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To create a modal with a slide-in animation using React Native Reanimated and a spring mass effect, you'll need to follow these steps. This example assumes you're using React Native Reanimated 2.x.

1. Install Dependencies: Make sure you have the necessary dependencies installed. Run the following command to install react-native-reanimated if you haven't done so yet:

   npm install react-native-reanimated
   

2. Setup Reanimated: Ensure that you follow any platform-specific installation steps mentioned in the Reanimated documentation.

3. Implement the Modal: Below is a complete example code to create a modal that slides in from the bottom using spring animation.

import React, { useState } from 'react';
import { View, Text, Button, Modal, StyleSheet, TouchableOpacity } from 'react-native';
import Animated, { useSharedValue, useAnimatedStyle, withSpring } from 'react-native-reanimated';

const App = () => {
  const [isVisible, setIsVisible] = useState(false);
  const translateY = useSharedValue(1000); // Offscreen position

  const showModal = () => {
    setIsVisible(true);
    translateY.value = withSpring(0, { mass: 0.5, tension: 100, friction: 10 }); // Spring animation
  };

  const hideModal = () => {
    translateY.value = withSpring(1000, { mass: 0.5, tension: 100, friction: 10 }, () => {
      setIsVisible(false);
    });
  };

  const animatedStyle = useAnimatedStyle(() => {
    return {
      transform: [{ translateY: translateY.value }],
    };
  });

  return (
    <View style={styles.container}>
      <Button title="Show Modal" onPress={showModal} />

      <Modal
        transparent
        visible={isVisible}
        animationType="none" // Disable default animation
        onRequestClose={hideModal} // Handle back button on Android
      >
        <TouchableOpacity style={styles.overlay} onPress={hideModal}>
          <Animated.View style={[styles.modalContainer, animatedStyle]}>
            <Text style={styles.modalText}>This is a modal!</Text>
            <Button title="Close" onPress={hideModal} />
          </Animated.View>
        </TouchableOpacity>
      </Modal>
    </View>
  );
};

const styles = StyleSheet.create({
  container: {
    flex: 1,
    justifyContent: 'center',
    alignItems: 'center',
  },
  overlay: {
    flex: 1,
    backgroundColor: 'rgba(0, 0, 0, 0.5)',
    justifyContent: 'flex-end',
  },
  modalContainer: {
    backgroundColor: 'white',
    padding: 20,
    borderTopLeftRadius: 20,
    borderTopRightRadius: 20,
  },
  modalText: {
    fontSize: 18,
    marginBottom: 10,
  },
});

export default App;

Explanation:

• State Management: We manage the modal's visibility with a state variable (isVisible).
• Animated Values: We use useSharedValue to hold the vertical translation value (translateY).
• Animation Trigger: The showModal function sets the translation to animate it into view, while hideModal animates it out and then hides the modal.
• Animated Style: We create an animated style that updates based on the translateY value.
• Modal Overlay: The modal can be dismissed by tapping outside of it (the overlay).

Remember:

• Always run your project on a real device or an emulator that supports React Native Reanimated features.
• Customize the styles and transition settings as per your design needs.