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Magnetometer

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

This API is pre-installed in managed apps. To use it in a bare React Native app, follow its installation instructions.

// in managed apps:
import { Magnetometer } from 'expo';

// in bare apps:
import { Magnetometer } from 'expo-sensors';

Returns whether the magnetometer is enabled on the device.

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

Subscribe for updates to the Magnetometer.

  • 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.

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

Remove all listeners.

Subscribe for updates to the Magnetometer.

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

import React from 'react';
import { Magnetometer } from 'expo';
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,
  },
});