Problem with react native animation onScroll using reanimated, redash

Question

Hello I am trying to implement animation in React Native. When I scroll up from scrollview, I want one of children in scrollview which is buttonContainer fade out. So, When I start to scroll up, buttonContainer's opacity may be changing from 1 to 0. But, nothing is happening. I don't fully understand what to put input ranges and output ranges.

Here is code in Snack

and here also

import * as React from 'react';
import { Text, View, StyleSheet, Dimensions, Button } from 'react-native';
import Animated from 'react-native-reanimated';
import { onScroll } from 'react-native-redash';

const { height } = Dimensions.get('window');
const BUTTON_CONTAINER_HEIGHT = height / 1.7;
const { Value, interpolate, Extrapolate } = Animated;

export default class App extends React.Component {
  render() {
    const scrollY = new Value(0);
    const opacity = interpolate(scrollY, {
      inputRange: [BUTTON_CONTAINER_HEIGHT, height - 30],
      outputRange: [1, 0],
      extrapolate: Extrapolate.CLAMP,
    });
    return (
      <View style={{ flex: 1 }}>
        <View style={styles.container}>
          <Animated.ScrollView
            onScroll={onScroll({ scrollY })}
            showVerticalScrollIndicator={false}
            scrollEventThrottle={1}>
            <Animated.View
              style={[styles.buttonContainer, { opacity: opacity }]}>
              <Text>Hello World! </Text>
              <Button title="Click Me!" />
            </Animated.View>
            <View>
              <Text style={styles.textStyle}>This is content</Text>
              <Text style={styles.textStyle}>This is content</Text>
              <Text style={styles.textStyle}>This is content</Text>
              <Text style={styles.textStyle}>This is content</Text>
              <Text style={styles.textStyle}>This is content</Text>
              <Text style={styles.textStyle}>This is content</Text>
              <Text style={styles.textStyle}>This is content</Text>
              <Text style={styles.textStyle}>This is content</Text>
            </View>
          </Animated.ScrollView>
        </View>
      </View>
    );
  }
}

const styles = StyleSheet.create({
  container: {
    flex: 1,
    flexDirection: 'column',
    position: 'relative',
  },
  buttonContainer: {
    flex: 1,
    flexDirection: 'column',
    justifyContent: 'flex-end',
    alignItems: 'center',
    height: BUTTON_CONTAINER_HEIGHT,
    marginBottom: 30,
  },
  textStyle: {
    height: 100,
    fontSize: 24,
    marginTop: 10,
  },
});

Answer 1

You're defining the animated value in the wrong way. Don't define values in render method since they will be created every time component re-render. Instead, define them in the constructor.

Use below code:

import * as React from 'react';
import {Text, View, StyleSheet, Dimensions, Button} from 'react-native';
import {onScroll} from 'react-native-redash';
import Animated from 'react-native-reanimated';

const {height} = Dimensions.get('window');
const BUTTON_CONTAINER_HEIGHT = height / 1.7;
const {Value, interpolate, Extrapolate} = Animated;

export default class App extends React.Component<{}> {
  constructor(props: Readonly<{}>) {
    super(props);
    this.scrollY = new Value(0);
  }
  render() {
    const {scrollY} = this;
    const opacity = interpolate(scrollY, {
      inputRange: [0, BUTTON_CONTAINER_HEIGHT, height - 30],
      outputRange: [1, 0, 0],
      extrapolate: Extrapolate.CLAMP,
    });
    return (
      <View style={{flex: 1}}>
        <View style={styles.container}>
          <Animated.ScrollView
            onScroll={onScroll({y: scrollY})}
            showsVerticalScrollIndicator={false}
            scrollEventThrottle={1}>
            <Animated.View style={[styles.buttonContainer, {opacity}]}>
              <Text>Hello World! </Text>
              <Button onPress={() => {}} title="Click Me!" />
            </Animated.View>
            <View>
              <Text style={styles.textStyle}>This is content</Text>
              <Text style={styles.textStyle}>This is content</Text>
              <Text style={styles.textStyle}>This is content</Text>
              <Text style={styles.textStyle}>This is content</Text>
              <Text style={styles.textStyle}>This is content</Text>
              <Text style={styles.textStyle}>This is content</Text>
              <Text style={styles.textStyle}>This is content</Text>
              <Text style={styles.textStyle}>This is content</Text>
            </View>
          </Animated.ScrollView>
        </View>
      </View>
    );
  }
}

const styles = StyleSheet.create({
  container: {
    flex: 1,
    flexDirection: 'column',
    position: 'relative',
  },
  buttonContainer: {
    flex: 1,
    flexDirection: 'column',
    justifyContent: 'flex-end',
    alignItems: 'center',
    height: BUTTON_CONTAINER_HEIGHT,
    marginBottom: 30,
  },
  textStyle: {
    height: 100,
    fontSize: 24,
    marginTop: 10,
  },
});