Source: https://docs.expo.dev/versions/v54.0.0/sdk/magnetometer

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Expo Magnetometer

GitHub Changelog npm

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

GitHub Changelog npm

Android

iOS

Bundled version:

~15.0.7

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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

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Terminal

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- npx expo install expo-sensors

If you are installing this in an existing React Native app , make sure to install expo in your project.

Usage

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Magnetometer

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Open in Snack

import { 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', }, });

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API

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

Classes

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Magnetometer

Only for: 

Android

iOS

Type: Class extends [DeviceSensor](https://docs.expo.dev/versions/latest/sdk/sensors) <[MagnetometerMeasurement](https://docs.expo.dev/versions/v54.0.0/sdk/magnetometer#magnetometermeasurement) >

Magnetometer Methods

addListener(listener)

| Parameter | Type | Description | | --- | --- | --- | | listener | Listener<[MagnetometerMeasurement](https://docs.expo.dev/versions/v54.0.0/sdk/magnetometer#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.

getListenerCount()

Returns the registered listeners count.

Returns:

number

getPermissionsAsync()

Checks user's permissions for accessing sensor.

Returns:

[Promise](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Promise) <PermissionResponse>

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](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Promise) <boolean>

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

removeAllListeners()

Removes all registered listeners.

Returns:

void

removeSubscription(subscription)

| Parameter | Type | Description | | --- | --- | --- | | subscription | EventSubscription | A subscription to remove. |

Removes the given subscription.

Returns:

void

requestPermissionsAsync()

Asks the user to grant permissions for accessing sensor.

Returns:

[Promise](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Promise) <PermissionResponse>

setUpdateInterval(intervalMs)

| Parameter | Type | Description | | --- | --- | --- | | intervalMs | number | Desired interval in milliseconds between sensor updates.<br><br>> Starting from Android 12 (API level 31), the system has a 200ms limit for each sensor updates.<br>> <br>> If you need an update interval less than 200ms, you should:<br>> <br>> * add android.permission.HIGH_SAMPLING_RATE_SENSORS to app.json permissions field<br>> <br>> * 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

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Subscription

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

Subscription Methods

remove()

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.

Returns:

void

Types

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MagnetometerMeasurement

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

| Property | Type | Description | | --- | --- | --- | | timestamp | number | Timestamp of the measurement in seconds. | | x | number | Value representing strength of magnetic field recorded in X axis. | | y | number | Value representing strength of magnetic field recorded in Y axis. | | z | number | Value representing strength of magnetic field recorded in Z axis. |

PermissionExpiration

Literal Type: union

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

Acceptable values are: 'never' | number

PermissionResponse

An object obtained by permissions get and request functions.

| Property | Type | Description | | --- | --- | --- | | canAskAgain | boolean | 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. | | expires | [PermissionExpiration](https://docs.expo.dev/versions/v54.0.0/sdk/magnetometer#permissionexpiration) | Determines time when the permission expires. | | granted | boolean | A convenience boolean that indicates if the permission is granted. | | status | [PermissionStatus](https://docs.expo.dev/versions/v54.0.0/sdk/magnetometer#permissionstatus) | Determines the status of the permission. |

Enums

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PermissionStatus

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.