how to set Promise data to react material chart-CodePudding

I'm trying to read data from a CSV file and draw a React chart on the UI. I have the following code.

import Paper from "@material-ui/core/Paper";
import {ArgumentAxis, Chart, LineSeries, ValueAxis} from '@devexpress/dx-react-chart-material-ui';
    
const App =  () => {
    let data4 = []
    ReadString().then(a => {
        data4 = a
    }).catch(e => {
        console.log('e =>'   e)
    })
    return (
        <Paper>
            <Chart data={data4}>
                <ArgumentAxis/>
                <ValueAxis/>

                <LineSeries valueField="y" argumentField="x"/>
            </Chart>
        </Paper>
    );}

Since the ReadString() is a Promise I can't directly assign the retuning array to Chart data={}. I was thinking when the data is available it will set data4 variable as I do the a=> data= a in the ReradString() function. It shows an empty chart. What is the issue with the code?

CodePudding user response：

The issue is data4 is just a local variable, not a state variable so updating the variable does not trigger page re-render.

To do it properly, you need to make data4 a state variable.

Also, calling the ReadString() promise in the main function body will cause it to be called each time page re-renders. Instead, use useEffect to call it only on page mount.

Something like this:

import { useEffect, useState } from 'react';

// ...
const [data4, setData4] = useState([]);

useEffect(() => {
  // call `ReadString()` and set `data4` by calling `setData4`
}, []);

// ...