Show View when scroll up Scrollview

Question

How to limit the quantity of View inside of a scrollview.

My component take too much time to render, because the map function renders too many views. I need to show only 10 views, and when scroll up, renders more 10.

I'm using react native, hooks and typescript.

Answer 1

First of all, if you have a large number of list data don't use scrollview. Because initially, it loads all the data to scrollview component & it costs performance as well.

Use flatlist in react-native instead of scrollview & you can limit the number of items to render in the initially using initialNumToRender. When you reach the end of the scroll position you can call onEndReached method to load more data.

A sample will like this

import React, { Component } from "react";
import { View, Text, FlatList, ActivityIndicator } from "react-native";
import { List, ListItem, SearchBar } from "react-native-elements";

class FlatListDemo extends Component {
  constructor(props) {
    super(props);

    this.state = {
      loading: false,
      data: [],
      page: 1,
      seed: 1,
      error: null,
      refreshing: false
    };
  }

  componentDidMount() {
    this.makeRemoteRequest();
  }

  makeRemoteRequest = () => {
    const { page, seed } = this.state;
    const url = `https://randomuser.me/api/?seed=${seed}&page=${page}&results=20`;
    this.setState({ loading: true });

    fetch(url)
      .then(res => res.json())
      .then(res => {
        this.setState({
          data: page === 1 ? res.results : [...this.state.data, ...res.results],
          error: res.error || null,
          loading: false,
          refreshing: false
        });
      })
      .catch(error => {
        this.setState({ error, loading: false });
      });
  };

  handleRefresh = () => {
    this.setState(
      {
        page: 1,
        seed: this.state.seed + 1,
        refreshing: true
      },
      () => {
        this.makeRemoteRequest();
      }
    );
  };

  handleLoadMore = () => {
    this.setState(
      {
        page: this.state.page + 1
      },
      () => {
        this.makeRemoteRequest();
      }
    );
  };

  renderSeparator = () => {
    return (
      <View
        style={{
          height: 1,
          width: "86%",
          backgroundColor: "#CED0CE",
          marginLeft: "14%"
        }}
      />
    );
  };

  renderHeader = () => {
    return <SearchBar placeholder="Type Here..." lightTheme round />;
  };

  renderFooter = () => {
    if (!this.state.loading) return null;

    return (
      <View
        style={{
          paddingVertical: 20,
          borderTopWidth: 1,
          borderColor: "#CED0CE"
        }}
      >
        <ActivityIndicator animating size="large" />
      </View>
    );
  };

  render() {
    return (
      <List containerStyle={{ borderTopWidth: 0, borderBottomWidth: 0 }}>
        <FlatList
          data={this.state.data}
          renderItem={({ item }) => (
            <ListItem
              roundAvatar
              title={`${item.name.first} ${item.name.last}`}
              subtitle={item.email}
              avatar={{ uri: item.picture.thumbnail }}
              containerStyle={{ borderBottomWidth: 0 }}
            />
          )}
          keyExtractor={item => item.email}
          ItemSeparatorComponent={this.renderSeparator}
          ListHeaderComponent={this.renderHeader}
          ListFooterComponent={this.renderFooter}
          onRefresh={this.handleRefresh}
          refreshing={this.state.refreshing}
          onEndReached={this.handleLoadMore}
          onEndReachedThreshold={50}
        />
      </List>
    );
  }
}

export default FlatListDemo;

Check this for more informations.