Axios源码笔记 | Adapters 适配器系统深度解析，多环境HTTP请求的工程实践

export default {
  getAdapter: (adapters) => {
    // 参数标准化
    adapters = utils.isArray(adapters) ? adapters : [adapters];

    // 核心选择逻辑
    const {length} = adapters;
    let nameOrAdapter;
    let adapter;
    const rejectedReasons = {};

    // 遍历适配器列表
    for (let i = 0; i < length; i++) {
      // ...选择逻辑...
    }

    // 错误处理
    if (!adapter) {
      // ...构造错误信息...
      throw new AxiosError(...);
    }

    return adapter;
  }
}

adapters = utils.isArray(adapters) ? adapters : [adapters];

for (let i = 0; i < length; i++) {
  nameOrAdapter = adapters[i];
  let id;
  
  adapter = nameOrAdapter;

  if (!isResolvedHandle(nameOrAdapter)) {
    adapter = knownAdapters[(id = String(nameOrAdapter)).toLowerCase()];
    
    if (adapter === undefined) {
      throw new AxiosError(`Unknown adapter '${id}'`);
    }
  }

  if (adapter) {
    break;
  }

  rejectedReasons[id || '#' + i] = adapter;
}

const reasons = Object.entries(rejectedReasons)
  .map(([id, state]) => `adapter ${id} ` +
    (state === false ? 
     'is not supported by the environment' : 
     'is not available in the build')
  );

let s = length ? 
  (reasons.length > 1 ? 
   'since :\n' + reasons.map(renderReason).join('\n') : 
   ' ' + renderReason(reasons[0])) : 
  'as no adapter specified';

throw new AxiosError(
  `There is no suitable adapter to dispatch the request ` + s,
  'ERR_NOT_SUPPORT'
);

Error: There is no suitable adapter to dispatch the request since:
- adapter http is not supported by the environment
- adapter fetch is not available in the build

const _config = resolveConfig(config);
let requestData = _config.data;
const requestHeaders = AxiosHeaders.from(_config.headers).normalize();

let request = new XMLHttpRequest();
request.open(_config.method.toUpperCase(), _config.url, true);
request.timeout = _config.timeout;

const responseHeaders = AxiosHeaders.from(
  'getAllResponseHeaders' in request && request.getAllResponseHeaders()
);
const responseData = !responseType || responseType === 'text' || responseType === 'json' ?
  request.responseText : request.response;

// 请求流处理
data = trackStream(_request.body, DEFAULT_CHUNK_SIZE, onProgress, flush);

// 响应流处理
response = new Response(trackStream(response.body...))

composeSignals([signal, cancelToken..., timeout])

const supportsRequestStream = ... // 通过构造测试请求检测duplex支持

const getBodyLength = async (body) => {
  // 处理8种不同数据类型的长度计算
}

// Cloudflare Workers特殊处理
const isCredentialsSupported = "credentials" in Request.prototype;
credentials: isCredentialsSupported ? ...

const resolvers = {
  stream: ...,
  text: ...,
  arrayBuffer: ...
}

throw AxiosError.from(err...)

await Promise.all([
  resolveBodyLength(),
  prepareRequestStream()
])

const streams = [res];
streams.push(transformStream);
responseStream = stream.pipeline(streams, utils.noop);

composeSignals([config.cancelToken, config.signal, timeout])

// FormData处理
data = formDataToStream(data, headerSetter);

// Blob处理
data = stream.Readable.from(readBlob(data));

new AxiosTransformStream({ maxRate })

switch(res.headers['content-encoding']) {
  case 'gzip': 
    streams.push(zlib.createUnzip());
}

const transport = followRedirects(https);

config.maxBodyLength = Infinity; // 解除默认限制

agents: { 
  http: config.httpAgent,
  https: config.httpsAgent 
}

if (data.length > config.maxBodyLength) {
  throw new AxiosError();
}

utils.stripBOM(responseData);

if (protocol === 'data:') { ... }