React Native通讯原理

yulei_xiu 3年前
   <p>在此文章的基础上分析和总结下RN与Native的通讯流程。</p>    <p>本文基于Android代码分析,iOS实现原理类似。</p>    <h3><strong>1. 通讯框架图</strong></h3>    <p style="text-align:center"><img src="https://simg.open-open.com/show/4c91acb74303a4faf80937b8f18a7dd3.png"></p>    <p style="text-align:center">通讯框架图</p>    <p>先来解析下各个模块的角色与作用:</p>    <p>Java层,这块的实现在ReactAndroid中</p>    <ul>     <li>ReactContext : Android上下文子类,包含一个CatalystInstance实例,用于获取NativeModule,JSModule、添加各种回调、处理异常等</li>     <li>ReactInstanceManager : 管理CatalystInstance的实例,处理RN Root View,启动JS页面,管理生命周期</li>     <li>CatalystInstance : 通讯的关键类,提供调用JS Module也支持JS调用Native Module,与Bridge进行交互,对开发者不可见</li>    </ul>    <p>C++层,这块实现在ReactCommon中,供Android与iOS使用</p>    <ul>     <li>NativeToJsBridge : native与JS的桥接,负责调用JS Module、回调Native(调用JsToNativeBridge)、加载JS代码(调用JavaScriptCore)</li>     <li>JsToNativeBridge : 调用Native Module的方法</li>     <li>JSCExecutor : 加载/执行JS代码(调用JavaScriptCore)、调用JS Module、回调native、性能统计等,都是比较核心的功能</li>    </ul>    <p>JS层,实现在Libraries中,RN JS相关的实现在都这个文件夹中</p>    <ul>     <li>MessageQueue : 管理JS的调用队列、调用Native/JS Module的方法、执行callback、管理JS Module等</li>     <li>JavaScriptModule : 代指所有的JSModule实现,在java层中也有对应的代码(都是interface),使用动态代理调用,统一入口在CatalystInstance中</li>    </ul>    <h3><strong>2. C++与JS间通讯</strong></h3>    <p>Native与JS通讯无非就是Java/OC与JS跨语言间的调用,在分析Native与JS通讯前先来了解下Java/OC与JS跨语言间的调用。</p>    <p>在ReactNative中使用JavaScriptCore来执行JS,这部分的关键就是如何利用JavaScriptCore。</p>    <p>看一下Android编译脚本:</p>    <ul>     <li>ReactAndroid/build.gradle</li>    </ul>    <pre>  <code class="language-javascript">compile 'org.webkit:android-jsc:r174650'        task downloadJSCHeaders(type: Download) {          def jscAPIBaseURL = 'https://svn.webkit.org/repository/webkit/  !svn/bc/174650/trunk/Source/JavaScriptCore/API/'          def jscHeaderFiles = ['JavaScript.h', 'JSBase.h', 'JSContextRef.h', 'JSObjectRef.h',    'JSRetainPtr.h', 'JSStringRef.h', 'JSValueRef.h', 'WebKitAvailability.h']          def output = new File(downloadsDir, 'jsc')          output.mkdirs()          src(jscHeaderFiles.collect { headerName -> "$jscAPIBaseURL$headerName" })          onlyIfNewer true          overwrite false          dest output      }        // Create Android.mk library module based on so files from mvn + include headers fetched from webkit    .org      task prepareJSC(dependsOn: downloadJSCHeaders) << {          copy {              from zipTree(configurations.compile.fileCollection { dep -> dep.name == 'android-jsc' }.    singleFile)              from {downloadJSCHeaders.dest}              from 'src/main/jni/third-party/jsc/Android.mk'              include 'jni/**/*.so', '*.h', 'Android.mk'              filesMatching('*.h', { fname -> fname.path = "JavaScriptCore/${fname.path}"})              into "$thirdPartyNdkDir/jsc";          }      }</code></pre>    <ul>     <li>ReactAndroid/src/main/jni/third-party/jsc/Android.mk</li>    </ul>    <pre>  <code class="language-javascript">LOCAL_SRC_FILES := jni/$(TARGET_ARCH_ABI)/libjsc.so</code></pre>    <p>从这里可以看出RN并没有用系统自带的webkit,WebKit主要包括WebCore排版引擎和JSCore引擎,这里主要使用了JSCore引擎,排版交给Native去做。</p>    <p>在RN中通过下面的方法设置native方法和属性:</p>    <pre>  <code class="language-javascript">JSObjectSetProperty(m_context, globalObject, jsPropertyName, valueToInject, 0, NULL);</code></pre>    <p>这个方法正是上面gradle脚本下载的JSObjectRef.h中,实现在libjsc.so中。这样就可以在Native设置,然后在JS中取出执行,反过来也是同样的。</p>    <h3><strong>3. Native与JS通讯</strong></h3>    <p>加载bundle文件</p>    <p>Native与JS的通讯首先需要加载Bundle文件,是在native初始化完成的时候,而Bundle文件的位置是可配置的。</p>    <pre>  <code class="language-javascript">public abstract class ReactNativeHost {      ...      /**     * Returns the name of the main module. Determines the URL used to fetch the JS bundle     * from the packager server. It is only used when dev support is enabled.     * This is the first file to be executed once the {@link ReactInstanceManager} is created.     * e.g. "index.android"     */    protected String getJSMainModuleName() {      return "index.android";    }      /**     * Returns a custom path of the bundle file. This is used in cases the bundle should be loaded     * from a custom path. By default it is loaded from Android assets, from a path specified     * by {@link getBundleAssetName}.     * e.g. "file://sdcard/myapp_cache/index.android.bundle"     */    protected @Nullable String getJSBundleFile() {      return null;    }      /**     * Returns the name of the bundle in assets. If this is null, and no file path is specified for     * the bundle, the app will only work with {@code getUseDeveloperSupport} enabled and will     * always try to load the JS bundle from the packager server.     * e.g. "index.android.bundle"     */    protected @Nullable String getBundleAssetName() {      return "index.android.bundle";    }      /**     * Returns whether dev mode should be enabled. This enables e.g. the dev menu.     */    protected abstract boolean getUseDeveloperSupport();      ...    }</code></pre>    <p>ReactNativeHost中的这些方法会根据需要在Application中重载,这些方法决定了从哪里加载Bundle,方法的注释写的非常清晰,不再介绍了,先看一下流程图:</p>    <p style="text-align:center"><img src="https://simg.open-open.com/show/f804464a1a9fde5d287bae61d6bd090c.png"></p>    <p style="text-align:center">Bundle加载流程图</p>    <p>JSBundleLoader从哪里加载,也是根据文件的位置,可以看看其loadScript方法,最终都会调用CatalystIntance去加载,有三个实现</p>    <pre>  <code class="language-javascript">/* package */ native void loadScriptFromAssets(AssetManager assetManager, String assetURL);    /* package */ native void loadScriptFromFile(String fileName, String sourceURL);    /* package */ native void loadScriptFromOptimizedBundle(String path, String sourceURL, int flags);</code></pre>    <p>最后一个支持加载优化后的Bundle,目前没有用到。这些方法都是c++实现,主要看一下前两个</p>    <pre>  <code class="language-javascript">void CatalystInstanceImpl::loadScriptFromAssets(jobject assetManager,                                                  const std::string& assetURL) {    const int kAssetsLength = 9;  // strlen("assets://");    auto sourceURL = assetURL.substr(kAssetsLength);      auto manager = react::extractAssetManager(assetManager);    auto script = react::loadScriptFromAssets(manager, sourceURL);    if (JniJSModulesUnbundle::isUnbundle(manager, sourceURL)) {      instance_->loadUnbundle(        folly::make_unique<JniJSModulesUnbundle>(manager, sourceURL),        std::move(script),        sourceURL);      return;    } else {      instance_->loadScriptFromString(std::move(script), sourceURL);    }  }    void CatalystInstanceImpl::loadScriptFromFile(jni::alias_ref<jstring> fileName,                                                const std::string& sourceURL) {    return instance_->loadScriptFromFile(fileName ? fileName->toStdString() : "",                                         sourceURL);  }</code></pre>    <p>从assets中加载就是先读取bundle的内容,当作一个字符串,这里有一个UnBundle,是RN打包的一种方式,除了生成整合 <strong>JS</strong> 文件 <strong>index.android.bundle</strong> 外,还会生成各个单独的未整合 <strong>JS</strong> 文件(但会被优化),全部放在 <strong>js-modules</strong> 目录下,同时会生成一个名为 <strong>UNBUNDLE</strong> 的标识文件,一并放在其中。 <strong>UNBUNDLE</strong> 标识文件的前 <strong>4</strong> 个字节固定为 <strong>0xFB0BD1E5</strong> ,用于加载前的校验。需要注意的是, <strong>js-modules</strong> 目录会一并打包到 <strong>apk</strong> 的 <strong>assets</strong> 文件夹中,这里就是处理这种情况的,后面具体的加载暂不分析。</p>    <p>对于开发模式有点特殊,在创建ReactInstanceManager之前会从server下载Bundle文件,然后保存起来,demo程序的路径为:</p>    <p>/data/user/0/com.awesomeproject/files/ReactNativeDevBundle.js</p>    <p>下载完成后调用CatalystInstance.loadScriptFromFile(),传递缓存后的径路,这个方法也是先读取文件内容,存为字符串,也是调用loadScriptFromString</p>    <pre>  <code class="language-javascript">void Instance::loadScriptFromString(std::unique_ptr<const JSBigString> string,                                      std::string sourceURL) {    callback_->incrementPendingJSCalls();    SystraceSection s("reactbridge_xplat_loadScriptFromString",                      "sourceURL", sourceURL);    // TODO mhorowitz: ReactMarker around loadApplicationScript    nativeToJsBridge_->loadApplicationScript(std::move(string), std::move(sourceURL));  }  --------------------------------------  void NativeToJsBridge::loadApplicationScript(std::unique_ptr<const JSBigString> script,                                               std::string sourceURL) {    // TODO(t11144533): Add assert that we are on the correct thread    m_mainExecutor->loadApplicationScript(std::move(script), std::move(sourceURL));  }</code></pre>    <p>loadApplicationScript的作用请参考 ReactNative Android源码分析 。</p>    <p>这里JS代码已经被执行了。</p>    <p><strong>如何调用JS</strong></p>    <p>在Native中调用JS的方式如下 :</p>    <pre>  <code class="language-javascript">ReactContext.getJSModule(JSModule类名.class).方法名(params);</code></pre>    <p>ReactContext调用的是CatalystInstance的同名方法</p>    <pre>  <code class="language-javascript">@Override    public <T extends JavaScriptModule> T getJSModule(Class<T> jsInterface) {      return getJSModule(mMainExecutorToken, jsInterface);    }      @Override    public <T extends JavaScriptModule> T getJSModule(ExecutorToken executorToken, Class<T> jsInterface) {      return Assertions.assertNotNull(mJSModuleRegistry)          .getJavaScriptModule(this, executorToken, jsInterface);    }</code></pre>    <p>mMainExecutorToken是在initializeBridge时创建的,根据注释是和 <strong>web workers</strong> 相关,是JS多线程相关的,即使用Token来区分线程。当前这种情况使用mMainExecutorToken就可以。看一下CatalystInstance的实现:</p>    <pre>  <code class="language-javascript">public synchronized <T extends JavaScriptModule> T getJavaScriptModule(      CatalystInstance instance,      ExecutorToken executorToken,      Class<T> moduleInterface) {      HashMap<Class<? extends JavaScriptModule>, JavaScriptModule> instancesForContext =          mModuleInstances.get(executorToken);      if (instancesForContext == null) {        instancesForContext = new HashMap<>();        mModuleInstances.put(executorToken, instancesForContext);      }        JavaScriptModule module = instancesForContext.get(moduleInterface);      if (module != null) {        return (T) module;      }        JavaScriptModuleRegistration registration =          Assertions.assertNotNull(              mModuleRegistrations.get(moduleInterface),              "JS module " + moduleInterface.getSimpleName() + " hasn't been registered!");      JavaScriptModule interfaceProxy = (JavaScriptModule) Proxy.newProxyInstance(          moduleInterface.getClassLoader(),          new Class[]{moduleInterface},          new JavaScriptModuleInvocationHandler(executorToken, instance, registration));      instancesForContext.put(moduleInterface, interfaceProxy);      return (T) interfaceProxy;    }</code></pre>    <p>在CatalystInstance创建的时候会把所有JavaScriptModule都收集到JavaScriptModuleRegistry的Map(mModuleRegistrations)中。而mModuleInstances是缓存已经调用过的JS Module的代理对象,如果已经调用过,则从map中直接返回,否则创建其代理对象,然后缓存起来。</p>    <p>这里使用的是动态代理模式,先创建一个interface的代理对象,当调用其方法时会InvocationHandler的invoke()方法。</p>    <pre>  <code class="language-javascript">@Override    public @Nullable Object invoke(Object proxy, Method method, @Nullable Object[] args) throws   Throwable {      ExecutorToken executorToken = mExecutorToken.get();      if (executorToken == null) {        FLog.w(ReactConstants.TAG, "Dropping JS call, ExecutorToken went away...");        return null;      }      NativeArray jsArgs = args != null ? Arguments.fromJavaArgs(args) : new WritableNativeArray();      mCatalystInstance.callFunction(        executorToken,        mModuleRegistration.getName(),        method.getName(),        jsArgs      );      return null;    }</code></pre>    <p>这个调用流程已经在 ReactNative Android源码分析 中分析了。这次会走到JS的MessageQueue.callFunctionReturnFlushedQueue()中了。</p>    <p><strong>JS接收调用和处理</strong></p>    <p>先来解释下为什么会走到callFunctionReturnFlushedQueue。</p>    <ol>     <li>在生成的bundle.js中会把MessageQueue对象放到一个全局的属性中 <pre>  <code class="language-javascript">Object.defineProperty(global,"__fbBatchedBridge",{configurable:!0,value:BatchedBridge})</code></pre> 这里明明是BatchedBridge,为什么说是MessageQueue的对象呢,原来在BatchedBridge.js中有这样几句代码 <pre>  <code class="language-javascript">const BatchedBridge = new MessageQueue(  () => global.__fbBatchedBridgeConfig,  serializeNativeParams  );</code></pre> </li>     <li> <p>在上面加载bundle文件的时候,会执行下面的方法</p> <pre>  <code class="language-javascript">void JSCExecutor::bindBridge() throw(JSException) {     auto global = Object::getGlobalObject(m_context);     auto batchedBridgeValue = global.getProperty("__fbBatchedBridge");     if (batchedBridgeValue.isUndefined()) {       throwJSExecutionException("Could not get BatchedBridge, make sure your bundle is packaged   correctly");     }       auto batchedBridge = batchedBridgeValue.asObject();     m_callFunctionReturnFlushedQueueJS = batchedBridge.getProperty("callFunctionReturnFlushedQueue"). asObject();     m_invokeCallbackAndReturnFlushedQueueJS = batchedBridge.getProperty(  "invokeCallbackAndReturnFlushedQueue").asObject();     m_flushedQueueJS = batchedBridge.getProperty("flushedQueue").asObject();  }</code></pre> <p>这里会把MessageQueue的三个方法会当作对象保存在c++中,当我们调用JS的方法时会直接用到。</p> <pre>  <code class="language-javascript">void JSCExecutor::callFunction(const std::string& moduleId, const std::string& methodId, const folly::dynamic& arguments) {     try {       auto result = m_callFunctionReturnFlushedQueueJS->callAsFunction({         Value(m_context, String::createExpectingAscii(moduleId)),         Value(m_context, String::createExpectingAscii(methodId)),         Value::fromDynamic(m_context, std::move(arguments))       });       auto calls = Value(m_context, result).toJSONString();       m_delegate->callNativeModules(*this, std::move(calls), true);     } catch (...) {       std::throw_with_nested(std::runtime_error("Error calling function: " + moduleId + ":" + methodId));     }  }</code></pre> <pre>  <code class="language-javascript">Value Object::callAsFunction(JSObjectRef thisObj, int nArgs, const JSValueRef args[]) const {     JSValueRef exn;     JSValueRef result = JSObjectCallAsFunction(m_context, m_obj, thisObj, nArgs, args, &exn);     if (!result) {       std::string exceptionText = Value(m_context, exn).toString().str();       throwJSExecutionException("Exception calling object as function: %s", exceptionText.c_str());     }     return Value(m_context, result);  }</code></pre> <p>最终还是通过JavaScriptCore的方法JSObjectCallAsFunction来调用JS的。下面就好办了,直接分析JS代码吧。</p> </li>    </ol>    <p>在callFunctionReturnFlushedQueue这个方法主要调用了__callFunction,来看一下它的实现:</p>    <pre>  <code class="language-javascript">__callFunction(module: string, method: string, args: any) {      ...      const moduleMethods = this._callableModules[module];      ...      const result = moduleMethods[method].apply(moduleMethods, args);      Systrace.endEvent();      return result;    }</code></pre>    <p>方法是从_callableModules中取出来的,那他的值是从哪里来的呢,看了下这个文件原来答案是有往里添加的方法</p>    <pre>  <code class="language-javascript">registerCallableModule(name, methods) {      this._callableModules[name] = methods;    }</code></pre>    <p>也就是说所有的JS Module都需要把该Module中可供Native调用的方法都放到这里来,这样才能够执行。以AppRegistry.js为例,来看看它是怎么往里添加的</p>    <pre>  <code class="language-javascript">var AppRegistry = {    registerConfig: function(config: Array<AppConfig>) {...},      registerComponent: function(appKey: string, getComponentFunc: ComponentProvider): string {...},      registerRunnable: function(appKey: string, func: Function): string {...},      getAppKeys: function(): Array<string> {...},      runApplication: function(appKey: string, appParameters: any): void {...},      unmountApplicationComponentAtRootTag: function(rootTag : number) {...},    };    BatchedBridge.registerCallableModule(    'AppRegistry',    AppRegistry  );</code></pre>    <p>到这里Native调用JS就已经完成了。</p>    <p>总结一下整个流程:</p>    <ol>     <li>MessageQueue把Native调用的方法放到JavaScriptCore中</li>     <li>JS Module把可以调用的方法放到MessageQueue的一个对列中</li>     <li>Native从JavaScriptCore中拿到JS的调用入口,并把Module Name、Method Name、Parameters传过去</li>     <li>执行JS Module的方法</li>    </ol>    <h3><strong>4. JS与Native通讯</strong></h3>    <p><strong>JS处理Native Module列表</strong></p>    <p>在 ReactNative Android源码分析 中分析了Native的初始化流程,这里总结一下对Native 模块的处理。</p>    <ol>     <li>在初始化CatalystInstance时会把所有的Native Module放在一个列表中,并在C++(ModuleRegistry)和Java(NativeModuleRegistry)中都保存了</li>     <li>在JavaScriptCore中设置了全局属性__fbBatchedBridgeConfig,其值为Module Name列表</li>    </ol>    <p>那么问题来了,在JS中只能取到Native Module的名字,怎么调用它的方法呢。下面来分析下这个问题。</p>    <p>在JSCExecutor初始化的时候,向JavaScriptCore中注册了几个c++的方法供JS调用,其中就有获取Native Module详细信息的方法</p>    <pre>  <code class="language-javascript">void JSCExecutor::initOnJSVMThread() throw(JSException) {      ....    installNativeHook<&JSCExecutor::nativeRequireModuleConfig>("nativeRequireModuleConfig");    installNativeHook<&JSCExecutor::nativeFlushQueueImmediate>("nativeFlushQueueImmediate");    ...    }    JSValueRef JSCExecutor::nativeRequireModuleConfig(      size_t argumentCount,      const JSValueRef arguments[]) {    if (argumentCount != 1) {      throw std::invalid_argument("Got wrong number of args");    }      std::string moduleName = Value(m_context, arguments[0]).toString().str();    folly::dynamic config = m_delegate->getModuleConfig(moduleName);    return Value::fromDynamic(m_context, config);  }</code></pre>    <p>从nativeRequireModuleConfig的入参和返回结果就可以看出来</p>    <p>是供JS调用的,用于获取Native Module详情信息,m_delegate-> getModuleConfig的实现下面会分析。</p>    <p>接着来分析下JS是如何处理Native Module的。入口是在MessageQueue.js中处理的。</p>    <pre>  <code class="language-javascript">class MessageQueue {    constructor(configProvider: () => Config, serializeNativeParams: boolean) {      ....      lazyProperty(this, 'RemoteModules', () => {        const {remoteModuleConfig} = configProvider();        const modulesConfig = this._genModulesConfig(remoteModuleConfig);        const modules = this._genModules(modulesConfig)        ...        return modules;      });    }      ...      function lazyProperty(target: Object, name: string, f: () => any) {    Object.defineProperty(target, name, {      configurable: true,      enumerable: true,      get() {        const value = f();        Object.defineProperty(target, name, {          configurable: true,          enumerable: true,          writeable: true,          value: value,        });        return value;      }    });  }</code></pre>    <p>在它的构造函数中定义了一个RemoteModules的属性,使用了懒加载的机制,只有真正使用的时候才会为其赋值。返回的是所有Modle列表,只添加了module id,其他信息并没有。</p>    <p>这个RemoteModules是在哪里使用,Module的其他信息又是怎么获取呢,路漫漫其修远兮,接着分析吧</p>    <p>搜了下代码,是在NativeModule.js中</p>    <pre>  <code class="language-javascript">const BatchedBridge = require('BatchedBridge');  const RemoteModules = BatchedBridge.RemoteModules;    ...    /**   * Define lazy getters for each module.   * These will return the module if already loaded, or load it if not.   */  const NativeModules = {};  Object.keys(RemoteModules).forEach((moduleName) => {    Object.defineProperty(NativeModules, moduleName, {      configurable: true,      enumerable: true,      get: () => {        let module = RemoteModules[moduleName];        if (module && typeof module.moduleID === 'number' && global.nativeRequireModuleConfig) {          // The old bridge (still used by iOS) will send the config as          //  a JSON string that needs parsing, so we set config according          //  to the type of response we got.          const rawConfig = global.nativeRequireModuleConfig(moduleName);          const config = typeof rawConfig === 'string' ? JSON.parse(rawConfig) : rawConfig;          module = config && BatchedBridge.processModuleConfig(config, module.moduleID);          RemoteModules[moduleName] = module;        }        Object.defineProperty(NativeModules, moduleName, {          configurable: true,          enumerable: true,          value: module,        });        return module;      },    });  });    module.exports = NativeModules;</code></pre>    <p>这块会遍历RemoteModules中所有的模块名,每个模块名都定义一个对象,使用的时候才会为其赋值。看到在赋值的时候会调用c++的nativeRequireModuleConfig,也就是获取每个Module的详细信息。</p>    <p>获取详细信息就是调用上面提到的m_delegate->getModuleConfig(moduleName),m_delegate是JsToNativeBridge对象,getModuleConfig直接调用了ModuleRegistry::getConfig(name)</p>    <pre>  <code class="language-javascript">folly::dynamic ModuleRegistry::getConfig(const std::string& name) {    SystraceSection s("getConfig", "module", name);    auto it = modulesByName_.find(name);    if (it == modulesByName_.end()) {      return nullptr;    }    CHECK(it->second < modules_.size());      NativeModule* module = modules_[it->second].get();      // string name, [object constants,] array methodNames (methodId is index), [array asyncMethodIds]    folly::dynamic config = folly::dynamic::array(name);      {      SystraceSection s("getConstants");      folly::dynamic constants = module->getConstants();      if (constants.isObject() && constants.size() > 0) {        config.push_back(std::move(constants));      }    }      {      SystraceSection s("getMethods");      std::vector<MethodDescriptor> methods = module->getMethods();        folly::dynamic methodNames = folly::dynamic::array;      folly::dynamic asyncMethodIds = folly::dynamic::array;      folly::dynamic syncHookIds = folly::dynamic::array;        for (auto& descriptor : methods) {        methodNames.push_back(std::move(descriptor.name));        if (descriptor.type == "remoteAsync") {          asyncMethodIds.push_back(methodNames.size() - 1);        } else if (descriptor.type == "syncHook") {          syncHookIds.push_back(methodNames.size() - 1);        }      }        if (!methodNames.empty()) {        config.push_back(std::move(methodNames));        config.push_back(std::move(asyncMethodIds));        if (!syncHookIds.empty()) {          config.push_back(std::move(syncHookIds));        }      }    }      if (config.size() == 1) {      // no constants or methods      return nullptr;    } else {      return config;    }  }</code></pre>    <p>这里需要解释两个数据结构,modules_是所有Native模块对象的数组,而modulesByName_是一个Map,key值是模块名字,value是该模块在modules_中的索引值。这个方法返回值是一个数组,它的格式是</p>    <pre>  <code class="language-javascript">[    "Module Name",    [Object Constants],    [Method Name Array],    [Async Method Ids],    [Sync Hook Ids]  ]</code></pre>    <p>前三个好理解,来解释后两是什么含意,asyncMethod字面意思是异步方法,也就是方法参数是Promise的。而syncHook类的方法,目前是没有遇到,这种方法可以JS线程中直接调用,而其他的方法是扔到后台线程队列,然后等待被调用。</p>    <p>下面重点看一下NativeModule的getConstants和getMethods的实现。</p>    <p style="text-align:center"><img src="https://simg.open-open.com/show/892d5a63b6cded3d64c7fda3062494b6.png"></p>    <p style="text-align:center">NativeModule类图</p>    <p>从NativeModule的类图中看出它有两个子类,JavaNativeModule就是普通的Java模块,而NewJavaNativeModule是指c++跨平台模块,目前还未使用到,所以现在只分析下JavaNativeModule。</p>    <pre>  <code class="language-javascript">std::vector<MethodDescriptor> getMethods() override {      static auto getMDMethod =        wrapper_->getClass()->getMethod<jni::JList<JMethodDescriptor::javaobject>::javaobject()>(          "getMethodDescriptors");        std::vector<MethodDescriptor> ret;      auto descs = getMDMethod(wrapper_);      for (const auto& desc : *descs) {        static auto nameField =          JMethodDescriptor::javaClassStatic()->getField<jstring>("name");        static auto typeField =          JMethodDescriptor::javaClassStatic()->getField<jstring>("type");          ret.emplace_back(          desc->getFieldValue(nameField)->toStdString(),          desc->getFieldValue(typeField)->toStdString()        );      }      return ret;    }      folly::dynamic getConstants() override {      static auto constantsMethod =        wrapper_->getClass()->getMethod<NativeArray::javaobject()>("getConstants");      auto constants = constantsMethod(wrapper_);      if (!constants) {        return nullptr;      } else {        // See JavaModuleWrapper#getConstants for the other side of this hack.        return cthis(constants)->array[0];      }    }</code></pre>    <p>这里的wrapper_是指的JavaModuleWrapper,而wrapper_->getClass()是指的Java类:"Lcom/非死book/react/cxxbridge/JavaModuleWrapper;",也就是说上面是使用反射,调用JavaModuleWrapper的getMethodDescriptors和getConstants。</p>    <p>getContants就是调用Java具体模块的getConstants方法,并把返回的map组装成RN可以接受的WritableNativeMap的结构返回,具体看一下getMethodDescriptors的实现</p>    <pre>  <code class="language-javascript">@DoNotStrip  public class MethodDescriptor {    @DoNotStrip    Method method;    @DoNotStrip    String signature;    @DoNotStrip    String name;    @DoNotStrip    String type;  }  @DoNotStrip  public List<MethodDescriptor> getMethodDescriptors() {    ArrayList<MethodDescriptor> descs = new ArrayList<>();    for (Map.Entry<String, BaseJavaModule.NativeMethod> entry :           mModule.getMethods().entrySet()) {      MethodDescriptor md = new MethodDescriptor();      md.name = entry.getKey();      md.type = entry.getValue().getType();      BaseJavaModule.JavaMethod method = (BaseJavaModule.JavaMethod) entry.getValue();      mMethods.add(method);      descs.add(md);    }    return descs;  }</code></pre>    <p>mModule.getMethods()是在BaseJavaModule中,也是使用反射查找当前模块的方法,方法必须有@ReactMethod的注解才会收集。最终把拿到的信息封装成一个MethodDescriptor的类。</p>    <p>到这里就已经分析了JS是如何拿到Native模块的详细信息的。</p>    <p><strong>如何调用Native</strong></p>    <p>这里演示下在JS中如何调用Native的module,先假设一个场景,用户点击一个TextView,然后弹个Toast提示。</p>    <p>以demo工程的代码为例:</p>    <pre>  <code class="language-javascript">class AwesomeProject extends Component {     render() {       return (         <View style={styles.container}>           <Text style={styles.welcome} onPress={onClick} >             Welcome to React Native!           </Text>           <Text style={styles.instructions}>             To get started, edit index.android.js           </Text>           <Text style={styles.instructions}>             Double tap R on your keyboard to reload,{'\n'}             Shake or press menu button for dev menu           </Text>           <TextInput />         </View>       );     }   }     function onClick(){      var ToastAndroid = require('ToastAndroid')      ToastAndroid.show('Click TextView...', ToastAndroid.SHORT);   }</code></pre>    <p>来看一下ToastAndroid的实现</p>    <pre>  <code class="language-javascript">var RCTToastAndroid = require('NativeModules').ToastAndroid;  ...  var ToastAndroid = {    ...    show: function (      message: string,      duration: number    ): void {      RCTToastAndroid.show(message, duration);    },    ...  };</code></pre>    <p>这里调用的是RCTToastAndroid.show(),而RCTToastAndroid是从NativeModules中取出的。 在前面分析JS如何收集Native模块的时候会生成modules属性,调用Native方法时就是执行它里面的函数,看一下这个函数是如何生成的</p>    <pre>  <code class="language-javascript">_genMethod(module, method, type) {      let fn = null;      const self = this;      if (type === MethodTypes.remoteAsync) {        ...      } else if (type === MethodTypes.syncHook) {        ...      } else {        fn = function(...args) {          const lastArg = args.length > 0 ? args[args.length - 1] : null;          const secondLastArg = args.length > 1 ? args[args.length - 2] : null;          const hasSuccCB = typeof lastArg === 'function';          const hasErrorCB = typeof secondLastArg === 'function';          hasErrorCB && invariant(            hasSuccCB,            'Cannot have a non-function arg after a function arg.'          );          const numCBs = hasSuccCB + hasErrorCB;          const onSucc = hasSuccCB ? lastArg : null;          const onFail = hasErrorCB ? secondLastArg : null;          args = args.slice(0, args.length - numCBs);          return self.__nativeCall(module, method, args, onFail, onSucc);        };      }      fn.type = type;      return fn;    }</code></pre>    <p>就是准备好参数,然后__nativeCall。</p>    <pre>  <code class="language-javascript">__nativeCall(module, method, params, onFail, onSucc) {      if (onFail || onSucc) {        ...        onFail && params.push(this._callbackID);        this._callbacks[this._callbackID++] = onFail;        onSucc && params.push(this._callbackID);        this._callbacks[this._callbackID++] = onSucc;      }      var preparedParams = this._serializeNativeParams ? JSON.stringify(params) : params;      ...      this._callID++;        this._queue[MODULE_IDS].push(module);      this._queue[METHOD_IDS].push(method);      this._queue[PARAMS].push(preparedParams);        const now = new Date().getTime();      if (global.nativeFlushQueueImmediate &&          now - this._lastFlush >= MIN_TIME_BETWEEN_FLUSHES_MS) {        global.nativeFlushQueueImmediate(this._queue);        this._queue = [[], [], [], this._callID];        this._lastFlush = now;      }      Systrace.counterEvent('pending_js_to_native_queue', this._queue[0].length);      ...    }</code></pre>    <p>把模块名、方法名、调用参数放到数组里存起来,如果上次调用和本次调用想着超过5ms则调用c++的nativeFlushQueueImmediate方法,如果小于5ms就直接返回了。</p>    <p><strong>Native接收调用和处理</strong></p>    <p>Native接收JS调用分两种情况:</p>    <ul>     <li> <p>两次调用超过5ms时,进入nativeFlushQueueImmediate</p> </li>     <li> <p>Native调用JS的时候会把之前存的调用返回到JSCExecutor::flush()</p> </li>    </ul>    <p>先来看第一种情况</p>    <pre>  <code class="language-javascript">JSValueRef JSCExecutor::nativeFlushQueueImmediate(      size_t argumentCount,      const JSValueRef arguments[]) {    if (argumentCount != 1) {      throw std::invalid_argument("Got wrong number of args");    }      std::string resStr = Value(m_context, arguments[0]).toJSONString();    flushQueueImmediate(std::move(resStr));    return JSValueMakeUndefined(m_context);  }    void JSCExecutor::flushQueueImmediate(std::string queueJSON) {    m_delegate->callNativeModules(*this, std::move(queueJSON), false);  }</code></pre>    <p>再来看一下第二种情况</p>    <pre>  <code class="language-javascript">void JSCExecutor::flush() {    auto result = m_flushedQueueJS->callAsFunction({});    try {      auto calls = Value(m_context, result).toJSONString();      m_delegate->callNativeModules(*this, std::move(calls), true);    } catch (...) {      std::string message = "Error in flush()";      try {        message += ":" + Value(m_context, result).toString().str();      } catch (...) {        // ignored      }      std::throw_with_nested(std::runtime_error(message));    }  }</code></pre>    <p>结果都是一样的,把JS的调用转成一个Json字符串,然后再调用JsToNativeBridge.callNativeModules().</p>    <p>这个Json字符串,是一个数组,包含四个元素,格式如下:</p>    <p><img src="https://simg.open-open.com/show/0072fb885ceede9898bb55f86c18a74c.png"></p>    <pre>  <code class="language-javascript">void callNativeModules(        JSExecutor& executor, std::string callJSON, bool isEndOfBatch) override {      ExecutorToken token = m_nativeToJs->getTokenForExecutor(executor);      m_nativeQueue->runOnQueue([this, token, callJSON=std::move(callJSON), isEndOfBatch] {        // An exception anywhere in here stops processing of the batch.  This        // was the behavior of the Android bridge, and since exception handling        // terminates the whole bridge, there's not much point in continuing.        for (auto& call : react::parseMethodCalls(callJSON)) {          m_registry->callNativeMethod(            token, call.moduleId, call.methodId, std::move(call.arguments), call.callId);        }        if (isEndOfBatch) {          m_callback->onBatchComplete();          m_callback->decrementPendingJSCalls();        }      });    }</code></pre>    <p>这里根据ModuleId 和 MethodId调用Native模块的方法。m_registry是c++的ModuleRegistry,先介绍它是怎么创建的。在CatalystInstance.initializeBridge()的时候传递一个Java层的ModuleRegistryHolder,同样在c++中也有一个同名的对象,在创建的时候会把Native的Module列表保存起来并创建一个c++的ModuleRegistry,把Native的模块列表也传过去了。</p>    <pre>  <code class="language-javascript">void ModuleRegistry::callNativeMethod(ExecutorToken token, unsigned int moduleId, unsigned int methodId,                                        folly::dynamic&& params, int callId) {      if (moduleId >= modules_.size()) {        throw std::runtime_error(          folly::to<std::string>("moduleId ", moduleId,                                 " out of range [0..", modules_.size(), ")"));      }      #ifdef WITH_FBSYSTRACE      if (callId != -1) {        fbsystrace_end_async_flow(TRACE_TAG_REACT_APPS, "native", callId);      }    #endif      modules_[moduleId]->invoke(token, methodId, std::move(params));  }</code></pre>    <p>moduleId就是模块在列表中的索引,modules_的类型是</p>    <pre>  <code class="language-javascript">std::vector<std::unique_ptr<NativeModule>> modules_;</code></pre>    <p>也就是在创建ModuleRegistryHolder的时候会根据Java层的ModuleRegistryHolder创建c++的NativeModule。来看一下它的invoke方法</p>    <pre>  <code class="language-javascript">void invoke(ExecutorToken token, unsigned int reactMethodId, folly::dynamic&& params) override {      static auto invokeMethod =        wrapper_->getClass()->getMethod<void(JExecutorToken::javaobject, jint, ReadableNativeArray::javaobject)>("invoke");      invokeMethod(wrapper_, JExecutorToken::extractJavaPartFromToken(token).get(), static_cast<jint>(reactMethodId),                   ReadableNativeArray::newObjectCxxArgs(std::move(params)).get());    }</code></pre>    <p>这里主要是通过反射,调用JavaModuleWrapper的invoke方法,同时把methodId和参数传过去。</p>    <pre>  <code class="language-javascript">/* package */ class JavaModuleWrapper {    ...    private final ArrayList<BaseJavaModule.JavaMethod> mMethods;      ...      @DoNotStrip    public void invoke(ExecutorToken token, int methodId, ReadableNativeArray parameters) {      if (mMethods == null || methodId >= mMethods.size()) {        return;      }        mMethods.get(methodId).invoke(mCatalystInstance, token, parameters);    }  }</code></pre>    <p>在JavaModuleWrapper中有一个List,包含了这个module中所有JS可以调用的方法,methodId就是方法的索引和MessageQueue里获取的模块方法id是一致的。JavaMethod的invoke就是通过反射调用相关的方法。至此JS调用Native的流程就完成了。</p>    <p> </p>    <p> </p>    <p>来自:http://www.jianshu.com/p/17d6f6c57a5c</p>    <p> </p>