Expo

Magnetometer

Magnetometer from expo-sensors provides access to the device magnetometer sensor(s) to respond to and measure the changes in the magnetic field. You can access the calibrated values with Magnetometer. and uncalibrated raw values with MagnetometerUncalibrated.

Platform Compatibility

Android DeviceAndroid EmulatoriOS DeviceiOS SimulatorWeb

Installation

expo install expo-sensors

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

API

import { Magnetometer } from 'expo-sensors';

Methods

Magnetometer.isAvailableAsync()

You should always check the sensor availability before attempting to use it.
Returns whether the magnetometer is enabled on the device.
OSAvailability
iOSiOS 8+
AndroidAndroid 2.3+ (API Level 9+)
WebN/A

Returns

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

Magnetometer.addListener(listener)

Subscribe for updates to the Magnetometer.

Arguments

  • listener (function) -- A callback that is invoked when an Magnetometer update is available. When invoked, the listener is provided a single argumument that is an object containing keys x, y, z.

Returns

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

Magnetometer.removeAllListeners()

Remove all listeners.

Magnetometer.setUpdateInterval(intervalMs)

Subscribe for updates to the Magnetometer.

Arguments

  • intervalMs (number) Desired interval in milliseconds between Magnetometer updates.

Example: basic subscription

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

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

  React.useEffect(() => {
    _toggle();
    return () => {
      _unsubscribe();
    };
  }, []);

  const _toggle = () => {
    if (subscription) {
      _unsubscribe();
    } else {
      _subscribe();
    }
  };

  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);
  };

  const { x, y, z } = data;
  return (
    <View style={styles.sensor}>
      <Text>Magnetometer:</Text>
      <Text>
        x: {round(x)} y: {round(y)} z: {round(z)}
      </Text>

      <View style={styles.buttonContainer}>
        <TouchableOpacity onPress={_toggle} style={styles.button}>
          <Text>Toggle</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>
  );
}

function round(n) {
  if (!n) {
    return 0;
  }
  return Math.floor(n * 100) / 100;
}

const styles = StyleSheet.create({
  container: {
    flex: 1,
  },
  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',
  },
  sensor: {
    marginTop: 15,
    paddingHorizontal: 10,
  },
});