Add items to FlatList dynamically in React Native

Question

I have a FlatList with two items. I need to append this list with another elements. When the user clicks on the button, the data from the text inputs should appear in the end of the FlatList. So, I've tried to push data object to the end of the list's array, but new item replaces the last one.

import React, { useState } from 'react';
import { Text, View, StyleSheet, Button } from 'react-native';
import { FlatList } from 'react-native-gesture-handler';

export default function HomeScreen() {

  var initialElements = [
    { id : "0", text : "Object 1"},
    { id : "1", text : "Object 2"},
  ]

  const [exampleState, setExampleState] = useState(initialElements);
  const [idx, incr] = useState(2);

  const addElement = () => {
    var newArray = [...initialElements , {id : toString(idx), text: "Object " + (idx+1) }];
    initialElements.push({id : toString(idx), text: "Object " + (idx+1) });
    incr(idx + 1);
    setExampleState(newArray);
  }

  return (
    <View style={styles.container}>
        <FlatList
            keyExtractor = {item => item.id}  
            data={exampleState}
            renderItem = {item => (<Text>{item.item.text}</Text>)} />
        <Button
          title="Add element"
          onPress={addElement} />
    </View>
  );
}

const styles = StyleSheet.create({
  container: {
    flex: 1,
    backgroundColor: '#fff',
    width: '100%',
    borderWidth: 1
  },
});

Answer 1

import React, { useState } from 'react';
import { Text, View, StyleSheet, Button } from 'react-native';
import { FlatList } from 'react-native-gesture-handler';

export default function HomeScreen() {

  var initialElements = [
    { id : "0", text : "Object 1"},
    { id : "1", text : "Object 2"},
  ]

  const [exampleState, setExampleState] = useState(initialElements)

  const addElement = () => {
    var newArray = [...initialElements , {id : "2", text: "Object 3"}];
    setExampleState(newArray);
  }

  return (
    <View style={styles.container}>
        <FlatList
            keyExtractor = {item => item.id}  
            data={exampleState}
            renderItem = {item => (<Text>{item.item.text}</Text>)} />
        <Button
          title="Add element"
          onPress={addElement} />
    </View>
  );
}

const styles = StyleSheet.create({
  container: {
    flex: 1,
    backgroundColor: '#fff',
    width: '100%',
    borderWidth: 1
  },
});

You are just changing the listElements array. This will NOT trigger the re rendering of the component and hence the flat list will be unchanged.

Create a state variable in the component and store your data in that so that any modification results in re rendering.

Answer 2

I fixed the problem of replacing elements by changing array into a state variable.

import React, { useState } from 'react';
import { Text, View, StyleSheet, Button } from 'react-native';
import { FlatList } from 'react-native-gesture-handler';

export default function HomeScreen() {

  const [initialElements, changeEl]  = useState([
    { id : "0", text : "Object 1"},
    { id : "1", text : "Object 2"},
  ]);

  const [exampleState, setExampleState] = useState(initialElements);
  const [idx, incr] = useState(2);

  const addElement = () => {
    var newArray = [...initialElements , {id : idx, text: "Object " + (idx+1) }];
    incr(idx + 1);
    console.log(initialElements.length);
    setExampleState(newArray);
    changeEl(newArray);
  }

  return (
    <View style={styles.container}>
        <FlatList
            keyExtractor = {item => item.id}  
            data={exampleState}
            renderItem = {item => (<Text>{item.item.text}</Text>)} />
        <Button
          title="Add element"
          onPress={addElement} />
    </View>
  );
}

const styles = StyleSheet.create({
  container: {
    flex: 1,
    backgroundColor: '#fff',
    width: '100%',
    borderWidth: 1
  },
});