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我有一个我的博览会应用程序正在查询的节点后端。node-express-mongo 后端工作得非常完美，我可以使用 Postman 的 GET 请求进行验证，但我的应用程序中出现未处理的承诺拒绝网络失败错误

完全错误：

[Unhandled promise rejection: TypeError: Network request failed]
- node_modules/whatwg-fetch/dist/fetch.umd.js:473:29 in xhr.onerror
- node_modules/event-target-shim/dist/event-target-shim.js:818:39 in EventTarget.prototype.dispatchEvent
- node_modules/react-native/Libraries/Network/XMLHttpRequest.js:574:29 in setReadyState
- node_modules/react-native/Libraries/Network/XMLHttpRequest.js:388:25 in __didCompleteResponse
- node_modules/react-native/Libraries/vendor/emitter/EventEmitter.js:190:12 in emit
- node_modules/react-native/Libraries/BatchedBridge/MessageQueue.js:436:47 in __callFunction
- node_modules/react-native/Libraries/BatchedBridge/MessageQueue.js:111:26 in __guard$argument_0
- node_modules/react-native/Libraries/BatchedBridge/MessageQueue.js:384:10 in __guard
- node_modules/react-native/Libraries/BatchedBridge/MessageQueue.js:110:17 in __guard$argument_0
* [native code]:null in callFunctionReturnFlushedQueue

这是我的代码：

api.js

export const fetchMeetups = () =>
    fetch('http://localhost:3000/api/meetups')
        .then(res => res.json());

应用程序.js

import * as React from 'react';
import { Platform, StyleSheet, Text, View, …

我第一次尝试阿波罗。我的后端服务器是 Phoenix 框架（长生不老药）。并在http://localhost:4000/api 中运行 所以我尝试在我的代码中使用 apollo 进行第一个查询。

import React from 'react';
import { StyleSheet, Text, View } from 'react-native';
import ApolloClient from 'apollo-boost';
import { gql } from 'apollo-boost';
const client = new ApolloClient(
  {
    uri: 'http://localhost:4000/api',
  }
);
client.query({
   query: gql`
     { 
        users {
          name
          email
        } 
     `}
}).then(result => console.log(result));

但我有一个错误。

[Network error]: TypeError: Network request failed

    node_modules/expo/build/logs/LogSerialization.js:166:14 in _captureConsoleStackTrace
    node_modules/expo/build/logs/LogSerialization.js:41:24 in serializeLogDataAsync
    ... 9 more stack frames from framework internals

[Unhandled promise rejection: Error: Network error: …