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Expo Magnetometer iconExpo Magnetometer

GitHub

npm

A library that provides access to the device's magnetometer sensor.

Android
iOS

Magnetometer from expo-sensors provides access to the device magnetometer sensor(s) to respond to and measure the changes in the magnetic field measured in microtesla (μT).

You can access the calibrated values with Magnetometer and uncalibrated raw values with MagnetometerUncalibrated.

Installation

Terminal
npx expo install expo-sensors

If you're installing this in a bare React Native app, you should also follow these additional installation instructions.

Usage

Magnetometer
import React, { useState, useEffect } from 'react';
import { StyleSheet, Text, TouchableOpacity, View } from 'react-native';
import { Magnetometer } from 'expo-sensors';

export default function Compass() {
  const [{ x, y, z }, setData] = useState({
    x: 0,
    y: 0,
    z: 0,
  });
  const [subscription, setSubscription] = useState(null);

  const _slow = () => Magnetometer.setUpdateInterval(1000);
  const _fast = () => Magnetometer.setUpdateInterval(16);

  const _subscribe = () => {
    setSubscription(
      Magnetometer.addListener(result => {
        setData(result);
      })
    );
  };

  const _unsubscribe = () => {
    subscription && subscription.remove();
    setSubscription(null);
  };

  useEffect(() => {
    _subscribe();
    return () => _unsubscribe();
  }, []);

  return (
    <View style={styles.container}>
      <Text style={styles.text}>Magnetometer:</Text>
      <Text style={styles.text}>x: {x}</Text>
      <Text style={styles.text}>y: {y}</Text>
      <Text style={styles.text}>z: {z}</Text>
      <View style={styles.buttonContainer}>
        <TouchableOpacity onPress={subscription ? _unsubscribe : _subscribe} style={styles.button}>
          <Text>{subscription ? 'On' : 'Off'}</Text>
        </TouchableOpacity>
        <TouchableOpacity onPress={_slow} style={[styles.button, styles.middleButton]}>
          <Text>Slow</Text>
        </TouchableOpacity>
        <TouchableOpacity onPress={_fast} style={styles.button}>
          <Text>Fast</Text>
        </TouchableOpacity>
      </View>
    </View>
  );
}

const styles = StyleSheet.create({
  container: {
    flex: 1,
    justifyContent: 'center',
    paddingHorizontal: 10,
  },
  text: {
    textAlign: 'center',
  },
  buttonContainer: {
    flexDirection: 'row',
    alignItems: 'stretch',
    marginTop: 15,
  },
  button: {
    flex: 1,
    justifyContent: 'center',
    alignItems: 'center',
    backgroundColor: '#eee',
    padding: 10,
  },
  middleButton: {
    borderLeftWidth: 1,
    borderRightWidth: 1,
    borderColor: '#ccc',
  },
});

API

import { Magnetometer, MagnetometerUncalibrated } from 'expo-sensors';

Classes

Only for:
Android
iOS

Magnetometer

Type: Class extends DeviceSensor<MagnetometerMeasurement>

Magnetometer Methods

addListener(listener)

NameTypeDescription
listenerListener<MagnetometerMeasurement>

A callback that is invoked when a magnetometer update is available. When invoked, the listener is provided with a single argument that is MagnetometerMeasurement.


Subscribe for updates to the magnetometer.

Returns:

Subscription

A subscription that you can call remove() on when you would like to unsubscribe the listener.

getListenerCount()

Returns the registered listeners count.

Returns:

number

getPermissionsAsync()

Checks user's permissions for accessing sensor.

hasListeners()

Returns boolean which signifies if sensor has any listeners registered.

Returns:

boolean

isAvailableAsync()

You should always check the sensor availability before attempting to use it.

Check the availability of the device magnetometer. Requires at least Android 2.3 (API Level 9) and iOS 8.

Returns:

Promise<boolean>

A promise that resolves to a boolean denoting the availability of the sensor.

removeAllListeners()

Removes all registered listeners.

Returns:

void

removeSubscription(subscription)

NameTypeDescription
subscriptionSubscription

A subscription to remove.


Removes the given subscription.

Returns:

void

requestPermissionsAsync()

Asks the user to grant permissions for accessing sensor.

setUpdateInterval(intervalMs)

NameTypeDescription
intervalMsnumber

Desired interval in milliseconds between sensor updates.

Starting from Android 12 (API level 31), the system has a 200ms limit for each sensor updates.

If you need an update interval less than 200ms, you should:

  • add android.permission.HIGH_SAMPLING_RATE_SENSORS to app.json permissions field
  • or if you are using bare workflow, add <uses-permission android:name="android.permission.HIGH_SAMPLING_RATE_SENSORS"/> to AndroidManifest.xml.

Set the sensor update interval.

Returns:

void

Interfaces

PermissionResponse

An object obtained by permissions get and request functions.

PermissionResponse Properties

NameTypeDescription
canAskAgainboolean

Indicates if user can be asked again for specific permission. If not, one should be directed to the Settings app in order to enable/disable the permission.

expiresPermissionExpiration

Determines time when the permission expires.

grantedboolean

A convenience boolean that indicates if the permission is granted.

statusPermissionStatus

Determines the status of the permission.


Types

MagnetometerMeasurement

Each of these keys represents the strength of magnetic field along that particular axis measured in microteslas (μT).

NameTypeDescription
xnumber

Value representing strength of magnetic field recorded in X axis.

ynumber

Value representing strength of magnetic field recorded in Y axis.

znumber

Value representing strength of magnetic field recorded in Z axis.

PermissionExpiration

Literal Type: multiple types

Permission expiration time. Currently, all permissions are granted permanently.

Acceptable values are: 'never' | number

Subscription

NameTypeDescription
remove() => void

A method to unsubscribe the listener.

Enums

PermissionStatus

PermissionStatus Values

DENIED

PermissionStatus.DENIED = "denied"

User has denied the permission.

GRANTED

PermissionStatus.GRANTED = "granted"

User has granted the permission.

UNDETERMINED

PermissionStatus.UNDETERMINED = "undetermined"

User hasn't granted or denied the permission yet.