这是因为autofocus第一次在componentDidMount上触发。要在从B导航回A后手动触发它，您必须使用withNavigationFocus HOC。因此，当用户聚焦屏幕A时，您可以使用以下代码来显示键盘。

import React from 'react';
import { Text } from 'react-native';
import { withNavigationFocus } from 'react-navigation';
class FocusStateLabel extends React.Component {
   componentDidUpdate() {
       if(this.props.isFocused){
           this.inputField.focus();
       }
   }  
   render() {
     return (
       <TextInput
         ref={(input) => { this.inputField = input; }}
         placeholder = "inputField"
       />
     )
  }
}
// withNavigationFocus returns a component that wraps FocusStateLabel and passes
// in the navigation prop
export default withNavigationFocus(FocusStateLabel);

自动对焦 prop 只有在组件被挂载时才会触发。当你导航回A时，如果A仍然被挂载（只是隐藏），那么自动对焦将不会再次工作。
在从B导航回A时，应该使用ref（添加一个新状态，例如此处的ref）并添加一个处理程序来触发this.state.ref.focus()

<TextInput
          ref={ref => ref && this.setState({ref})}
          style={styles.textInput}
          textStyle={styles.inputTextStyle}
          autoFocus={true}
          placeholder='Enter Code'
          keyboard={'numeric'}
          placeholderTextColor={AppStyles.color.grey}
          value={code}
          onChangeText={this.updateCode}
          underline={false}
        />

基于Jarret的注解（V6不赞成使用withNavigationFocus）和此StackOverflow答案（Can't focus TextInput when navigating to a screen）：

import {useFocusEffect} from 'react-navigation/native';
const TestFocus = (props) => {
  const textRef = useRef(null);
  useFocusEffect(
    useCallback(() => {
     // When the screen is focused
     const focus = () => {
      setTimeout(() => {
       textRef?.current?.focus();
      }, 1);
     };
     focus();
     return focus; // cleanup
    }, []),
  );
  return (
   <TextInput
     ref={textRef}
   />
  )
  
}