Gesture Responder System

The Gesture Responder System in React Native is a layered system for managing touch events and gestures across views in a way that captures the complexity of user interactions on a touch screen. It allows components to become responders to touch gestures, managing which component should respond to a particular touch at any given time. This system is critical for creating seamless and interactive user interfaces across different input methods.

Key Concepts

1. Touchable Components:

   • React Native provides components like TouchableOpacity, TouchableHighlight, and TouchableWithoutFeedback which are built on the Gesture Responder System.

2. Responder Lifecycle:

   • The lifecycle defines how a component can become a responder to touch events:
     • On Start: When a touch is initiated, a component can declare its interest in becoming the responder.
     • Grant: If no other component is a better candidate, the interested component becomes the responder.
     • Move: While a touch is ongoing, the system can decide to transfer responder status to another component if needed.
     • Release/Terminate: Once the touch interaction is finished, the component releases the responder status.

3. Responder Callbacks:

   • Components handle touch events through callback props. Some key callbacks include:
     • onStartShouldSetResponder: Determines if a component should become responder when a touch begins.
     • onMoveShouldSetResponder: Called when a touch moves. Determines if a component should become responder.
     • onResponderGrant: Called when a component becomes the responder.
     • onResponderMove: Called repeatedly while the responder is active for handling touch movements.
     • onResponderRelease: Called when the touch interaction ends.
     • onResponderTerminate: Called if the responder status is taken away from the component by other controls.

4. Event Bubbling and Capturing:

   • Similar to the event system in web development, React Native's Gesture Responder System supports event capture and bubbling, allowing events to propagate through the hierarchy according to responder status.

5. Handling Complex Gestures:

   • For complex gestures like swipes or pinches, it's better to use react-native-gesture-handler, which complements the built-in Gesture Responder System by offering higher-level gesture recognizers.

Implementing Gesture Responder System

import React, { useState } from 'react';
import { View, Text, StyleSheet } from 'react-native';

const GestureResponderExample = () => {
  const [gestureState, setGestureState] = useState('None');

  const handleStartShouldSetResponder = (evt) => true;

  const handleMoveShouldSetResponder = (evt) => true;

  const handleResponderGrant = (evt) => {
    setGestureState('Touch Started');
  };

  const handleResponderMove = (evt) => {
    setGestureState('Moving...');
  };

  const handleResponderRelease = (evt) => {
    setGestureState('Touch Released');
  };

  return (
    <View
      style={styles.container}
      onStartShouldSetResponder={handleStartShouldSetResponder}
      onMoveShouldSetResponder={handleMoveShouldSetResponder}
      onResponderGrant={handleResponderGrant}
      onResponderMove={handleResponderMove}
      onResponderRelease={handleResponderRelease}
    >
      <Text>{gestureState}</Text>
    </View>
  );
};

const styles = StyleSheet.create({
  container: {
    flex: 1,
    justifyContent: 'center',
    alignItems: 'center',
    backgroundColor: '#f2f2f2',
  },
});

export default GestureResponderExample;

Gesture Event Propagation

Bubblig Phase Handlers

onStartShouldSetResponder and onMoveShouldSetResponder are indeed triggered in the bubbling phase, which means that the deepest child component (the one closest to the touch point) gets the first opportunity to handle the touch event.

it ensures that the innermost component — often a button, control, or interactive element — gets the chance to become the responder if multiple components return true for these handlers. This behavior is in line with typical user expectations, ensuring that touch interactions on interactive elements like buttons are properly handled.

Capture Phase Handlers

onStartShouldSetResponderCapture and onMoveShouldSetResponderCapture are similar to their counterparts, but they are specifically used during the capturing phase of the touch event.

These handlers provide developers with the ability to decide early in the event lifecycle whether a parent component should handle touch input, allowing for more controlled and prioritized touch handling within a React Native application.