React Native Debug原理浅析

摘要：过程涉及到三个对象，一个是或，一个是，另外一个就是浏览器或或其他。在中进行配置了，也就是会执行脚本。然后会给这个注册一些监听在收到消息时会回调。发出一个消息让浏览器准备的运行环境在收到消息会调用。

第一次在segmentfault写博客，很紧张~~~公司项目上ReactNative,之前也是没有接触过，所以也是一边学习一边做项目了，最近腾出手来更新总结了一下RN的Debug的一个小知识点，不是说怎么去Debug，而是Debug的代码原理，下面开始正文。

Debug过程涉及到三个对象，一个是App(Android或iOS)，一个是Server，另外一个就是浏览器(Chrome或FireFox或其他)。Server是App和浏览器之间通信的桥梁，比如App发Http请求给Server，Server再通过WebSocket发送给浏览器，反过来也是。首先肯定需要准备一下中介，就是Server

这里的Server不用专门准备一台服务器，只需要配置一个Node.js环境，然后启动npm start就行。npm start在package.json中进行配置了，也就是会执行cli.js脚本。

"scripts": {
    "start": "node node_modules/react-native/local-cli/cli.js start"
  },

然后cli.js会执行runServer.js，在这里启动一个NodeJS Server:

const serverInstance = args.https
    ? https.createServer(
        {
          key: fs.readFileSync(args.key),
          cert: fs.readFileSync(args.cert),
        },
        app,
      )
    : http.createServer(app);

  serverInstance.listen(args.port, args.host, 511, function() {
    attachHMRServer({
      httpServer: serverInstance,
      path: "/hot",
      packagerServer,
    });

    wsProxy = webSocketProxy.attachToServer(serverInstance, "/debugger-proxy");
    ms = messageSocket.attachToServer(serverInstance, "/message");
    readyCallback(reporter);
  });

有了中介Server后就可以建立App与浏览器之间的关系了。

在手机菜单中点击Debug JS Remotely,App就会发出一个Http请求

GET /launch-js-devtools HTTP/1.1

Server接收到这个请求会执行opn操作，主要做两件事：

打开Chrome的一个tab
让这个tab打开urlhttp://localhost:8081/debugger-ui/

这个界面就是我们打开Debug时在浏览器见到的第一个界面

这个界面的文件就是Server的index.html,我截取了body的代码：

  

    
       Dark Theme
    
    
       Maintain Priority
    
    

      React Native JS code runs as a web worker inside this tab.
    
    
Press ⌘⌥I to open Developer Tools. Enable Pause On Caught Exceptions for a better debugging experience.
    
You may also install the standalone version of React Developer Tools to inspect the React component hierarchy, their props, and state.
    
Status: Loading...
  

浏览器在执行index.html的时候会发出下面的请求：

GET /debugger-proxy?role=debugger&name=Chrome HTTP/1.1

我们来看看发出这个请求有什么目的，扒一扒源码：

function connectToDebuggerProxy() {
    const ws = new WebSocket("ws://" + window.location.host + "/debugger-proxy?role=debugger&name=Chrome"); //Chrome通过websocket和Packager保持通讯
  
      //WebSocket注册监听
    ws.onopen = function() {
      Page.setState({status: {type: "connecting"}});
    };

    ws.onmessage = async function(message) {
      if (!message.data) {
        return;
      }
      const object = JSON.parse(message.data);

      if (object.$event === "client-disconnected") {
        shutdownJSRuntime();
        Page.setState({status: {type: "disconnected"}});
        return;
      }

      if (!object.method) {
        return;
      }

      // Special message that asks for a new JS runtime
      if (object.method === "prepareJSRuntime") {
        shutdownJSRuntime();
        console.clear();
        createJSRuntime();
        ws.send(JSON.stringify({replyID: object.id}));
        Page.setState({status: {type: "connected", id: object.id}});
      } else if (object.method === "$disconnected") {
        shutdownJSRuntime();
        Page.setState({status: {type: "disconnected"}});
      } else if (object.method === "executeApplicationScript") {
        worker.postMessage({
          ...object,
          url: await getBlobUrl(object.url),
        });
      } else {
        // Otherwise, pass through to the worker.
        worker.postMessage(object);
      }
    };

    ws.onclose = function(error) {
      shutdownJSRuntime();
      Page.setState({status: {type: "error", error}});
      if (error.reason) {
        console.warn(error.reason);
      }
      setTimeout(connectToDebuggerProxy, 500);
    };

    // Let debuggerWorker.js know when we"re not visible so that we can warn about
    // poor performance when using remote debugging.
    document.addEventListener("visibilitychange", updateVisibility, false);
  }

首先就是通过new WebSocket浏览器建立与Server的联系，WebSocket就是可以保持长连接的全双工通信协议，在握手阶段通过Http进行，后面就和Http没有什么关系了。然后会给这个webSocket注册一些监听：

ws.onopen 
ws.onmessage
ws.onclose

在webSocket收到消息时会回调ws.onmessage。

到这里App和浏览器之间就已经建立连接了，接下来App会发出几个消息让浏览器加载需要调试的代码, 接着往下看。

首先需要强调的就是浏览器加载项目代码肯定不能在UI线程加载吧，要不然肯定影响浏览器的正常工作。那怎么去加载？启一个后台线程，有的小伙伴就要不信了，别急，我们接着去扒一扒源码。
App发出一个消息让浏览器准备JS的运行环境：

在收到‘prepareJSRuntime’消息会调用createJSRuntime。
// Special message that asks for a new JS runtime
      if (object.method === "prepareJSRuntime") {
        shutdownJSRuntime();
        console.clear();
        createJSRuntime();
        ws.send(JSON.stringify({replyID: object.id}));
        Page.setState({status: {type: "connected", id: object.id}});
      } else if (object.method === "$disconnected") {
        shutdownJSRuntime();
        Page.setState({status: {type: "disconnected"}});
      } else if (object.method === "executeApplicationScript") {
        worker.postMessage({
          ...object,
          url: await getBlobUrl(object.url),
        });
      } else {
        // Otherwise, pass through to the worker.
        worker.postMessage(object);
      }

接着看‘createJSRuntime’这个函数, 主要工作就是‘new Worker’，看下Worker的定义：

Web Workers is a simple means for web content to run scripts in 
background threads. The worker thread can perform tasks without 
interfering with the user interface. 
也就是会起一个后台线程，来运行‘debuggerWorker.js’这个脚本。

function createJSRuntime() {
      // This worker will run the application JavaScript code,
      // making sure that it"s run in an environment without a global
      // document, to make it consistent with the JSC executor environment.
      worker = new Worker("debuggerWorker.js");
      worker.onmessage = function(message) {
        ws.send(JSON.stringify(message.data));
      };
      window.onbeforeunload = function() {
        return "If you reload this page, it is going to break the debugging session. " +
          "You should press" + refreshShortcut + "in simulator to reload.";
      };
      updateVisibility();
}

接着看看debuggerWorker.js，主要就是一个消息的监听，可以看到在messageHandlers里主要处理两类消息：

"executeApplicationScript", "setDebuggerVisibility"

/* global __fbBatchedBridge, self, importScripts, postMessage, onmessage: true */
/* eslint no-unused-vars: 0 */

"use strict";

onmessage = (function() {
  var visibilityState;
  var showVisibilityWarning = (function() {
    var hasWarned = false;
    return function() {
      // Wait until `YellowBox` gets initialized before displaying the warning.
      if (hasWarned || console.warn.toString().includes("[native code]")) {
        return;
      }
      hasWarned = true;
      console.warn(
        "Remote debugger is in a background tab which may cause apps to " +
        "perform slowly. Fix this by foregrounding the tab (or opening it in " +
        "a separate window)."
      );
    };
  })();

  var messageHandlers = {
    "executeApplicationScript": function(message, sendReply) {
      for (var key in message.inject) {
        self[key] = JSON.parse(message.inject[key]);
      }
      var error;
      try {
        importScripts(message.url);
      } catch (err) {
        error = err.message;
      }
      sendReply(null /* result */, error);
    },
    "setDebuggerVisibility": function(message) {
      visibilityState = message.visibilityState;
    },
  };

  return function(message) {
    if (visibilityState === "hidden") {
      showVisibilityWarning();
    }

    var object = message.data;

    var sendReply = function(result, error) {
      postMessage({replyID: object.id, result: result, error: error});
    };

    var handler = messageHandlers[object.method];
    if (handler) {
      // Special cased handlers
      handler(object, sendReply);
    } else {
      // Other methods get called on the bridge
      var returnValue = [[], [], [], 0];
      var error;
      try {
        if (typeof __fbBatchedBridge === "object") {
          returnValue = __fbBatchedBridge[object.method].apply(null, object.arguments);
        } else {
          error = "Failed to call function, __fbBatchedBridge is undefined";
        }
      } catch (err) {
        error = err.message;
      } finally {
        sendReply(JSON.stringify(returnValue), error);
      }
    }
  };
})();

App在点击调试的时候会给浏览器还发送这么一个‘executeApplicationScript’消息，让浏览器去加载项目代码：

这个messageEvent的数据比较多，我就截取一部分,里面包含了方法名，url(这个url就是后面浏览器需要去下载bundle的地方)，inject包含的数据最多，主要是会赋值给浏览器全局对象的方法。

{
  "id": 1,
  "method": "executeApplicationScript",
  "url": "http://localhost:8081/index.android.bundle?platform=android&dev=true&minify=false",
  "inject": {
    "__fbBatchedBridgeConfig": "{"remoteModuleConfig":[["AccessibilityInfo",{},["isTouchExplorationEnabled"]],["LocationObserver",{},["getCurrentPosition","startObserving","stopObserving"]],["CameraRollManager",{},["getPhotos","saveToCameraRoll"],[0,1]],["NetInfo",{},["getCurrentConnectivity","isConnectionMetered"],[0,1]],["PlatformConstants",{"ServerHost":"localhost:8081","reactNativeVersion":{"patch":0,"prerelease":null,"minor":51,"major":0},"Version":21,"isTesting":false}],["TimePickerAndroid",{}
}

webSocket首先接收到这个消息, 然后通过worker.postMessage给上面的worker发送‘executeApplicationScript’消息

ws.onmessage = async function(message) {
     ......
      // Special message that asks for a new JS runtime
      if (object.method === "prepareJSRuntime") {
        shutdownJSRuntime();
        console.clear();
        createJSRuntime();
        ws.send(JSON.stringify({replyID: object.id}));
        Page.setState({status: {type: "connected", id: object.id}});
      } else if (object.method === "$disconnected") {
        shutdownJSRuntime();
        Page.setState({status: {type: "disconnected"}});
      } else if (object.method === "executeApplicationScript") {
        worker.postMessage({
          ...object,
          url: await getBlobUrl(object.url),
        });
      } else {
        // Otherwise, pass through to the worker.
        worker.postMessage(object);
      }
    };

worker接收到这个消息在messageHandlers找到相应的处理方法，在里面首选循环取出inject里面的字段和value然后赋值给self，在这里我理解就是这个worker线程的全局对象，然后通过 importScripts(message.url)去加载bundle。

var messageHandlers = {
    "executeApplicationScript": function(message, sendReply) {
      for (var key in message.inject) {
        self[key] = JSON.parse(message.inject[key]);
      }
      var error;
      try {
        importScripts(message.url);
      } catch (err) {
        error = err.message;
      }
      sendReply(null /* result */, error);
    },
    ......
  };

为了证明我上面的分析没错，决定捉包看下发起的请求是不是这样的：

在加载bundle后面还有一个map，体积也很大，有1.74MB的体积，这个是用于映射bundle里面的代码成一个个工程项目里的类文件，这样就和在代码编译器里面调试效果一样了。

根据上面的捉包请求简单总结下建立连接的过程，首先通过/launch-jsdevtools打开调试Tab，浏览器通过/debugger-proxy建立与Server的WebSocket连接，然后浏览器打开index.html文件，发起/debugger-ui/debuggerWorker.js建立后台线程，通过这个后台线程加载bundle。

到这里建立Debug连接的原理分析就差不多了，希望对小伙伴们有帮助，欢迎点赞和关注哈。

谢谢大家！