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This is documentation for the next SDK version. For up-to-date documentation, see the latest version (SDK 51).

Expo Magnetometer iconExpo Magnetometer

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 are installing this in an existing React Native app (bare workflow), start by installing expo in your project. Then, follow the additional instructions as mentioned by library's README under "Installation in bare React Native projects" section.

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

Android
iOS

Magnetometer

Type: Class extends DeviceSensor<MagnetometerMeasurement>

Magnetometer Methods

Android
iOS

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:

EventSubscription

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

Android
iOS

getListenerCount()

Returns the registered listeners count.

Returns:

number

Android
iOS

getPermissionsAsync()

Checks user's permissions for accessing sensor.

Android
iOS

hasListeners()

Returns boolean which signifies if sensor has any listeners registered.

Returns:

boolean

Android
iOS

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.

Android
iOS

removeAllListeners()

Removes all registered listeners.

Returns:

void

Android
iOS

removeSubscription(subscription)

NameTypeDescription
subscriptionEventSubscription

A subscription to remove.


Removes the given subscription.

Returns:

void

Android
iOS

requestPermissionsAsync()

Asks the user to grant permissions for accessing sensor.

Android
iOS

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

Android
iOS

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

Android
iOS

MagnetometerMeasurement

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

NameTypeDescription
timestampnumber

Timestamp of the measurement in seconds.

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.

Android
iOS

PermissionExpiration

Literal Type: multiple types

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

Acceptable values are: 'never' | number

Android
iOS

Subscription

A subscription object that allows to conveniently remove an event listener from the emitter.

NameTypeDescription
remove() => void

Removes an event listener for which the subscription has been created. After calling this function, the listener will no longer receive any events from the emitter.

Enums

Android
iOS

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.

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