原生分析的是安卓端

核心代码

//react-native/vendor/core/wehatwg-fetch.js
self.fetch = function(input, init) {
    return new Promise(function(resolve, reject) {
      var request = new Request(input, init);
      var xhr = new XMLHttpRequest();

      xhr.onload = function() {
        var options = {
          status: xhr.status,
          statusText: xhr.statusText,
          headers: parseHeaders(xhr.getAllResponseHeaders() || ''),
        };
        options.url =
          'responseURL' in xhr
            ? xhr.responseURL
            : options.headers.get('X-Request-URL');
        var body = 'response' in xhr ? xhr.response : xhr.responseText;
        resolve(new Response(body, options));
      };

      xhr.onerror = function() {
        reject(new TypeError('Network request failed'));
      };

      xhr.ontimeout = function() {
        reject(new TypeError('Network request failed'));
      };

      xhr.open(request.method, request.url, true);

      if (request.credentials === 'include') {
        xhr.withCredentials = true;
      } else if (request.credentials === 'omit') {
        xhr.withCredentials = false;
      }

      request.headers.forEach(function(value, name) {
        xhr.setRequestHeader(name, value);
      });

      xhr.send(
        typeof request._bodyInit === 'undefined' ? null : request._bodyInit,
      );
    });
  };

// node_modules/react-native/Libraries/Network/XMLHttpRequest.js
send(data: any): void {
    if (this.readyState !== this.OPENED) {
      throw new Error('Request has not been opened');
    }
    if (this._sent) {
      throw new Error('Request has already been sent');
    }
    this._sent = true;
    const incrementalEvents =
      this._incrementalEvents || !!this.onreadystatechange || !!this.onprogress;

    this._subscriptions.push(
      RCTNetworking.addListener('didSendNetworkData', args =>
        this.__didUploadProgress(...args),
      ),
    );
    this._subscriptions.push(
      RCTNetworking.addListener('didReceiveNetworkResponse', args =>
        this.__didReceiveResponse(...args),
      ),
    );
    this._subscriptions.push(
      RCTNetworking.addListener('didReceiveNetworkData', args =>
        this.__didReceiveData(...args),
      ),
    );
    this._subscriptions.push(
      RCTNetworking.addListener('didReceiveNetworkIncrementalData', args =>
        this.__didReceiveIncrementalData(...args),
      ),
    );
    this._subscriptions.push(
      RCTNetworking.addListener('didReceiveNetworkDataProgress', args =>
        this.__didReceiveDataProgress(...args),
      ),
    );
    this._subscriptions.push(
      RCTNetworking.addListener('didCompleteNetworkResponse', args =>
        this.__didCompleteResponse(...args),
      ),
    );

    let nativeResponseType: NativeResponseType = 'text';
    if (this._responseType === 'arraybuffer') {
      nativeResponseType = 'base64';
    }
    if (this._responseType === 'blob') {
      nativeResponseType = 'blob';
    }

    invariant(this._method, 'Request method needs to be defined.');
    invariant(this._url, 'Request URL needs to be defined.');
    RCTNetworking.sendRequest(
      this._method,
      this._trackingName,
      this._url,
      this._headers,
      data,
      /* $FlowFixMe(>=0.78.0 site=react_native_android_fb) This issue was found
       * when making Flow check .android.js files. */
      nativeResponseType,
      incrementalEvents,
      this.timeout,
      this.__didCreateRequest.bind(this),
      this.withCredentials,
    );
  }

// node_modules/react-native/Libraries/Network/RCTNetworking.js
sendRequest(
    method: string,
    trackingName: string,
    url: string,
    headers: Object,
    data: RequestBody,
    responseType: 'text' | 'base64',
    incrementalUpdates: boolean,
    timeout: number,
    callback: (requestId: number) => any,
    withCredentials: boolean,
  ) {
    const body = convertRequestBody(data);
    if (body && body.formData) {
      body.formData = body.formData.map(part => ({
        ...part,
        headers: convertHeadersMapToArray(part.headers),
      }));
    }
    const requestId = generateRequestId();
    RCTNetworkingNative.sendRequest(
      method,
      url,
      requestId,
      convertHeadersMapToArray(headers),
      {...body, trackingName},
      responseType,
      incrementalUpdates,
      timeout,
      withCredentials,
    );
    callback(requestId);
  }

应用到XMLHttpRequest

https://developer.mozilla.org/zh-CN/docs/Web/API/XMLHttpRequest

调用流程

fetch
xhr.send -->
RCTNetworking.android.js
RCTNetworking.sendRequest -->
走到native调用到NetworkingModule
NetworkingModule.sendRequest

取消请求

XMLHttpRequest.js
abort(): void {
    this._aborted = true;
    if (this._requestId) {
      RCTNetworking.abortRequest(this._requestId);
    }
    // only call onreadystatechange if there is something to abort,
    // below logic is per spec
    if (
      !(
        this.readyState === this.UNSENT ||
        (this.readyState === this.OPENED && !this._sent) ||
        this.readyState === this.DONE
      )
    ) {
      this._reset();
      this.setReadyState(this.DONE);
    }
    // Reset again after, in case modified in handler
    this._reset();
  }