Axios源码研究学习系列---xhr请求模块大概分析

 改变是人生的定律，专注于过去和现在的人，必将错过未来

首先文件结构，插件标准模式，严格模式，用ES6语法promise返回，所以就是常见的axios.get(url).then(res=>{})形式

'use strict';

var utils = require('./../utils');
var settle = require('./../core/settle');
var buildURL = require('./../helpers/buildURL');
var parseHeaders = require('./../helpers/parseHeaders');
var isURLSameOrigin = require('./../helpers/isURLSameOrigin');
var createError = require('../core/createError');

module.exports = function xhrAdapter(config) {
  return new Promise(function dispatchXhrRequest(resolve, reject) {
     //启动请求
     request.open(config.method.toUpperCase(), buildURL(config.url, config.params,         config.paramsSerializer), true);
     ...
     settle(resolve, reject, response) //给promise返回resolve和reject两种状态
     ...
     // 发送请求
     request.send(requestData);  
  }
}

由于用到promise，那么resolve和reject状态就显得非常重要，我们先看下settle函数 settle(resolve, reject, response)，位于settle.js中

'use strict';

var createError = require('./createError');

/**
 * Resolve or reject a Promise based on response status.
 *
 * @param {Function} resolve A function that resolves the promise.
 * @param {Function} reject A function that rejects the promise.
 * @param {object} response The response.
 */
module.exports = function settle(resolve, reject, response) {
  var validateStatus = response.config.validateStatus;
  if (!validateStatus || validateStatus(response.status)) { //验证响应状态是否通过
    resolve(response);
  } else {
    reject(createError(
      'Request failed with status code ' + response.status,
      response.config,
      null,
      response.request,
      response
    ));
  }
};

settle函数里面对resolve和reject进行了条件判断，在reject时候，有调用createError函数，进行输出。我们使用axios请求接口，不通，控制台会输出相关报错信息就是该函数封装的

我们先看输出resolve的条件，使用了response.config.validateStatus，它到底是什么，它就是defaults.js中一个验证状态码是否是[200,300)区间，表示请求通过

var defaults = {
  ...
  //验证状态码是否处于200-300之间，就是请求通过状态
  validateStatus: function validateStatus(status) {
    return status >= 200 && status < 300;
  }
};

但是为何会response.config.validateStatus这样写，那么分析发现，这是xhr.js文件传参config的配置

module.exports = function xhrAdapter(config) {
  return new Promise(function dispatchXhrRequest(resolve, reject) {
     var response = {
        data: responseData,
        status: request.status,
        statusText: request.statusText,
        headers: responseHeaders,
        config: config, //传进来的参数
        request: request
      };

      settle(resolve, reject, response); 
  }
}

那么我们再简单看下cofig.validateStatus中间做了些什么，还是defaults.js文件

function getDefaultAdapter() {
  var adapter;
  // Only Node.JS has a process variable that is of [[Class]] process
  if (typeof process !== 'undefined' && Object.prototype.toString.call(process) === '[object process]') {
    // For node use HTTP adapter
    adapter = require('./adapters/http');
  } else if (typeof XMLHttpRequest !== 'undefined') {
    // For browsers use XHR adapter
    adapter = require('./adapters/xhr'); //这里引用了xhr.js文件
  }
  return adapter;
}

getDefaultAdapter函数是一个区分node和浏览器两种环境，使用node的http模块或者使用浏览器的XMLHttpRequest，先到这里。继续分析settle文件reject返回结果，里面出现一个createError函数，这是使用axios时候，请求接口报错，会在控制台输出相关信息

'use strict';

var enhanceError = require('./enhanceError');

/**
 * Create an Error with the specified message, config, error code, request and response.
 *
 * @param {string} message The error message.
 * @param {Object} config The config.
 * @param {string} [code] The error code (for example, 'ECONNABORTED').
 * @param {Object} [request] The request.
 * @param {Object} [response] The response.
 * @returns {Error} The created error.
 */
module.exports = function createError(message, config, code, request, response) {
  var error = new Error(message);
  return enhanceError(error, config, code, request, response);
};

createError文件是创建一个由指定信息Error对象，返回enchanceError函数。

JavaScript 原生提供了7种错误类型，分别是 Error, EvalError, SyntaxError, RangeError, ReferenceError, TypeError, 和 URIError。当我们为封装自己的库时候，通常会自定义错误异常提示，自定义异常最好是继承js提供的的Error类，这样便可以通过try catch捕获错误异常

'use strict';

/**
 * Update an Error with the specified config, error code, and response.
 *
 * @param {Error} error The error to update.
 * @param {Object} config The config.
 * @param {string} [code] The error code (for example, 'ECONNABORTED').
 * @param {Object} [request] The request.
 * @param {Object} [response] The response.
 * @returns {Error} The error.
 */
module.exports = function enhanceError(error, config, code, request, response) {
  error.config = config;
  if (code) {
    error.code = code;
  }

  error.request = request;
  error.response = response;
  error.isAxiosError = true;

  error.toJSON = function() {
    return {
      // Standard
      message: this.message,
      name: this.name,
      // Microsoft
      description: this.description,
      number: this.number,
      // Mozilla
      fileName: this.fileName,
      lineNumber: this.lineNumber,
      columnNumber: this.columnNumber,
      stack: this.stack,
      // Axios
      config: this.config,
      code: this.code
    };
  };
  return error;
};

以上是enchanceError文件的代码，功能就是将自定义的异常信息，通过Error类抛出，所以xhr返回就是验证通过的信息或者错误异常提示

axios中浏览器端请求和jQuery等插件请求都是使用XMLHttpRequest对象，整个创建流程差不多，放整个xhr源码先

'use strict';

var utils = require('./../utils');
var settle = require('./../core/settle');
var buildURL = require('./../helpers/buildURL');
var parseHeaders = require('./../helpers/parseHeaders');
var isURLSameOrigin = require('./../helpers/isURLSameOrigin');
var createError = require('../core/createError');

module.exports = function xhrAdapter(config) {
  return new Promise(function dispatchXhrRequest(resolve, reject) {
    var requestData = config.data;
    var requestHeaders = config.headers;

    if (utils.isFormData(requestData)) {
      delete requestHeaders['Content-Type']; // Let the browser set it
    }

    var request = new XMLHttpRequest();

    // HTTP basic authentication
    if (config.auth) {
      var username = config.auth.username || '';
      var password = config.auth.password || '';
      requestHeaders.Authorization = 'Basic ' + btoa(username + ':' + password);
    }

    request.open(config.method.toUpperCase(), buildURL(config.url, config.params, config.paramsSerializer), true);

    // Set the request timeout in MS
    request.timeout = config.timeout;

    // Listen for ready state
    request.onreadystatechange = function handleLoad() {
      if (!request || request.readyState !== 4) {
        return;
      }

      // The request errored out and we didn't get a response, this will be
      // handled by onerror instead
      // With one exception: request that using file: protocol, most browsers
      // will return status as 0 even though it's a successful request
      if (request.status === 0 && !(request.responseURL && request.responseURL.indexOf('file:') === 0)) {
        return;
      }

      // Prepare the response
      var responseHeaders = 'getAllResponseHeaders' in request ? parseHeaders(request.getAllResponseHeaders()) : null;
      var responseData = !config.responseType || config.responseType === 'text' ? request.responseText : request.response;
      var response = {
        data: responseData,
        status: request.status,
        statusText: request.statusText,
        headers: responseHeaders,
        config: config,
        request: request
      };

      settle(resolve, reject, response);

      // Clean up request
      request = null;
    };

    // Handle browser request cancellation (as opposed to a manual cancellation)
    request.onabort = function handleAbort() {
      if (!request) {
        return;
      }

      reject(createError('Request aborted', config, 'ECONNABORTED', request));

      // Clean up request
      request = null;
    };

    // Handle low level network errors
    request.onerror = function handleError() {
      // Real errors are hidden from us by the browser
      // onerror should only fire if it's a network error
      reject(createError('Network Error', config, null, request));

      // Clean up request
      request = null;
    };

    // Handle timeout
    request.ontimeout = function handleTimeout() {
      reject(createError('timeout of ' + config.timeout + 'ms exceeded', config, 'ECONNABORTED',
        request));

      // Clean up request
      request = null;
    };

    // Add xsrf header
    // This is only done if running in a standard browser environment.
    // Specifically not if we're in a web worker, or react-native.
    if (utils.isStandardBrowserEnv()) {
      var cookies = require('./../helpers/cookies');

      // Add xsrf header
      var xsrfValue = (config.withCredentials || isURLSameOrigin(config.url)) && config.xsrfCookieName ?
        cookies.read(config.xsrfCookieName) :
        undefined;

      if (xsrfValue) {
        requestHeaders[config.xsrfHeaderName] = xsrfValue;
      }
    }

    // Add headers to the request
    if ('setRequestHeader' in request) {
      utils.forEach(requestHeaders, function setRequestHeader(val, key) {
        if (typeof requestData === 'undefined' && key.toLowerCase() === 'content-type') {
          // Remove Content-Type if data is undefined
          delete requestHeaders[key];
        } else {
          // Otherwise add header to the request
          request.setRequestHeader(key, val);
        }
      });
    }

    // Add withCredentials to request if needed
    if (config.withCredentials) {
      request.withCredentials = true;
    }

    // Add responseType to request if needed
    if (config.responseType) {
      try {
        request.responseType = config.responseType;
      } catch (e) {
        // Expected DOMException thrown by browsers not compatible XMLHttpRequest Level 2.
        // But, this can be suppressed for 'json' type as it can be parsed by default 'transformResponse' function.
        if (config.responseType !== 'json') {
          throw e;
        }
      }
    }

    // Handle progress if needed
    if (typeof config.onDownloadProgress === 'function') {
      request.addEventListener('progress', config.onDownloadProgress);
    }

    // Not all browsers support upload events
    if (typeof config.onUploadProgress === 'function' && request.upload) {
      request.upload.addEventListener('progress', config.onUploadProgress);
    }

    if (config.cancelToken) {
      // Handle cancellation
      config.cancelToken.promise.then(function onCanceled(cancel) {
        if (!request) {
          return;
        }

        request.abort();
        reject(cancel);
        // Clean up request
        request = null;
      });
    }

    if (requestData === undefined) {
      requestData = null;
    }

    // Send the request
    request.send(requestData);
  });
};

这里面有几个函数，我觉得应该解释下

isFormData：用来判断是否是FormData对象

FormData对象用以将数据编译成键值对，以便用XMLHttpRequest来发送数据。其主要用于发送表单数据，但亦可用于发送带键数据(keyed data)，而独立于表单使用。如果表单enctype属性设为multipart/form-data ，则会使用表单的submit()方法来发送数据，从而，发送数据具有同样形式。

/**
 * Determine if a value is a FormData
 *
 * @param {Object} val The value to test
 * @returns {boolean} True if value is an FormData, otherwise false
 */
function isFormData(val) {
  return (typeof FormData !== 'undefined') && (val instanceof FormData);
}

buildURL(config.url, config.params, config.paramsSerializer)：创建URL函数，简单的讲就是对传进来的URL做标准化处理，axios很多时候都是使用传pram对象方式，这个buildURL就是帮你把参数转换成合法的URL接口，放代码

'use strict';

var utils = require('./../utils');

function encode(val) {
  return encodeURIComponent(val).
    replace(/%40/gi, '@').
    replace(/%3A/gi, ':').
    replace(/%24/g, '$').
    replace(/%2C/gi, ',').
    replace(/%20/g, '+').
    replace(/%5B/gi, '[').
    replace(/%5D/gi, ']');
}

/**
 * Build a URL by appending params to the end
 *
 * @param {string} url The base of the url (e.g., http://www.google.com)
 * @param {object} [params] The params to be appended
 * @returns {string} The formatted url
 */
module.exports = function buildURL(url, params, paramsSerializer) {
  /*eslint no-param-reassign:0*/
  if (!params) {
    return url;
  }

  var serializedParams;
  if (paramsSerializer) {
    serializedParams = paramsSerializer(params);
  } else if (utils.isURLSearchParams(params)) {
    serializedParams = params.toString();
  } else {
    var parts = [];

    utils.forEach(params, function serialize(val, key) {
      if (val === null || typeof val === 'undefined') {
        return;
      }

      if (utils.isArray(val)) {
        key = key + '[]';
      } else {
        val = [val];
      }

      utils.forEach(val, function parseValue(v) {
        if (utils.isDate(v)) {
          v = v.toISOString();
        } else if (utils.isObject(v)) {
          v = JSON.stringify(v);
        }
        parts.push(encode(key) + '=' + encode(v));
      });
    });

    serializedParams = parts.join('&');
  }

  if (serializedParams) {
    var hashmarkIndex = url.indexOf('#');
    if (hashmarkIndex !== -1) {
      url = url.slice(0, hashmarkIndex);
    }

    url += (url.indexOf('?') === -1 ? '?' : '&') + serializedParams;
  }

  return url;
};

首先判断是否有配置的参数序列化器，有就是用配置的序列化参数，接着用isURLSearchParams判断参数是否属于URLSearchParams对象，如果存在，那么就说明不需要进行序列化，接下来才是对参数进行遍历，过滤替换一些字符，拼接

/**
 * Determine if a value is a URLSearchParams object
 *
 * @param {Object} val The value to test
 * @returns {boolean} True if value is a URLSearchParams object, otherwise false
 */
function isURLSearchParams(val) {
  return typeof URLSearchParams !== 'undefined' && val instanceof URLSearchParams;
}

/************
*URLSearchParams接口定义了与URL的查询字符串一起使用的实用程序方法。
*对象实现URLSearchParams可以直接在for...of结构中使用
******************/

var paramsString = "q=URLUtils.searchParams&topic=api";
var searchParams = new URLSearchParams(paramsString);

//Iterate the search parameters.
for (let p of searchParams) {
  console.log(p);
}

searchParams.has("topic") === true; // true
searchParams.get("topic") === "api"; // true
searchParams.getAll("topic"); // ["api"]
searchParams.get("foo") === null; // true
searchParams.append("topic", "webdev");
searchParams.toString(); // "q=URLUtils.searchParams&topic=api&topic=webdev"
searchParams.set("topic", "More webdev");
searchParams.toString(); // "q=URLUtils.searchParams&topic=More+webdev"
searchParams.delete("topic");
searchParams.toString(); // "q=URLUtils.searchParams"

xhr.js大概粗略的分析完了，该模块主要功能是创建一个请求，最最重要是，axios发起的是异步请求，里面涵盖了传参，参数处理，请求错误异常的自定义提示