-React Native 中的应用开发- 您好， 我遇到了 Expo Camera 问题。当您想要拍照时，会出现错误。

“TypeError：undefined 不是对象（评估“_this.camera = _ref”）”/Scan.js。

如果应用程序刚刚使用 Expo 更新，则一切正常。但是，只要您继续编程并出现另一个错误，就会出现此错误，并且直到您再次刷新应用程序才会消失。

我尝试了很多方法，但我需要帮助。

Scan.js

import React, { Component, useState, useEffect } from 'react';
import { View, Text, StyleSheet, TouchableOpacity, Image } from 'react-native';
import {launchCamera, launchImageLibrary} from 'react-native-image-picker';
import {Camera, Constants} from 'expo-camera';
import * as MediaLibrary from 'expo-media-library';
import * as Haptics from 'expo-haptics';


import Images from '../assets/icon/index'



const Scan = () => {
  const [hasPermission, setHasPermission] = useState(null);
  const [type, setType] = useState(Camera.Constants.Type.back);
  const [status, requestPermission] = MediaLibrary.usePermissions();
  


  useEffect(() => {
    (async () => {
      const { status } = await Camera.requestCameraPermissionsAsync();
      setHasPermission(status === 'granted');
    })();
  }, []);

  if (hasPermission === null) {
    return <View/>;
  }
  if (hasPermission === false) {
    return <Text>No access to camera</Text>;
  }
  
  takePicture = async () => {
    if (this.camera) {
      let photo = await this.camera.takePictureAsync();

      Haptics.impactAsync(Haptics.ImpactFeedbackStyle.Medium);

      console.log(photo.uri);
      MediaLibrary.saveToLibraryAsync(photo.uri);
    }
  };

  
  
  
  

  
  return (
    <View style={styles.container}>
      <Camera style={styles.camera}
        type={type}
        ref={ref => {
          this.camera = ref;
        }}>
        <View style={styles.buttonContainer}>
          <TouchableOpacity
            style={styles.button}
            onPress={() => {
              setType(
                type === Camera.Constants.Type.back
                  ? Camera.Constants.Type.front
                  : Camera.Constants.Type.back
              );
            }}
            >
            <Image source={Images.camera} style={styles.icon}></Image>
          </TouchableOpacity>
          <TouchableOpacity
          style={styles.button}
          onPress={takePicture}
          >
            <Text style={styles.text}>Take</Text>
          </TouchableOpacity>
        </View>
      </Camera>
    </View>
  );
}

const styles = StyleSheet.create({
  container: {
    flex: 1,
  },
  camera: {
    flex: 1,
  },
  buttonContainer: {
    flex: 1,
    backgroundColor: 'transparent',
    flexDirection: 'row',
    margin: 20,
    top: 0,
  },
  button: {
    flex: 0.1,
    alignSelf: 'flex-end',
    alignItems: 'center',
  },
  text: {
    fontSize: 18,
    color: 'white',
  },
  icon : {
    tintColor: 'white',
  },
})

export default Scan; ```