RN 组件在导出时，命名首字母必须大写，不然报错

例如：

  1. import React from 'react';
  2. import { ViewProps, TouchableOpacity, TextInput, View, Dimensions, Image} from 'react-native';
  4. const {width: viewportWidth, height: viewportHeight} = Dimensions.get('window');
  6. const singleImg = () => {
  7.     return(
  8.         <View>
  9.             <Image
  10.                 style={{ height: 20, width: viewportWidth}}
  11.                 resizeMode='cover'
  12.                 source={{uri: 'https://shop.leyuyouxiang.com/uploads/2020102823235983d782442.jpg'}}
  13.             />
  14.         </View>
  15.     )
  16. }
  17. export default singleImg

默认导出了 singleImg ，当引入这个组件时， 就会报错
编译之后，报错信息如下： 

  1.   Invariant Violation: 
  2. View config getter callback for component `singleImg` must be a 
  3. function (received `undefined`).
  4. Make sure to start component names with a capital letter.

解决办法就是把默认导出的 singleImg 改成 SingleImg 就可以了