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[英]TypeError: undefined is not an object (evaluating 'this.state')
[英]react-native : TypeError:undefined is not an object (evaluating 'this.state')
我是react-native的新手,學習完整代碼。 但我無法理解導出前和渲染后“const”之間的區別。 例如:
const { height, width } = Dimensions.get("window");
export default class App extends React.Component {
state = {
newToDo: ""
};
render() {
const { newToDo } = this.state;
為什么我問這個是因為我的第一個 init 不是“export default class App extends React.Component {”而是“export default function App() {”。 所以我不能分配 const 或分配它,它會導致顯示消息的錯誤
TypeError:undefined 不是 object(評估 'this.state')
這是我的代碼:
import React from "react";
import {
StyleSheet,
Text,
View,
Dimensions,
TextInput,
Platform,
ScrollView
} from "react-native";
import ToDo from "./ToDo";
const { height, width } = Dimensions.get("window");
export default function App() {
const state = {
newToDo: ""
};
const { newToDO } = this.state;
return (
<View style={styles.container}>
<View style={styles.titleContainer}>
<Text style={styles.title}>Good or Bad</Text>
<Text style={styles.subTitle}>Check Daily your habit</Text>
</View>
<View style={styles.content}>
<Text style={styles.contentTitle}>To Do List</Text>
<TextInput
style={styles.input}
placeholder={"New to do"}
value={newToDO}
onChangeText={this._controllNewToDo}
returnKeyType={"done"}
autoCorrect={false}
/>
<ScrollView>
<ToDo />
</ScrollView>
</View>
</View>
);
_controllNewToDo = text => {
this.setState({
newToDO: text
});
};
}
在功能組件中,您使用 useState 掛鈎來管理 state。試試這個,
import React, { useState } from "react";
import {
StyleSheet,
Text,
View,
Dimensions,
TextInput,
Platform,
ScrollView
} from "react-native";
import ToDo from "./ToDo";
const { height, width } = Dimensions.get("window");
export default function App() {
const [newToDo, setNewToDo] = useState("");
return (
<View style={styles.container}>
<View style={styles.titleContainer}>
<Text style={styles.title}>Good or Bad</Text>
<Text style={styles.subTitle}>Check Daily your habit</Text>
</View>
<View style={styles.content}>
<Text style={styles.contentTitle}>To Do List</Text>
<TextInput
style={styles.input}
placeholder={"New to do"}
value={newToDo}
onChangeText={_controllNewToDo}
returnKeyType={"done"}
autoCorrect={false}
/>
<ScrollView>
<ToDo />
</ScrollView>
</View>
</View>
);
const _controllNewToDo = text => {
setNewToDo(text);
};
}
您不能在功能組件中使用this.setState({})
。 所以你應該使用一個普通的 class 組件來調用this.setState
或使用 Hooks 來更新功能組件內部的 state 。
import React, {Component} from "react";
import {
StyleSheet,
Text,
View,
Dimensions,
TextInput,
Platform,
ScrollView
} from "react-native";
import ToDo from "./ToDo";
const { height, width } = Dimensions.get("window");
class App extends Component {
state = {
newToDo: ""
};
_controllNewToDo = text => {
this.setState({
newToDO: text
});
};
render(){
const { newToDO } = this.state;
return (
<View style={styles.container}>
<View style={styles.titleContainer}>
<Text style={styles.title}>Good or Bad</Text>
<Text style={styles.subTitle}>Check Daily your habit</Text>
</View>
<View style={styles.content}>
<Text style={styles.contentTitle}>To Do List</Text>
<TextInput
style={styles.input}
placeholder={"New to do"}
value={newToDO}
onChangeText={this._controllNewToDo}
returnKeyType={"done"}
autoCorrect={false}
/>
<ScrollView>
<ToDo />
</ScrollView>
</View>
</View>
);
}
}
export default App;
試試這個代碼!
正如我在下面提到的,如果您需要使用state和setState({}) ,您應該在 class 組件內部使用。 否則你應該使用Hooks ,檢查這個。我認為這會幫助你理解。
import React from 'react';
import {
StyleSheet,
Text,
View,
Dimensions,
TextInput,
Platform,
ScrollView,
} from 'react-native';
const { height, width } = Dimensions.get('window');
export default class App extends React.Component {
state = {
newToDo: 'wwe',
};
_controllNewToDo = text => {
this.setState({
newToDO: text,
});
};
render() {
const { newToDO } = this.state;
return (
<View>
<View>
<Text>Good or Bad</Text>
<Text>Check Daily your habit</Text>
</View>
<View>
<Text>To Do List</Text>
<TextInput
style={{ height: 60 }}
placeholder={'New to do'}
value={newToDO}
onChangeText={this._controllNewToDo}
returnKeyType={'done'}
autoCorrect={false}
/>
<ScrollView
style={{ justifyContent: 'center', alignItems: 'center' }}>
<Text>{newToDO}</Text>
</ScrollView>
</View>
</View>
);
}
}
有疑問請隨意
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