javascript - Undefined 不是一个对象(评估 'this. state.input' )

Question

我正在尝试从 json 文件读取 key 并在文本字段中显示其值。用户将在 TextInput 字段中输入键值。以下是我使用的代码。输入文本并按下提交按钮后，它会抛出“未定义不是对象(评估'this.state.input')”错误。我认为将值绑定(bind)/传递给 showMeaning() 函数存在一些问题。请帮忙。

import React, {Component} from 'react';
import {AppRegistry, StyleSheet, Text, TextInput, View} from 'react-native';

var english_german = 'english_german.json';
class Dictionary extends Component {
  constructor(props) {
    super(props);
    this.state = {
      input: '',
      output: ''
    };
  }

  showMeaning() {
    var meaning = this.state.input in english_german ? english_german[this.state.input] : "Not Found";
    this.setState({
      output: meaning
    });
  }

  render() {
    return (
      <View style={styles.parent}>
        <Text>
          Type something in English:
        </Text>
        <TextInput value={this.state.input}
          onChangeText={(input) => this.setState({ input }) }
          onSubmitEditing = {this.showMeaning}
          />
        <Text style={styles.germanLabel}>
          Its German equivalent is:
        </Text>
        <Text style={styles.germanWord}>
          {this.state.output}
        </Text>
      </View>
    );
  }
};

var styles = StyleSheet.create({
  parent: {
    padding: 16
  },
  germanLabel: {
    marginTop: 20,
    fontWeight: 'bold'
  },
  germanWord: {
    marginTop: 15,
    fontSize: 30,
    fontStyle: 'italic'
  }
});

AppRegistry.registerComponent('Dictionary', function () {
  return Dictionary;
})

Answer 1

当您调用 showMeaning 时，它未绑定(bind)到正确的上下文 (this)。你应该bind它或使用具有词法 this 的箭头函数，例如:

onSubmitEditing = {() => this.showMeaning()}