React Native Debug原理浅析

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

1.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。

2.建立连接

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：

<body>

  <div class="content">

    <label for="dark">

      <input type="checkbox" id="dark" onclick="Page.toggleDarkTheme()"> Dark Theme

    </label>

    <label for="maintain-priority">

      <input type="checkbox" id="maintain-priority" onclick="Page.togglePriorityMaintenance()"> Maintain Priority

    </label>

    <p>

      React Native JS code runs as a web worker inside this tab.

    </p>

    <p>Press <kbd id="shortcut" class="shortcut">⌘⌥I</kbd> to open Developer Tools. Enable <a href="https://stackoverflow.com/a/17324511/232122" target="_blank">Pause On Caught Exceptions</a> for a better debugging experience.</p>

    <p>You may also install <a href="https://github.com/facebook/react-devtools/tree/master/packages/react-devtools" target="_blank">the standalone version of React Developer Tools</a> to inspect the React component hierarchy, their props, and state.</p>

    <p>Status: <span id="status">Loading...</span></p>

  </div>

</body>

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。

3.加载调试代码

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。

4.总结

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！