调用导致Native Module Crash的NativeModuleCallExceptionHandler，在初始化ReactInstanceManager的时候传入，并且要关闭DeveloperSupport后才可以启用，假如不传，则默认交由DevSupportManger去处理；

3.创建ReactPackage。ReactPackage主要通过createNativeModules、createJSModules和createViewManagers等API去创建本地模块，JS模块及视图组件等。ReactPackage分为framework的CoreModulesPackage和业务方可选的基础MainReactPackage，CoreModulesPackage封装了大部分通信，调试核心类，如UIManagerModule，这个负责控制Js层Dom到Native View的核心类；

4.创建CatalystInstance。CatalystInstance并不直接面向开发者，开发者通ReactInstanceManger间接操作CatalystInstance。CatalystInstance持有对ReactBridge的引用，主要通过ReactBridge这个JNI类去实现Java层与Js层的通信，ReactBridge由CatalystInstance的Constructor创建。同时初始化的时候调用了ReactQueueConfigurationSpec.createDefault创建了ReactNative通信的两个线程 JsQueueThread&NativeModulesQueueThread；

5.调用reactContext.initializeWithInstance进一步将创建完的CatalystInstance及线程等缓存在ReactContext中；

6.调用catalystInstance.runJSBundle加载解析Jsbundle；

/**
 * ReactInstanceManagerImpl.java
 *
 * @return instance of {@link ReactContext} configured a {@link CatalystInstance} set
 */
private ReactApplicationContext createReactContext(
    JavaScriptExecutor jsExecutor,
    JSBundleLoader jsBundleLoader) {
  
  mSourceUrl = jsBundleLoader.getSourceUrl();
  NativeModuleRegistry.Builder nativeRegistryBuilder = new NativeModuleRegistry.Builder();
  JavaScriptModulesConfig.Builder jsModulesBuilder = new JavaScriptModulesConfig.Builder();

  ReactApplicationContext reactContext = new ReactApplicationContext(mApplicationContext);
  if (mUseDeveloperSupport) {
    reactContext.setNativeModuleCallExceptionHandler(mDevSupportManager);
  }
  
  CoreModulesPackage coreModulesPackage =
        new CoreModulesPackage(this, mBackBtnHandler, mUIImplementationProvider);
  processPackage(coreModulesPackage, reactContext, nativeRegistryBuilder, jsModulesBuilder);
  
  for (ReactPackage reactPackage : mPackages) {
      processPackage(reactPackage, reactContext, nativeRegistryBuilder, jsModulesBuilder);
  }
  
  nativeModuleRegistry = nativeRegistryBuilder.build();
  javaScriptModulesConfig = jsModulesBuilder.build();

  NativeModuleCallExceptionHandler exceptionHandler = mNativeModuleCallExceptionHandler != null
      ? mNativeModuleCallExceptionHandler
      : mDevSupportManager;
  CatalystInstanceImpl.Builder catalystInstanceBuilder = new CatalystInstanceImpl.Builder()
      .setReactQueueConfigurationSpec(ReactQueueConfigurationSpec.createDefault())
      .setJSExecutor(jsExecutor)
      .setRegistry(nativeModuleRegistry)
      .setJSModulesConfig(javaScriptModulesConfig)
      .setJSBundleLoader(jsBundleLoader)
      .setNativeModuleCallExceptionHandler(exceptionHandler);
  
  CatalystInstance catalystInstance= catalystInstanceBuilder.build();
  if (mBridgeIdleDebugListener != null) {
    catalystInstance.addBridgeIdleDebugListener(mBridgeIdleDebugListener);
  }
  reactContext.initializeWithInstance(catalystInstance);
  catalystInstance.runJSBundle();
  
  return reactContext;
}

ReactBridge由CatalystInstance的Constructor创建。

/**
 *  CatalystInstanceImpl.java
 */
private CatalystInstanceImpl(
    final ReactQueueConfigurationSpec ReactQueueConfigurationSpec,
    final JavaScriptExecutor jsExecutor,
    final NativeModuleRegistry registry,
    final JavaScriptModulesConfig jsModulesConfig,
    final JSBundleLoader jsBundleLoader,
    NativeModuleCallExceptionHandler nativeModuleCallExceptionHandler) {
  mReactQueueConfiguration = ReactQueueConfigurationImpl.create(
      ReactQueueConfigurationSpec,
      new NativeExceptionHandler());
  mBridgeIdleListeners = new CopyOnWriteArrayList<>();
  mJavaRegistry = registry;
  mJSModuleRegistry = new JavaScriptModuleRegistry(CatalystInstanceImpl.this, jsModulesConfig);
  mJSBundleLoader = jsBundleLoader;
  mNativeModuleCallExceptionHandler = nativeModuleCallExceptionHandler;
  mTraceListener = new JSProfilerTraceListener();

  try {
    mBridge = mReactQueueConfiguration.getJSQueueThread().callOnQueue(
        new Callable() {
          @Override
          public ReactBridge call() throws Exception {
            Systrace.beginSection(Systrace.TRACE_TAG_REACT_JAVA_BRIDGE, "initializeBridge");
            try {
              return initializeBridge(jsExecutor, jsModulesConfig);
            } finally {
              Systrace.endSection(Systrace.TRACE_TAG_REACT_JAVA_BRIDGE);
            }
          }
        }).get();
  } catch (Exception t) {
    throw new RuntimeException("Failed to initialize bridge", t);
  }
}

ReactBridge将注册表信息存入与前端互通的全局变量 __fbBatchedBridgeConfig 中，使得Js层与Java层存在同样的模块注册表。

/**
 *  CatalystInstanceImpl.java
 */
private ReactBridge initializeBridge(
    JavaScriptExecutor jsExecutor,
    JavaScriptModulesConfig jsModulesConfig) {

  ReactBridge bridge = new ReactBridge(jsExecutor, new NativeModulesReactCallback(),
      mReactQueueConfiguration.getNativeModulesQueueThread());
  
  Systrace.beginSection(Systrace.TRACE_TAG_REACT_JAVA_BRIDGE, "setBatchedBridgeConfig");
  bridge.setGlobalVariable(
      "__fbBatchedBridgeConfig",
      buildModulesConfigJSONProperty(mJavaRegistry, jsModulesConfig));
  bridge.setGlobalVariable(
      "__RCTProfileIsProfiling",
    
  return bridge;
}

调用catalystInstance.runJSBundle加载解析Jsbundle。假如在解析过程中出现Exception，统一交给NativeModuleCallExceptionHandler处理，建议开发者设置自己的NativeModuleCallExceptionHandler，可以归避部分Crash（SyntaxError: Unexpected token ‘<‘ 或 syntaxerror: unexpected="" end="" of="" script<="" span="">）。

/**
 *  CatalystInstanceImpl.java
 */
public void runJSBundle() {
  try {
    mJSBundleHasLoaded = mReactQueueConfiguration.getJSQueueThread().callOnQueue(
        new Callable() {
          @Override
          public Boolean call() throws Exception {
            incrementPendingJSCalls();
            try {
              mJSBundleLoader.loadScript(mBridge);
              Systrace.registerListener(mTraceListener);
            } catch (JSExecutionException e) {
              mNativeModuleCallExceptionHandler.handleException(e);
            } finally {
              Systrace.endSection(Systrace.TRACE_TAG_REACT_JAVA_BRIDGE);
            }
            return true;
          }
        }).get();
  } catch (Exception t) {
    throw new RuntimeException(t);
  }
}

在创建完React Context后会执行ReactContextInitAsyncTask的onPostExecute，从而调用setUpReactContext，会将ReactRootView做为Root View传递给UIManagerModule，此后Js通过UIManager创建的View都会add到该View上。

/**
 * ReactInstanceManagerImpl.java
 */
@Override
public void attachMeasuredRootView(ReactRootView rootView) {
  UiThreadUtil.assertOnUiThread();
  if(mIsNeedDetachView){
    Log.d(ReactConstants.QZONE_REACT_SRC_TAG,"attachMeasuredRootView do add");
    mAttachedRootViews.add(rootView);
    // If react context is being created in the background, JS application will be started
    // automatically when creation completes, as root view is part of the attached root view list.
    if (!mIsContextInitAsyncTaskRunning && mCurrentReactContext != null) {
      attachMeasuredRootViewToInstance(rootView, mCurrentReactContext.getCatalystInstance());
    }
  }else{
    Log.d(ReactConstants.QZONE_REACT_SRC_TAG,"attachMeasuredRootView do nothing");
  }

}

在绑定完RootView后，通过CatalystInstance获取AppRegistry这个JSModule后，进一步调用runApplication启动Js Application。

 /**
 * ReactInstanceManagerImpl.java
 */
private void attachMeasuredRootViewToInstance(
    ReactRootView rootView,
    CatalystInstance catalystInstance) {

  rootView.removeAllViews();
  rootView.setId(View.NO_ID);

  UIManagerModule uiManagerModule = catalystInstance.getNativeModule(UIManagerModule.class);
  int rootTag = uiManagerModule.addMeasuredRootView(rootView);
  @Nullable Bundle launchOptions = rootView.getLaunchOptions();
  WritableMap initialProps = launchOptions != null
      ? Arguments.fromBundle(launchOptions)
      : Arguments.createMap();
  String jsAppModuleName = rootView.getJSModuleName();

  WritableNativeMap appParams = new WritableNativeMap();
  appParams.putDouble("rootTag", rootTag);
  appParams.putMap("initialProps", initialProps);
  catalystInstance.getJSModule(AppRegistry.class).runApplication(jsAppModuleName, appParams);
}

ReactNative中Java与Js通信不再赘述。至此，启动Js层AppRegistry的runApplication启动Js Application。

/**
* AppRegistry.js
*/
runApplication: function(appKey: string, appParameters: any): void {
  console.log(
    'Running application "' + appKey + '" with appParams: ' +
    JSON.stringify(appParameters) + '. ' +
    '__DEV__ === ' + String(__DEV__) +
    ', development-level warning are ' + (__DEV__ ? 'ON' : 'OFF') +
    ', performance optimizations are ' + (__DEV__ ? 'OFF' : 'ON')
  );
  invariant(
    runnables[appKey] && runnables[appKey].run,
    'Application ' + appKey + ' has not been registered. This ' +
    'is either due to a require() error during initialization ' +
    'or failure to call AppRegistry.registerComponent.'
  );
  runnables[appKey].run(appParameters);
},