未处理的承诺拒绝：TypeError：网络请求失败的博览会节点后端

Question

我有一个我的 expo 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());

App.js

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,
  },
});

index.js

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 块来获取并获取这个新错误

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

Answer 1

在 React-native Expo 和 Python Django 后端有同样的问题。 问题是关于模拟器本地主机和服务器本地主机之间的冲突。 您的后端服务器可能在 127.0.0.1:8000 上运行，但模拟器找不到。

在终端中使用命令“ipconfig”找到您的 IPv4 地址。 例如，它将是 192.138.1.40

在此之后将其放入您的 fetch ( 'http://192.138.1.40:8000/')。
还有一点也很重要 - 使用相同的主机和端口运行您的后端服务器。
以 python Django 为例：

py manage.py runserver 192.138.1.40:8000

在 Django 上，您还需要在 settings.py 中添加 ALLOWED_HOSTS = ['192.138.1.40']

Answer 2

在阅读了有关此问题的几个小时后，我发现这是 Expo 中的一个错误

要返回，我清除了 Android 上的 Expo 缓存

Answer 3

在您的 url 中代替 localhost，http://localhost:3000/api/meetups，提供您的 IP 地址，例如 http://192.168.49.108:3000/api/meetups