usestate 挂钩与计时器在 useeffect 挂钩内无法正常工作

Question

Here I used usestate hook for val

my code is updating val correctly through an extra global variable g when setinterval is calling tick function!!

import React, { useEffect, useState } from 'react';
import { TextInput } from 'react-native';
import { Button } from 'react-native';
import { ImageBackground } from 'react-native';
import { StyleSheet, View, FlatList, Alert, TouchableWithoutFeedback,TouchableOpacity, Keyboard,Text} from 'react-native';
import Icon from 'react-native-vector-icons/AntDesign';
import Slider from '@react-native-community/slider';
import * as Progress from 'react-native-progress';
export default function Status({route,navigation}) {
  let [val,setvalue]=useState(0);
  var g=0;
  useEffect(()=>{
    setTimeout(() => {
      navigation.navigate('Home',{xx:"hhh"});
    }, 5000);
   
  },[]);
  useEffect(() => {
    const x=setInterval(tick, 50);
     return ()=>{
            clearInterval(x);
     }
  },[])
  function tick()
  {
    console.log('hi');
    g=g+0.01;
    setvalue(g);
  }
  const {image,text}=route.params;
  return(
    <View style={styles.container}>
        <ImageBackground source={{uri:image}} imageStyle={styles.image} style={styles.border}>
          <Progress.Bar  progress={val} width={null} />
          <Text  style={styles.input}>{text}</Text>
        </ImageBackground>
    </View>
  );
}
const styles = StyleSheet.create({
    container: {
      flex: 1,
      backgroundColor: 'red'
    },
    image: {
        flex:1,
        resizeMode: 'cover'
      },
      border: {
        flex:1,
        justifyContent:'flex-end'
      },
      input: {
        flex:1,
        height: 40,
        fontSize:20,
        padding:2,
        color:'white',
        //width:null,
        marginTop: 500,
        marginBottom:10,
        borderWidth: 0,
        borderColor:null,
        borderRadius:10,
        alignSelf:'center'
      }
});

but here my code is not updating val when setinterval is calling tick function?? please help!!

import React, { useEffect, useState } from 'react';
import { TextInput } from 'react-native';
import { Button } from 'react-native';
import { ImageBackground } from 'react-native';
import { StyleSheet, View, FlatList, Alert, TouchableWithoutFeedback,TouchableOpacity, Keyboard,Text} from 'react-native';
import Icon from 'react-native-vector-icons/AntDesign';
import Slider from '@react-native-community/slider';
import * as Progress from 'react-native-progress';
export default function Status({route,navigation}) {
  let [val,setvalue]=useState(0);
  
  useEffect(()=>{
    setTimeout(() => {
      navigation.navigate('Home',{xx:"hhh"});
    }, 5500);
   
  },[]);
  useEffect(() => {
    const x=setInterval(tick, 50);
     return ()=>{
            clearInterval(x);
     }
  },[])
  function tick()
  {
    console.log('hi');
    setvalue(val+0.01);
  }
  const {image,text}=route.params;
  return(
    <View style={styles.container}>
        <ImageBackground source={{uri:image}} imageStyle={styles.image} style={styles.border}>
          <Progress.Bar  progress={val} width={null} />
          <Text  style={styles.input}>{text}</Text>
        </ImageBackground>
    </View>
  );
}
const styles = StyleSheet.create({
    container: {
      flex: 1,
      backgroundColor: 'red'
    },
    image: {
        flex:1,
        resizeMode: 'cover'
      },
      border: {
        flex:1,
        justifyContent:'flex-end'
      },
      input: {
        flex:1,
        height: 40,
        fontSize:20,
        padding:2,
        color:'white',
        //width:null,
        marginTop: 500,
        marginBottom:10,
        borderWidth: 0,
        borderColor:null,
        borderRadius:10,
        alignSelf:'center'
      }
});

Here I am using a progress bar and It should update progress automatically for 5 seconds and after I am navigating to another home screen.

I want to know why my second code is not working as expected as from the first code??

Answer 1

setvalue is asynchronous function and because of this value is not set immediately after call. You try to update value based on current value in short range of time which cause the issue. You should use callback passed to setvalue which get you correct previous value event if state will not have time to update yet.

setvalue( prevValue => prevValue + 0.01 )