Sensors #

Introduction #

Smartphones got a bunch of different sensors build in to it. If you think about it, it’s pretty amazing the amount of things that is crammed into such a small device.

What kind of sensors, and what are they used for? #

Besides the obvious like camera, microphone and fingerprint. There is also a whole slew of other sensors that you might not think about.

Light #

Your phone has a light sensor that is used to automatically adjust the screen brightness.

You might occasionally notice if you accidentally cover the sensor while being it a lit room that it will turn down the brightness.

Accelerometer and gyroscope #

There is an accelerometer and gyroscope that are used to determine orientation, so it can automatically rotate the screen layout when you flip the phone. They are also used for step counting.

Magnetometer #

It has a magnetometer, so it can show the direction you are facing on maps.

Location (A-GPS) #

Speaking of maps. There is A-GPS to determine your location. The A stands for Assisted. Which means that it uses cell towers / mobile network to assist the triangulation of location.

In addition, it can also use Wi-Fi signal strength to further assist in determining position. So, Apple and Google each have a database with the location of each Wi-Fi access point in world. Peoples phones send information about access points in its proximity to keep the database up to date. This all happens in the background on most devices. Laptops and other devices that don’t have GPS can also use this information to determine its approximate location.

Using sensors #

Here we will explore how to use the sensors.

Support for the various sensors can be added to Flutter project through plugins found on pub.dev.

Light #

This only works on Android.

Start by adding the light plugin to your project.

flutter pub add light

In your code, instantiate Light object then you can use lightSensorStream to get a stream of values from the sensor.

import 'package:light/light.dart';

final light = Light();
light.lightSensorStream.forEach((value) => print(value));

Example

The example uses the light sensor to fade between two versions of an images based on the level of ambient light.

Accelerometer, gyroscope and magnetometer #

They can all be accessed using the sensors_plus plugin.

It works both on iOS and Android.

On iOS you need to request access motions sensors through app metadata, otherwise the app will crash when attempting to read sensor values. Open ios/Runner/Info.plist and add the following:

<key>NSMotionUsageDescription</key>
<string>This app requires access to the barometer to provide altitude information.</string>

The sensors_plus plugin expose the following functions which can be used to read values from the sensors.

• accelerometerEventStream()
  • Returns a broadcast stream of events from the device accelerometer at the given sampling frequency.
• barometerEventStream()
  • Returns a broadcast stream of events from the device barometer at the given sampling frequency.
• gyroscopeEventStream()
  • Returns a broadcast stream of events from the device gyroscope at the given sampling frequency.
• magnetometerEventStream()
  • Returns a broadcast stream of events from the device magnetometer at the given sampling frequency.
• userAccelerometerEventStream()
  • Returns a broadcast stream of events from the device accelerometer with gravity removed at the given sampling frequency.

That each (except barometer) provides a stream of readings with values for X, Y and Z axis.

Example

In the example readings of each sensor is shown on a chart.

Some apps use the sensors to provide cool effects such as what can be achieved using flutter_tilt package.

flutter pub add flutter_tilt

Using flutter_tilt without accessing sensors directly doesn’t require you to add a dependency to sensors_plus, since flutter_tilt already depends on it.

Here is an example that uses the package to create a parallax effect when you move your phone.

Example

Location #

There are two popular plugins that can be used to access location services. There is location and geolocator. I’m using geolocator.

It works on iOS, Android, macOS, web and Windows.

There is a bit of platform specific setup to use it though.

android {
  compileSdkVersion 34

  ...
}

<uses-permission android:name="android.permission.ACCESS_FINE_LOCATION" />
<uses-permission android:name="android.permission.ACCESS_COARSE_LOCATION" />

<key>NSLocationWhenInUseUsageDescription</key>
<string>This app needs access to location when open.</string>

To access location service it both needs to be enabled on the phone, and it needs permission from the user.

if (!await Geolocator.isLocationServiceEnabled()) {
  throw Exception("Location disabled");
}

var permission = await Geolocator.checkPermission();
if (permission == LocationPermission.denied) {
  permission = await Geolocator.requestPermission();
  if (permission == LocationPermission.denied ||
      permission == LocationPermission.deniedForever) {
    throw Exception("Location denied");
  }
}

You can then use Geolocator to:

• getCurrentPosition()
  • Returns the current position.
• getCurrentPosition()
  • Returns the current position.
• getLastKnownPosition()
  • Returns the last known position stored on the users device.

It also has a convenient distanceBetween() method to calculate the distance between the supplied coordinates in meters.

• Example Cubit
• Example widget

Geolocator can obviously be used to show where the user is located on a map. Here I’m using [flutter_map] packages, since it doesn’t require API-keys.

• Example map

The examples use flutter_map_location_marker to add a marker for the current location. It also indicates the direction like on Google Maps.