Displaying lists with FlatList (renderItem & keyExtractor props) in React Native

In this tutorial, we'll se how to use the FlastList component in React Native to display a virtualized list of the data fetched in the previous part. We'll also see how to use keyExtractor to add unique keys for each list item.

React Native provides UI components like FlatList or SectionList for efficiently displaying the lists of data.

FlatList is an implementation of VirtualizedList which makes virtualization for improving the memory consumption and performance of large lists by maintaining a finite render window of active items and replacing all items outside of the render window with appropriately sized blank space.

According to the docs:

The FlatList component displays a scrolling list of changing, but similarly structured, data. FlatList works well for long lists of data, where the number of items might change over time. Unlike the more generic ScrollView, the FlatList only renders elements that are currently showing on the screen, not all the elements at once.

You need to pass the data and renderItem props to FlatList:

• data represents the source of data that will be displayed in the list,
• renderItem takes one item from the source and returns a formatted component to render.

First import the FlatList component:

import {
  View,
  Text,
  Image,
  ImageBackground,
  FlatList
} from 'react-native';

Next, change the HomeScreen component as follows:

const HomeScreen = (props) => {
  console.log("articles: ", props.articles);
  return (
    <View>
        <FlatList
          data={ props.articles }
  renderItem={({item}) => <Text> {item.title}</Text>}
        />
    </View>
  );
}

This is a screenshot of the UI:

Instead of displaying only the title of the article, let’s also display the description and encapsulate them inside their own component that will be rendered for each item.

Let’s create the ArticleItem component that will be rendered for each item:

const ArticleItem = ({article}) => {
  const { description, title } = article;
  return (
    <View>
      <Text>
        { title }
      </Text>
      <Text>
        { description }
      </Text>
    </View>
  )
}

The article will be passed as a prop. Using destructuring assignment, we unpack the title, and description properties from the article object. Next, we use a <Text> component for displaying them.

Read the reference for more info about FlatList.

Using unique keys with keyExtractor

You can see that FlatList throws this warning:

VirtualizedList: missing keys for items, make sure to specify a key property on each item or provide a custom keyExtractor.

This simply says that we need to specify a unique key for each item in the list. We can use a custom KeyExtractor for the FlatList as follows:

        <FlatList 
          data={ props.articles }
          keyExtractor={(item, index) => index.toString()}
  renderItem={({item}) => <ArticleItem article = { item }  />}
        />

See this link for more information.

Now, the warning should disappear.

• Date: 09 Aug 2019