Android 未处理的承诺拒绝：TypeError:网络请求失败expo节点后端

我的expo应用程序正在查询一个节点后端。node express mongo后端工作非常完美，我可以使用来自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

这是我的密码：

api.js

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

import * as React from 'react';
import { Platform, StyleSheet, Text, View, ActivityIndicator } from 'react-native';
import { fetchMeetups } from './constants/api'; 

const instructions = Platform.select({
  ios: `Press Cmd+R to reload,\nCmd+D or shake for dev menu`,
  android: `Double tap R on your keyboard to reload,\nShake or press menu button for dev menu`,
});


class App extends React.Component{
  static defaultProps = {
    fetchMeetups
  }

  state = {
    loading: false,
    meetups: []

  }

  async componentDidMount(){
    this.setState({loading: true});
    const data = await this.props.fetchMeetups();
    setTimeout(() => this.setState({loading: false, meetups: data.meetups}),2000)
  }


  render(){
    if(this.state.loading){
      return(
        <ActivityIndicator size="large"/>
      )
    }
    return (
      <View style={styles.container}>
        <Text>MeetupME</Text>
      </View>
    );
  }

}

const styles = StyleSheet.create({
  container: {
    flex: 1,
    justifyContent: 'center',
    alignItems: 'center',
    backgroundColor: '#F5FCFF',
  },
  welcome: {
    fontSize: 20,
    textAlign: 'center',
    margin: 10,
  },
  instructions: {
    textAlign: 'center',
    color: '#333333',
    marginBottom: 5,
  },
});

import { registerRootComponent } from 'expo';

import App from './App';

// registerRootComponent calls AppRegistry.registerComponent('main', () => App);
// It also ensures that whether you load the app in the Expo client or in a native build,
// the environment is set up appropriately
registerRootComponent(App);

App.js

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

import * as React from 'react';
import { Platform, StyleSheet, Text, View, ActivityIndicator } from 'react-native';
import { fetchMeetups } from './constants/api'; 

const instructions = Platform.select({
  ios: `Press Cmd+R to reload,\nCmd+D or shake for dev menu`,
  android: `Double tap R on your keyboard to reload,\nShake or press menu button for dev menu`,
});


class App extends React.Component{
  static defaultProps = {
    fetchMeetups
  }

  state = {
    loading: false,
    meetups: []

  }

  async componentDidMount(){
    this.setState({loading: true});
    const data = await this.props.fetchMeetups();
    setTimeout(() => this.setState({loading: false, meetups: data.meetups}),2000)
  }


  render(){
    if(this.state.loading){
      return(
        <ActivityIndicator size="large"/>
      )
    }
    return (
      <View style={styles.container}>
        <Text>MeetupME</Text>
      </View>
    );
  }

}

const styles = StyleSheet.create({
  container: {
    flex: 1,
    justifyContent: 'center',
    alignItems: 'center',
    backgroundColor: '#F5FCFF',
  },
  welcome: {
    fontSize: 20,
    textAlign: 'center',
    margin: 10,
  },
  instructions: {
    textAlign: 'center',
    color: '#333333',
    marginBottom: 5,
  },
});

import { registerRootComponent } from 'expo';

import App from './App';

// registerRootComponent calls AppRegistry.registerComponent('main', () => App);
// It also ensures that whether you load the app in the Expo client or in a native build,
// the environment is set up appropriately
registerRootComponent(App);

--------------更新------------------ 添加了一个catch块来获取和获取此新错误

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

---

代替url中的localhost，提供您的ip地址，就像在React native Expo和Python Django后端上有同样的问题一样。 问题是关于仿真器localhost和服务器localhost之间的冲突。 您的后端服务器可能正在127.0.0.1:8000上运行，但模拟器找不到

在终端中，使用命令“ipconfig”查找Ipv4地址。 例如，它将是192.138.1.40

在这之后，把它放进你的取物箱( ''。
还有一点很重要-使用相同的主机和端口运行后端服务器。
例如，在python Django上：

py manage.py运行服务器192.138.1.40:8000

---

在Django上，您还需要在settings.py中添加不起作用的允许的_HOSTS=['192.138.1.40']。localhost:3000/api/meetups是节点进程，现在也在浏览器中…这个地址没有返回json…我仍然在expo emulator apprun中得到未解决的promise错误，这在终端中-adb反向tcp:8163 tcp:8163和tryno没有帮助…我认为fetch有一些问题…因为它没有从fetch获取json对象…它应该获取json对象来自localhost:3000/api/meetups…就像在邮差的情况下，当我发出localhost:3000/api/meetups获取请求时…它在浏览器中也会返回json…所以我认为它在后端部分工作…同时反应部分也正确…唯一不工作的部分。。。。const data=wait this.props.fetchMeetups（）；