如何在tabNavigator (React Native)中的if语句中调用驻留在父屏幕中的函数

// ParentScreen.js

import React from 'react';
import { View, Text } from 'react-native';

export default class ParentScreen extends React.Component {
  parentFunction() {
    console.log('This is the parent function');
  }

  render() {
    return (
      <View>
        <Text>This is the parent screen</Text>
      </View>
    );
  }
}

// ChildScreen.js

import React from 'react';
import { View, Text, Button } from 'react-native';
import { createStackNavigator } from 'react-navigation';

import ParentScreen from './ParentScreen';

class ChildScreen extends React.Component {
  callParentFunction() {
    const parentFunction = this.props.navigation.getParam('parentFunction');
    parentFunction();
  }

  render() {
    return (
      <View>
        <Text>This is the child screen</Text>
        <Button title="Call Parent Function" onPress={() => this.callParentFunction()} />
      </View>
    );
  }
}

const AppNavigator = createStackNavigator({
  Child: {
    screen: ChildScreen,
  },
});

export default AppNavigator;