componentDidMount 或 componentWillMount 在 React-Native 中不起作用？

Question

Can anyone tell me what I am doing wrong here. I am trying to fetch cities using API but componentDidMount nor componentWillMount is working.

I have tested my getWeather function using button, the function is working but when I try to call the it using componentDidMount or componentWillMount it is not working...

My code below:

import React, { Component } from 'react';
import { StyleSheet, Text, View } from 'react-native';
import Header from './Header';
import { TextInput, Card, Button, Title } from 'react-native-paper';
import { useState } from 'react';
import { FlatList } from 'react-native-gesture-handler';

export default function Home() {

    const [info, setInfo] = useState([{
        city_name: "loading !!",
        temp: "loading",
        humidity: "loading",
        desc: "loading",
        icon: "loading"
    }]);

    getWeather = () => {
        console.log("Hello Weather");
        fetch("https://api.openweathermap.org/data/2.5/find?q=London&units=metric&appid={MY_API_KEY}")
        .then(res => res.json())
        .then(data => {
            console.log(data);
            /*setInfo([{
                city_name: data.name,
                temp: data.main.temp,
                humidity: data.main.humidity,
                desc: data.weather[0].description,
                icon: data.weather[0].icon}]);
                */
        })
    }

    componentWillMount = () => {
        console.log("Hello Will");
        this.getWeather();
    }
    
    componentDidMount = () => {
        console.log("Hello Did");
        this.getWeather();
    }

    return (
        <View style={styles.container}>
          <Header title="Current Weather"/>
          <Card style = {{margin: 20}}>
              <View>
                  <Title>{info.city_name}</Title>
                  <Title>{info.desc}</Title>
              </View>
          </Card>
          <Button onPress={() => this.getWeather()} style={{margin: 40, padding: 20}}>Testing</Button>
        </View>
    );
}

const styles = StyleSheet.create({
  container: {
    flex: 1,
  },
});

Answer 1

componentDidMount and componentWillMount only work in class based React components; what you have here is a functional component. You can use the useEffect hook to accomplish the same.

useEffect(() => {
  getWeather();
}, []);

Note that this does not exist in functional components; you can just call the function directly after you have declared it.

If you haven't used useEffect before, you may have questions about the array as the second argument. If it is empty, it will run on mount and will run what you return from the first argument on unmount. If you want to run your effect again, add an dependencies into the array.

Answer 2

ComponentDidMount would only work in class components. Using the useEffect React hook would achieve the same effect without issues