Expo

Documentation

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
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Installation

For managed apps, you'll need to run expo install expo-sensors. To use it in a bare React Native app, follow its 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 React from 'react';
import { Magnetometer } from 'expo-sensors';
import { StyleSheet, Text, TouchableOpacity, View } from 'react-native';

export default class MagnetometerSensor extends React.Component {
  state = {
    MagnetometerData: {},
  };

  componentDidMount() {
    this._toggle();
  }

  componentWillUnmount() {
    this._unsubscribe();
  }

  _toggle = () => {
    if (this._subscription) {
      this._unsubscribe();
    } else {
      this._subscribe();
    }
  };

  _slow = () => {
    Magnetometer.setUpdateInterval(1000);
  };

  _fast = () => {
    Magnetometer.setUpdateInterval(16);
  };

  _subscribe = () => {
    this._subscription = Magnetometer.addListener(result => {
      this.setState({ MagnetometerData: result });
    });
  };

  _unsubscribe = () => {
    this._subscription && this._subscription.remove();
    this._subscription = null;
  };

  render() {
    let { x, y, z } = this.state.MagnetometerData;

    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={this._toggle} style={styles.button}>
            <Text>Toggle</Text>
          </TouchableOpacity>
          <TouchableOpacity onPress={this._slow} style={[styles.button, styles.middleButton]}>
            <Text>Slow</Text>
          </TouchableOpacity>
          <TouchableOpacity onPress={this._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,
  },
});