如何进行ReactNative For Android 框架启动核心路径剖析，相信很多没有经验的人对此束手无策，为此本文总结了问题出现的原因和解决方法，通过这篇文章希望你能解决这个问题。
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。
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。
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<ReactContextInitParams, Void, Result<ReactApplicationContext>> {
@Override
protected void onPreExecute() {
if (mCurrentReactContext != null) {
tearDownReactContext(mCurrentReactContext);
mCurrentReactContext = null;
}
}
@Override
protected Result<ReactApplicationContext> 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<ReactApplicationContext> 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:
通过Builder构建上文概念讲过的NativeModuleRegistry及JavaScriptModuleConfig；
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；
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；
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；
调用reactContext.initializeWithInstance进一步将创建完的CatalystInstance及线程等缓存在ReactContext中；
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<ReactBridge>() {
@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）。
/**
* CatalystInstanceImpl.java
*/
public void runJSBundle() {
try {
mJSBundleHasLoaded = mReactQueueConfiguration.getJSQueueThread().callOnQueue(
new Callable<Boolean>() {
@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);
},看完上述内容，你们掌握如何进行ReactNative For Android 框架启动核心路径剖析的方法了吗？如果还想学到更多技能或想了解更多相关内容，欢迎关注亿速云行业资讯频道，感谢各位的阅读！
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