React Native – How To Manage The Lifecycle of Gestures In React Native ?

Hello Readers, CoolMonkTechie heartily welcomes you in this article (How To Manage The Lifecycle of Gestures In React Native ?).

In this article, We will learn how to manage the lifecycle of gestures in react native. The gesture responder system manages the lifecycle of gestures in react native application. A touch can go through several phases as the app determines what the user’s intention is. For example, the app needs to determine if the touch is scrolling, sliding on a widget, or tapping. This can even change during the duration of a touch. There can also be multiple simultaneous touches.

The touch responder system is needed to allow components to negotiate these touch interactions without any additional knowledge about their parent or child components.

A famous quote about learning is :

” Change is the end result of all true learning. “

So Let’s begin.

Best Practices

To make our app feel great, every action should have the following attributes:

• Feedback/highlighting – show the user what is handling their touch, and what will happen when they release the gesture
• Cancel-ability – when making an action, the user should be able to abort it mid-touch by dragging their finger away

These features make users more comfortable while using an app, because it allows people to experiment and interact without fear of making mistakes.

TouchableHighlight and Touchable

The responder system can be complicated to use. So we have provided an abstract Touchable implementation for things that should be “tappable“. This uses the responder system and allows us to configure tap interactions declaratively. Use TouchableHighlight anywhere where we would use a button or link on web.

Gesture Responder System Lifecycle

A view can become the touch responder by implementing the correct negotiation methods. There are two methods to ask the view if it wants to become responder:

• View.props.onStartShouldSetResponder: (evt) => true, – Does this view want to become responder on the start of a touch?
• View.props.onMoveShouldSetResponder: (evt) => true, – Called for every touch move on the View when it is not the responder: does this view want to “claim” touch responsiveness?

If the View returns true and attempts to become the responder, one of the following will happen:

• View.props.onResponderGrant: (evt) => {} – The View is now responding for touch events. This is the time to highlight and show the user what is happening.
• View.props.onResponderReject: (evt) => {} – Something else is the responder right now and will not release it.

If the view is responding, the following handlers can be called:

• View.props.onResponderMove: (evt) => {} – The user is moving their finger.
• View.props.onResponderRelease: (evt) => {} – Fired at the end of the touch, ie “touchUp”.
• View.props.onResponderTerminationRequest: (evt) => true – Something else wants to become responder. Should this view release the responder? Returning true allows release.
• View.props.onResponderTerminate: (evt) => {} – The responder has been taken from the View. Might be taken by other views after a call to onResponderTerminationRequest, or might be taken by the OS without asking (happens with control center/ notification center on iOS).

evt is a synthetic touch event with the following nativeEvent form:

• changedTouches – Array of all touch events that have changed since the last event
• identifier – The ID of the touch
• locationX – The X position of the touch, relative to the element
• locationY – The Y position of the touch, relative to the element
• pageX – The X position of the touch, relative to the root element
• pageY – The Y position of the touch, relative to the root element
• target – The node id of the element receiving the touch event
• timestamp – A time identifier for the touch, useful for velocity calculation
• touches – Array of all current touches on the screen

Capture ShouldSet Handlers

onStartShouldSetResponder and onMoveShouldSetResponder are called with a bubbling pattern, where the deepest node is called first. That means that the deepest component will become responder when multiple Views return true for ShouldSetResponder handlers. This is desirable in most cases, because it makes sure all controls and buttons are usable.

However, sometimes a parent will want to make sure that it becomes responder. This can be handled by using the capture phase. Before the responder system bubbles up from the deepest component, it will do a capture phase, firing on ShouldSetResponderCapture. So if a parent View wants to prevent the child from becoming responder on a touch start, it should have a onStartShouldSetResponderCapture handler which returns true.

• View.props.onStartShouldSetResponderCapture: (evt) => true,
• View.props.onMoveShouldSetResponderCapture: (evt) => true,

Pan Responder

PanResponder reconciles several touches into a single gesture. It makes single-touch gestures resilient to extra touches, and can be used to recognize basic multi-touch gestures.

By default, PanResponder holds an InteractionManager handle to block long-running JS events from interrupting active gestures.

It provides a predictable wrapper of the responder handlers provided by the gesture responder system. For each handler, it provides a new gestureState object alongside the native event object:

onPanResponderMove: (event, gestureState) => {}

A native event is a synthetic touch event with form of PressEvent.

A gestureState object has the following:

• stateID – ID of the gestureState- persisted as long as there at least one touch on screen
• moveX – the latest screen coordinates of the recently-moved touch
• moveY – the latest screen coordinates of the recently-moved touch
• x0 – the screen coordinates of the responder grant
• y0 – the screen coordinates of the responder grant
• dx – accumulated distance of the gesture since the touch started
• dy – accumulated distance of the gesture since the touch started
• vx – current velocity of the gesture
• vy – current velocity of the gesture
• numberActiveTouches – Number of touches currently on screen

API

Only a single component can respond to touch events at one time – the component responding to events owns a global “interaction lock”. The PanResponder API helps us manage what component owns this lock through a set of callbacks. Each of these callbacks is also passed an event and gestureState object containing info about the touch events (e.g. position and velocity).

To create a PanResponder, we call PanResponder.create(callbacksObject). The result is a set of props that can be passed to View as props (these are the lower-level touch event handling props). We’ll typically wrap the result with useRef, since we only want to create a single PanResponder for the lifecycle of the component.

The full set of callbacks we can pass is:

• onStartShouldSetPanResponder: (event, gestureState) => {}
• onStartShouldSetPanResponderCapture: (event, gestureState) => {}
• onMoveShouldSetPanResponder: (event, gestureState) => {}
• onMoveShouldSetPanResponderCapture: (event, gestureState) => {}
• onPanResponderReject: (event, gestureState) => {}
• onPanResponderGrant: (event, gestureState) => {}
• onPanResponderStart: (event, gestureState) => {}
• onPanResponderEnd: (event, gestureState) => {}
• onPanResponderRelease: (event, gestureState) => {}
• onPanResponderMove: (event, gestureState) => {}
• onPanResponderTerminate: (event, gestureState) => {}
• onPanResponderTerminationRequest: (event, gestureState) => {}
• onShouldBlockNativeResponder: (event, gestureState) => {}

Usage Pattern

const ExampleComponent = () => {
  const panResponder = React.useRef(
    PanResponder.create({
      // Ask to be the responder:
      onStartShouldSetPanResponder: (evt, gestureState) => true,
      onStartShouldSetPanResponderCapture: (evt, gestureState) =>
        true,
      onMoveShouldSetPanResponder: (evt, gestureState) => true,
      onMoveShouldSetPanResponderCapture: (evt, gestureState) =>
        true,

      onPanResponderGrant: (evt, gestureState) => {
        // The gesture has started. Show visual feedback so the user knows
        // what is happening!
        // gestureState.d{x,y} will be set to zero now
      },
      onPanResponderMove: (evt, gestureState) => {
        // The most recent move distance is gestureState.move{X,Y}
        // The accumulated gesture distance since becoming responder is
        // gestureState.d{x,y}
      },
      onPanResponderTerminationRequest: (evt, gestureState) =>
        true,
      onPanResponderRelease: (evt, gestureState) => {
        // The user has released all touches while this view is the
        // responder. This typically means a gesture has succeeded
      },
      onPanResponderTerminate: (evt, gestureState) => {
        // Another component has become the responder, so this gesture
        // should be cancelled
      },
      onShouldBlockNativeResponder: (evt, gestureState) => {
        // Returns whether this component should block native components from becoming the JS
        // responder. Returns true by default. Is currently only supported on android.
        return true;
      }
    })
  ).current;

  return <View {...panResponder.panHandlers} />;
};

Example

PanResponder works with Animated API to help build complex gestures in the UI. The following example contains an animated View component which can be dragged freely across the screen.

import React, { useRef } from "react";
import { Animated, View, StyleSheet, PanResponder, Text } from "react-native";

const App = () => {
  const pan = useRef(new Animated.ValueXY()).current;

  const panResponder = useRef(
    PanResponder.create({
      onMoveShouldSetPanResponder: () => true,
      onPanResponderGrant: () => {
        pan.setOffset({
          x: pan.x._value,
          y: pan.y._value
        });
      },
      onPanResponderMove: Animated.event(
        [
          null,
          { dx: pan.x, dy: pan.y }
        ]
      ),
      onPanResponderRelease: () => {
        pan.flattenOffset();
      }
    })
  ).current;

  return (
    <View style={styles.container}>
      <Text style={styles.titleText}>Drag this box!</Text>
      <Animated.View
        style={{
          transform: [{ translateX: pan.x }, { translateY: pan.y }]
        }}
        {...panResponder.panHandlers}
      >
        <View style={styles.box} />
      </Animated.View>
    </View>
  );
}

const styles = StyleSheet.create({
  container: {
    flex: 1,
    alignItems: "center",
    justifyContent: "center"
  },
  titleText: {
    fontSize: 14,
    lineHeight: 24,
    fontWeight: "bold"
  },
  box: {
    height: 150,
    width: 150,
    backgroundColor: "blue",
    borderRadius: 5
  }
});

export default App;

The output of the above example is :

That’s all about in this article.

Conclusion

In this article, We understood how to manage the lifecycle of gestures in react native. We also discussed about Best Practices and PanResponser usage in React Native platform.

Thanks for reading ! I hope you enjoyed and learned about the Gestures Concepts in React Native. Reading is one thing, but the only way to master it is to do it yourself.

Please follow and subscribe us on this blog and and support us in any way possible. Also like and share the article with others for spread valuable knowledge.

You can find Other articles of CoolmonkTechie as below link :

• React Native Articles
• CoolMonkTechie Home Page

You can also follow official website and tutorials of React Native as below links :

React Native Official Website

If you have any comments, questions, or think I missed something, feel free to leave them below in the comment box.

Thanks again Reading. HAPPY READING!!???