Android Sensor Tutorial

By Lars Vogel- vogella.com

Android Sensor

This tutorial describes how to use the Androids Sensor manager. The tutorial is based on Eclipse 3.6, Java 1.6 and Android 2.3.3 (Gingerbread).

1. Android Sensors

Android supports several sensors via the SensorManager, for example the accelerometer. Unfortunately you cannot test the accelerometer on the Android emulator.
You can access a SensorManager via getSystemService(SENSOR_SERVICE). The Sensor class defines several constants for accessing the different sensors.

Sensor.TYPE_GYROSCOPE
Sensor.TYPE_MAGNETIC_FIELD
Sensor.TYPE_ORIENTATION
Sensor.TYPE_ACCELEROMETER

You can access the Sensor via the sensorManager.getDefaultSensor() method, which take as parameters the sensor type and the deplay defined as constants on SensorManager.
Once you acquired a sensor, you can register a SensorEventListener object on it. This listener will get informed, if the sensor data changes.
To avoid the unnecessary usage of battery you register your listener in the onResume() method and de-register it in the onPause() method.

2. Tutorial: Accelerometer

We will build an application which will change its background color, if it is shuffled. Create a new Android project "de.vogella.android.sensor" with an Activity called SensorTestActivity.
Change your layout main.xml to the following.

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="fill_parent"
    android:layout_height="fill_parent"
    android:orientation="vertical" >

    <TextView
        android:id="@+id/textView"
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        android:text="Shake to get a toast and to switch color" />

</LinearLayout>

Change the your Activity to the following.

package de.vogella.android.sensor;

import android.app.Activity;
import android.graphics.Color;
import android.hardware.Sensor;
import android.hardware.SensorEvent;
import android.hardware.SensorEventListener;
import android.hardware.SensorManager;
import android.os.Bundle;
import android.view.View;
import android.view.Window;
import android.view.WindowManager;
import android.widget.Toast;

public class SensorTestActivity extends Activity implements SensorEventListener {
 private SensorManager sensorManager;
 private boolean color = false;
 private View view;
 private long lastUpdate;

 
/** Called when the activity is first created. */


@Override
 public void onCreate(Bundle savedInstanceState) {
  requestWindowFeature(Window.FEATURE_NO_TITLE);
  getWindow().setFlags(WindowManager.LayoutParams.FLAG_FULLSCREEN,
    WindowManager.LayoutParams.FLAG_FULLSCREEN);

  super.onCreate(savedInstanceState);
  setContentView(R.layout.main);
  view = findViewById(R.id.textView);
  view.setBackgroundColor(Color.GREEN);

  sensorManager = (SensorManager) getSystemService(SENSOR_SERVICE);
  sensorManager.registerListener(this,
    sensorManager.getDefaultSensor(Sensor.TYPE_ACCELEROMETER),
    SensorManager.SENSOR_DELAY_NORMAL);
  lastUpdate = System.currentTimeMillis();
 }

 @Override
 public void onSensorChanged(SensorEvent event) {
  if (event.sensor.getType() == Sensor.TYPE_ACCELEROMETER) {
   getAccelerometer(event);
  }

 }

 private void getAccelerometer(SensorEvent event) {
  float[] values = event.values;
  // Movement
  float x = values[0];
  float y = values[1];
  float z = values[2];

  float accelationSquareRoot = (x * x + y * y + z * z)
    / (SensorManager.GRAVITY_EARTH * SensorManager.GRAVITY_EARTH);
  long actualTime = System.currentTimeMillis();
  if (accelationSquareRoot >= 2) //
  {
   if (actualTime - lastUpdate < 200) {
    return;
   }
   lastUpdate = actualTime;
   Toast.makeText(this, "Device was shuffed", Toast.LENGTH_SHORT)
     .show();
   if (color) {
    view.setBackgroundColor(Color.GREEN);

   } else {
    view.setBackgroundColor(Color.RED);
   }
   color = !color;
  }
 }

 @Override
 public void onAccuracyChanged(Sensor sensor, int accuracy) {

 }

 @Override
 protected void onResume() {
  super.onResume();
  // register this class as a listener for the orientation and
  // accelerometer sensors
  sensorManager.registerListener(this,
    sensorManager.getDefaultSensor(Sensor.TYPE_ACCELEROMETER),
    SensorManager.SENSOR_DELAY_NORMAL);
 }

 @Override
 protected void onPause() {
  // unregister listener
  super.onPause();
  sensorManager.unregisterListener(this);
 }
}

3. Tutorial: Building a compass

Create a new Android project "de.vogella.android.sensor.compass" with an Activity called MainActivity.
Create the following custom View class.

package de.vogella.android.sensor.compass;

import android.content.Context;
import android.graphics.Canvas;
import android.graphics.Color;
import android.graphics.Paint;
import android.view.View;

public class MyCompassView extends View {

 private Paint paint;
 private float position = 0;

 public MyCompassView(Context context) {
  super(context);
  init();
 }

 private void init() {
  paint = new Paint();
  paint.setAntiAlias(true);
  paint.setStrokeWidth(2);
  paint.setTextSize(25);
  paint.setStyle(Paint.Style.STROKE);
  paint.setColor(Color.WHITE);
 }

 @Override
 protected void onDraw(Canvas canvas) {
  int xPoint = getMeasuredWidth() / 2;
  int yPoint = getMeasuredHeight() / 2;

  float radius = (float) (Math.max(xPoint, yPoint) * 0.6);
  canvas.drawCircle(xPoint, yPoint, radius, paint);
  canvas.drawRect(0, 0, getMeasuredWidth(), getMeasuredHeight(), paint);

  // 3.143 is a good approximation for the circle
  canvas.drawLine(xPoint,
    yPoint,
    (float) (xPoint + radius
      * Math.sin((double) (-position) / 180 * 3.143)),
    (float) (yPoint - radius
      * Math.cos((double) (-position) / 180 * 3.143)), paint);

  canvas.drawText(String.valueOf(position), xPoint, yPoint, paint);
 }

 public void updateData(float position) {
  this.position = position;
  invalidate();
 }

}

Change the coding of your Activity.

package de.vogella.android.sensor.compass;

import android.app.Activity;
import android.content.Context;
import android.hardware.Sensor;
import android.hardware.SensorEvent;
import android.hardware.SensorEventListener;
import android.hardware.SensorManager;
import android.os.Bundle;
import android.util.Log;
import android.widget.Toast;

public class MainActivity extends Activity {

 private static SensorManager sensorService;
 private MyCompassView compassView;
 private Sensor sensor;

 
/** Called when the activity is first created. */


@Override
 public void onCreate(Bundle savedInstanceState) {
  super.onCreate(savedInstanceState);
  compassView = new MyCompassView(this);
  setContentView(compassView);

  sensorService = (SensorManager) getSystemService(Context.SENSOR_SERVICE);
  sensor = sensorService.getDefaultSensor(Sensor.TYPE_ORIENTATION);
  if (sensor != null) {
   sensorService.registerListener(mySensorEventListener, sensor,
     SensorManager.SENSOR_DELAY_NORMAL);
   Log.i("Compass MainActivity", "Registerered for ORIENTATION Sensor");

  } else {
   Log.e("Compass MainActivity", "Registerered for ORIENTATION Sensor");
   Toast.makeText(this, "ORIENTATION Sensor not found",
     Toast.LENGTH_LONG).show();
   finish();
  }
 }

 private SensorEventListener mySensorEventListener = new SensorEventListener() {

  @Override
  public void onAccuracyChanged(Sensor sensor, int accuracy) {
  }

  @Override
  public void onSensorChanged(SensorEvent event) {
   // angle between the magnetic north directio
   // 0=North, 90=East, 180=South, 270=West
   float azimuth = event.values[0];
   compassView.updateData(azimuth);
  }
 };

 @Override
 protected void onDestroy() {
  super.onDestroy();
  if (sensor != null) {
   sensorService.unregisterListener(mySensorEventListener);
  }
 }

}