| page.title=Sensors Overview |
| parent.title=Sensors |
| parent.link=index.html |
| @jd:body |
| |
| <div id="qv-wrapper"> |
| <div id="qv"> |
| <h2>Quickview</h2> |
| <ul> |
| <li>Learn about the sensors that Android supports and the Android sensor framework.</li> |
| <li>Find out how to list sensors, determine sensor capabilities, and monitor sensor data.</li> |
| <li>Learn about best practices for accessing and using sensors.</li> |
| </ul> |
| <h2>In this document</h2> |
| <ol> |
| <li><a href="#sensors-intro">Introduction to Sensors</a></li> |
| <li><a href="#sensors-identify">Identifying Sensors and Sensor Capabilities</a></li> |
| <li><a href="#sensors-monitor">Monitoring Sensor Events</a></li> |
| <li><a href="#sensors-configs">Handling Different Sensor Configurations</a></li> |
| <li><a href="#sensors-coords">Sensor Coordinate System</a></li> |
| <li><a href="#sensors-practices">Best Practices for Accessing and Using Sensors</a></li> |
| </ol> |
| <h2>Key classes and interfaces</h2> |
| <ol> |
| <li>{@link android.hardware.Sensor}</li> |
| <li>{@link android.hardware.SensorEvent}</li> |
| <li>{@link android.hardware.SensorManager}</li> |
| <li>{@link android.hardware.SensorEventListener}</li> |
| </ol> |
| <h2>Related samples</h2> |
| <ol> |
| <li><a href="{@docRoot}resources/samples/AccelerometerPlay/index.html">Accelerometer |
| Play</a></li> |
| <li><a |
| href="{@docRoot}resources/samples/ApiDemos/src/com/example/android/apis/os/RotationVectorDemo.html"> |
| API Demos (OS - RotationVectorDemo)</a></li> |
| <li><a |
| href="{@docRoot}resources/samples/ApiDemos/src/com/example/android/apis/os/Sensors.html">API Demos |
| (OS - Sensors)</a></li> |
| </ol> |
| <h2>See also</h2> |
| <ol> |
| <li><a href="{@docRoot}guide/topics/sensors/index.html">Sensors</a></li> |
| <li><a href="{@docRoot}guide/topics/sensors/sensors_motion.html">Motion Sensors</a></li> |
| <li><a href="{@docRoot}guide/topics/sensors/sensors_position.html">Position |
| Sensors</a></li> |
| <li><a href="{@docRoot}guide/topics/sensors/sensors_environment.html">Environment |
| Sensors</a></li> |
| </ol> |
| </div> |
| </div> |
| |
| <p>Most Android-powered devices have built-in sensors that measure motion, orientation, |
| and various environmental conditions. These sensors are capable of providing raw data with high |
| precision and accuracy, and are useful if you want to monitor three-dimensional device movement or |
| positioning, or you want to monitor changes in the ambient environment near a device. For example, a |
| game might track readings from a device's gravity sensor to infer complex user gestures |
| and motions, such as tilt, shake, rotation, or swing. Likewise, a weather application might use a |
| device's temperature sensor and humidity sensor to calculate and report the dewpoint, or a travel |
| application might use the geomagnetic field sensor and accelerometer to report a compass |
| bearing.</p> |
| |
| <p>The Android platform supports three broad categories of sensors:</p> |
| |
| <ul> |
| <li>Motion sensors |
| <p>These sensors measure acceleration forces and rotational forces along three axes. This |
| category includes accelerometers, gravity sensors, gyroscopes, and rotational vector |
| sensors.</p> |
| </li> |
| <li>Environmental sensors |
| <p>These sensors measure various environmental parameters, such as ambient air temperature |
| and pressure, illumination, and humidity. This category includes barometers, photometers, and |
| thermometers.</p> |
| </li> |
| <li>Position sensors |
| <p>These sensors measure the physical position of a device. This category includes |
| orientation sensors and magnetometers.</p> |
| </li> |
| </ul> |
| |
| |
| <p>You can access sensors available on the device and acquire raw sensor data by using the Android |
| sensor framework. The sensor framework provides several classes and interfaces that help you perform a wide |
| variety of sensor-related tasks. For example, you can use the sensor framework to do the following:</p> |
| |
| <ul> |
| <li>Determine which sensors are available on a device.</li> |
| <li>Determine an individual sensor's capabilities, such as its maximum range, manufacturer, power |
| requirements, and resolution.</li> |
| <li>Acquire raw sensor data and define the minimum rate at which you acquire sensor data.</li> |
| <li>Register and unregister sensor event listeners that monitor sensor changes.</li> |
| </ul> |
| |
| <p>This topic provides an overview of the sensors that are available on the Android platform. |
| It also provides an introduction to the sensor framework.</p> |
| |
| <h2 id="sensors-intro">Introduction to Sensors</h2> |
| |
| <p>The Android sensor framework lets you access many types of sensors. Some of these sensors are |
| hardware-based and some are software-based. Hardware-based sensors are physical components built |
| into a handset or tablet device. They derive their data by directly measuring specific environmental |
| properties, such as acceleration, geomagnetic field strength, or angular change. Software-based |
| sensors are not physical devices, although they mimic hardware-based sensors. Software-based sensors |
| derive their data from one or more of the hardware-based sensors and are sometimes called virtual |
| sensors or synthetic sensors. The linear acceleration sensor and the gravity sensor are examples of |
| software-based sensors. Table 1 summarizes the sensors that are supported by the Android |
| platform.</p> |
| |
| <p>Few Android-powered devices have every type of sensor. For example, most handset devices and |
| tablets have an accelerometer and a magnetometer, but fewer devices have |
| barometers or thermometers. Also, a device can have more than one sensor of a given type. For |
| example, a device can have two gravity sensors, each one having a different range.</p> |
| |
| <p class="table-caption" id="table1"> |
| <strong>Table 1.</strong> Sensor types supported by the Android platform.</p> |
| <table> |
| <tr> |
| <th scope="col" style="white-space:nowrap">Sensor</th> |
| <th scope="col" style="white-space:nowrap">Type</th> |
| <th scope="col" style="white-space:nowrap">Description</th> |
| <th scope="col" style="white-space:nowrap">Common Uses</th> |
| </tr> |
| <tr> |
| <td>{@link android.hardware.Sensor#TYPE_ACCELEROMETER}</td> |
| <td>Hardware</td> |
| <td>Measures the acceleration force in m/s<sup>2</sup> that is applied to a device on |
| all three physical axes (x, y, and z), including the force of gravity.</td> |
| <td>Motion detection (shake, tilt, etc.).</td> |
| </tr> |
| <tr> |
| <td>{@link android.hardware.Sensor#TYPE_AMBIENT_TEMPERATURE}</td> |
| <td>Hardware</td> |
| <td>Measures the ambient room temperature in degrees Celsius (°C). See note below.</td> |
| <td>Monitoring air temperatures.</td> |
| <tr> |
| <td>{@link android.hardware.Sensor#TYPE_GRAVITY}</td> |
| <td>Software or Hardware</td> |
| <td>Measures the force of gravity in m/s<sup>2</sup> that is applied to a device on all |
| three physical axes (x, y, z).</td> |
| <td>Motion detection (shake, tilt, etc.).</td> |
| </tr> |
| <tr> |
| <td>{@link android.hardware.Sensor#TYPE_GYROSCOPE}</td> |
| <td>Hardware</td> |
| <td>Measures a device's rate of rotation in rad/s around each of the three |
| physical axes |
| (x, y, and z).</td> |
| <td>Rotation detection (spin, turn, etc.).</td> |
| </tr> |
| <tr> |
| <td>{@link android.hardware.Sensor#TYPE_LIGHT}</td> |
| <td>Hardware</td> |
| <td>Measures the ambient light level (illumination) in lx.</td> |
| <td>Controlling screen brightness.</td> |
| </tr> |
| <tr> |
| <td>{@link android.hardware.Sensor#TYPE_LINEAR_ACCELERATION}</td> |
| <td>Software or Hardware</td> |
| <td>Measures the acceleration force in m/s<sup>2</sup> that is |
| applied to a device on |
| all three physical axes (x, y, and z), excluding the force of gravity.</td> |
| <td>Monitoring acceleration along a single axis.</td> |
| </tr> |
| <tr> |
| <td>{@link android.hardware.Sensor#TYPE_MAGNETIC_FIELD}</td> |
| <td>Hardware</td> |
| <td>Measures the ambient geomagnetic field for all three physical axes (x, y, z) in |
| μT.</td> |
| <td>Creating a compass.</td> |
| </tr> |
| <tr> |
| <td>{@link android.hardware.Sensor#TYPE_ORIENTATION}</td> |
| <td>Software</td> |
| <td>Measures degrees of rotation that a device makes around all three physical axes (x, y, z). |
| As of API level 3 you can obtain the inclination matrix and rotation matrix for |
| a device by using the gravity sensor and the geomagnetic field sensor in conjunction with |
| the {@link android.hardware.SensorManager#getRotationMatrix getRotationMatrix()} |
| method.</td> |
| <td>Determining device position.</td> |
| </tr> |
| <tr> |
| <td>{@link android.hardware.Sensor#TYPE_PRESSURE}</td> |
| <td>Hardware</td> |
| <td>Measures the ambient air pressure in hPa or mbar.</td> |
| <td>Monitoring air pressure changes.</td> |
| </tr> |
| <tr> |
| <td>{@link android.hardware.Sensor#TYPE_PROXIMITY}</td> |
| <td>Hardware</td> |
| <td>Measures the proximity of an object in cm relative to the view screen of a |
| device. This sensor is typically used to determine whether a handset is being held up to |
| a person's ear.</td> |
| <td>Phone position during a call.</td> |
| </tr> |
| <tr> |
| <td>{@link android.hardware.Sensor#TYPE_RELATIVE_HUMIDITY}</td> |
| <td>Hardware</td> |
| <td>Measures the relative ambient humidity in percent (%).</td> |
| <td>Monitoring dewpoint, absolute, and relative humidity.</td> |
| </tr> |
| <tr> |
| <td>{@link android.hardware.Sensor#TYPE_ROTATION_VECTOR}</td> |
| <td>Software or Hardware</td> |
| <td>Measures the orientation of a device by providing the three elements of the device's |
| rotation vector.</td> |
| <td>Motion detection and rotation detection.</td> |
| </tr> |
| <tr> |
| <td>{@link android.hardware.Sensor#TYPE_TEMPERATURE}</td> |
| <td>Hardware</td> |
| <td>Measures the temperature of the device in degrees Celsius (°C). This sensor |
| implementation varies across devices and |
| this sensor was replaced with the {@link android.hardware.Sensor#TYPE_AMBIENT_TEMPERATURE} sensor in |
| API Level 14</td> |
| <td>Monitoring temperatures.</td> |
| </tr> |
| </table> |
| |
| <h3>Sensor Framework</h3> |
| |
| <p>You can access these sensors and acquire raw sensor data by using the Android sensor framework. |
| The sensor framework is part of the {@link android.hardware} package and includes the following |
| classes and interfaces:</p> |
| |
| <dl> |
| <dt>{@link android.hardware.SensorManager}</dt> |
| <dd>You can use this class to create an instance of the sensor service. This class provides |
| various methods for accessing and listing sensors, registering and unregistering sensor event |
| listeners, and acquiring orientation information. This class also provides several sensor constants |
| that are used to report sensor accuracy, set data acquisition rates, and calibrate sensors.</dd> |
| <dt>{@link android.hardware.Sensor}</dt> |
| <dd>You can use this class to create an instance of a specific sensor. This class provides various |
| methods that let you determine a sensor's capabilities.</dd> |
| <dt>{@link android.hardware.SensorEvent}</dt> |
| <dd>The system uses this class to create a sensor event object, which provides information about a |
| sensor event. A sensor event object includes the following information: the raw sensor data, the |
| type of sensor that generated the event, the accuracy of the data, and the timestamp for the |
| event.</dd> |
| <dt>{@link android.hardware.SensorEventListener}</dt> |
| <dd>You can use this interface to create two callback methods that receive notifications (sensor |
| events) when sensor values change or when sensor accuracy changes.</dd> |
| </dl> |
| |
| <p>In a typical application you use these sensor-related APIs to perform two basic tasks:</p> |
| |
| <ul> |
| <li><strong>Identifying sensors and sensor capabilities</strong> |
| <p>Identifying sensors and sensor capabilities at runtime is useful if your application has |
| features that rely on specific sensor types or capabilities. For example, you may want to |
| identify all of the sensors that are present on a device and disable any application features |
| that rely on sensors that are not present. Likewise, you may want to identify all of the sensors |
| of a given type so you can choose the sensor implementation that has the optimum performance |
| for your application.</p> |
| </li> |
| <li><strong>Monitor sensor events</strong> |
| <p>Monitoring sensor events is how you acquire raw sensor data. A sensor event occurs every time |
| a sensor detects a change in the parameters it is measuring. A sensor event provides you |
| with four pieces of information: the name of the sensor that triggered the event, the |
| timestamp for the event, the accuracy of the event, and the raw sensor data that triggered |
| the event.</p> |
| </li> |
| </ul> |
| |
| <h3>Sensor Availability</h3> |
| |
| <p>While sensor availability varies from device to device, it can also vary between Android |
| versions. This is because the Android sensors have been introduced over the course of several |
| platform releases. For example, many sensors were introduced in Android 1.5 (API Level 3), but some |
| were not implemented and were not available for use until Android 2.3 (API Level 9). Likewise, |
| several sensors were introduced in Android 2.3 (API Level 9) and Android 4.0 (API Level 14). Two |
| sensors have been deprecated and replaced by newer, better sensors.</p> |
| |
| <p>Table 2 summarizes the availability of each sensor on a platform-by-platform basis. Only four |
| platforms are listed because those are the platforms that involved sensor changes. Sensors that are |
| listed as deprecated are still available on subsequent platforms (provided the |
| sensor is present on a device), which is in line with Android's forward compatibility policy.</p> |
| |
| <p class="table-caption" id="table2"> |
| <strong>Table 2.</strong> Sensor availability by platform.</p> |
| <table> |
| <tr> |
| <th scope="col">Sensor</th> |
| <th scope="col">Android 4.0 <br>(API Level 14)</th> |
| <th scope="col">Android 2.3 <br>(API Level 9)</th> |
| <th scope="col">Android 2.2 <br>(API Level 8)</th> |
| <th scope="col">Android 1.5 <br>(API Level 3)</th> |
| </tr> |
| <tr> |
| <td>{@link android.hardware.Sensor#TYPE_ACCELEROMETER}</td> |
| <td><strong>Yes</strong></td> |
| <td><strong>Yes</strong></td> |
| <td><strong>Yes</strong></td> |
| <td><strong>Yes</strong></td> |
| </tr> |
| <tr> |
| <td>{@link android.hardware.Sensor#TYPE_AMBIENT_TEMPERATURE}</td> |
| <td><strong>Yes</strong></td> |
| <td>n/a</td> |
| <td>n/a</td> |
| <td>n/a</td> |
| </tr> |
| <tr> |
| <td>{@link android.hardware.Sensor#TYPE_GRAVITY}</td> |
| <td><strong>Yes</strong></td> |
| <td><strong>Yes</strong></td> |
| <td>n/a</td> |
| <td>n/a</td> |
| </tr> |
| <tr> |
| <td>{@link android.hardware.Sensor#TYPE_GYROSCOPE}</td> |
| <td><strong>Yes</strong></td> |
| <td><strong>Yes</strong></td> |
| <td>n/a<sup>1</sup></td> |
| <td>n/a<sup>1</sup></td> |
| </tr> |
| <tr> |
| <td>{@link android.hardware.Sensor#TYPE_LIGHT}</td> |
| <td><strong>Yes</strong></td> |
| <td><strong>Yes</strong></td> |
| <td><strong>Yes</strong></td> |
| <td><strong>Yes</strong></td> |
| </tr> |
| <tr> |
| <td>{@link android.hardware.Sensor#TYPE_LINEAR_ACCELERATION}</td> |
| <td><strong>Yes</strong></td> |
| <td><strong>Yes</strong></td> |
| <td>n/a</td> |
| <td>n/a</td> |
| </tr> |
| <tr> |
| <td>{@link android.hardware.Sensor#TYPE_MAGNETIC_FIELD}</td> |
| <td><strong>Yes</strong></td> |
| <td><strong>Yes</strong></td> |
| <td><strong>Yes</strong></td> |
| <td><strong>Yes</strong></td> |
| </tr> |
| <tr> |
| <td>{@link android.hardware.Sensor#TYPE_ORIENTATION}</td> |
| <td><strong>Yes</strong><sup>2</sup></td> |
| <td><strong>Yes</strong><sup>2</sup></td> |
| <td><strong>Yes</strong><sup>2</sup></td> |
| <td><strong>Yes</strong></td> |
| </tr> |
| <tr> |
| <td>{@link android.hardware.Sensor#TYPE_PRESSURE}</td> |
| <td><strong>Yes</strong></td> |
| <td><strong>Yes</strong></td> |
| <td>n/a<sup>1</sup></td> |
| <td>n/a<sup>1</sup></td> |
| </tr> |
| <tr> |
| <td>{@link android.hardware.Sensor#TYPE_PROXIMITY}</td> |
| <td><strong>Yes</strong></td> |
| <td><strong>Yes</strong></td> |
| <td><strong>Yes</strong></td> |
| <td><strong>Yes</strong></td> |
| </tr> |
| <tr> |
| <td>{@link android.hardware.Sensor#TYPE_RELATIVE_HUMIDITY}</td> |
| <td><strong>Yes</strong></td> |
| <td>n/a</td> |
| <td>n/a</td> |
| <td>n/a</td> |
| </tr> |
| <tr> |
| <td>{@link android.hardware.Sensor#TYPE_ROTATION_VECTOR}</td> |
| <td><strong>Yes</strong></td> |
| <td><strong>Yes</strong></td> |
| <td>n/a</td> |
| <td>n/a</td> |
| </tr> |
| <tr> |
| <td>{@link android.hardware.Sensor#TYPE_TEMPERATURE}</td> |
| <td><strong>Yes</strong><sup>2</sup></td> |
| <td><strong>Yes</strong></td> |
| <td><strong>Yes</strong></td> |
| <td><strong>Yes</strong></td> |
| </tr> |
| </table> |
| |
| <p class="note"><strong><sup>1</sup></strong> This sensor type was added in Android 1.5 (API Level |
| 3), |
| but it was not available for use until Android 2.3 (API Level 9).</p> |
| |
| <p class="note"><strong><sup>2</sup></strong> This sensor is available, but it has been |
| deprecated.</p> |
| |
| <h2 id="sensors-identify">Identifying Sensors and Sensor Capabilities</h2> |
| |
| <p>The Android sensor framework provides several methods that make it easy for you to determine at |
| runtime which sensors are on a device. The API also provides methods that let you determine the |
| capabilities of each sensor, such as its maximum range, its resolution, and its power |
| requirements.</p> |
| |
| <p>To identify the sensors that are on a device you first need to get a reference to the sensor |
| service. To do this, you create an instance of the {@link android.hardware.SensorManager} class by |
| calling the {@link android.content.Context#getSystemService getSystemService()} method and passing |
| in the {@link android.content.Context#SENSOR_SERVICE SENSOR_SERVICE} argument. For example:</p> |
| |
| <pre> |
| private SensorManager mSensorManager; |
| ... |
| mSensorManager = (SensorManager) getSystemService(Context.SENSOR_SERVICE); |
| </pre> |
| |
| <p>Next, you can get a listing of every sensor on a device by calling the |
| {@link android.hardware.SensorManager#getSensorList getSensorList()} method and using the {@link |
| android.hardware.Sensor#TYPE_ALL} constant. For example:</p> |
| <pre> |
| List<Sensor> deviceSensors = mSensorManager.getSensorList(Sensor.TYPE_ALL); |
| </pre> |
| |
| <p>If you want to list all of the sensors of a given type, you could use another constant instead of |
| {@link android.hardware.Sensor#TYPE_ALL} such as {@link android.hardware.Sensor#TYPE_GYROSCOPE}, |
| {@link android.hardware.Sensor#TYPE_LINEAR_ACCELERATION}, or |
| {@link android.hardware.Sensor#TYPE_GRAVITY}. |
| </p> |
| |
| <p>You can also determine whether a specific type of sensor exists on a device by using the {@link |
| android.hardware.SensorManager#getDefaultSensor getDefaultSensor()} method and passing in the type |
| constant for a specific sensor. If a device has more than one sensor of a given type, one of the |
| sensors must be designated as the default sensor. If a default sensor does not exist for a given |
| type of sensor, the method call returns null, which means the device does not have that type of |
| sensor. For example, the following code checks whether there's a magnetometer on a device:</p> |
| <pre> |
| private SensorManager mSensorManager; |
| ... |
| mSensorManager = (SensorManager) getSystemService(Context.SENSOR_SERVICE); |
| if (mSensorManager.getDefaultSensor(Sensor.TYPE_MAGNETIC_FIELD) != null){ |
| // Success! There's a magnetometer. |
| } |
| else { |
| // Failure! No magnetometer. |
| } |
| </pre> |
| |
| <p class="note"><strong>Note:</strong> Android does not require device manufacturers to build any |
| particular types of sensors into their Android-powered devices, so devices can have a wide range of |
| sensor configurations.</p> |
| |
| <p>In addition to listing the sensors that are on a device, you can use the public methods of the |
| {@link android.hardware.Sensor} class to determine the capabilities and attributes of individual |
| sensors. This is useful if you want your application to behave differently based on which sensors or |
| sensor capabilities are available on a device. For example, you can use the {@link |
| android.hardware.Sensor#getResolution} and {@link android.hardware.Sensor#getMaximumRange} |
| methods to obtain a sensor's resolution and maximum range of measurement. You can also use the |
| {@link android.hardware.Sensor#getPower} method to obtain a sensor's power requirements.</p> |
| |
| <p>Two of the public methods are particularly useful if you want to optimize your application for |
| different manufacturer's sensors or different versions of a sensor. For example, if your application |
| needs to monitor user gestures such as tilt and shake, you could create one set of data filtering |
| rules and optimizations for newer devices that have a specific vendor's gravity sensor, and another |
| set of data filtering rules and optimizations for devices that do not have a gravity sensor and have |
| only an accelerometer. The following code sample shows you how you can use the {@link |
| android.hardware.Sensor#getVendor} and {@link android.hardware.Sensor#getVersion} methods to do |
| this. In this sample, we're looking for a gravity sensor that lists Google Inc. as the vendor and |
| has a version number of 3. If that particular sensor is not present on the device, we try to use the |
| accelerometer.</p> |
| |
| <pre> |
| private SensorManager mSensorManager; |
| private Sensor mSensor; |
| |
| ... |
| |
| mSensorManager = (SensorManager) getSystemService(Context.SENSOR_SERVICE); |
| |
| if (mSensorManager.getDefaultSensor(Sensor.TYPE_GRAVITY) != null){ |
| List<Sensor> gravSensors = mSensorManager.getSensorList(Sensor.TYPE_GRAVITY); |
| for(int i=0; i<gravSensors.size(); i++) { |
| if ((gravSensors.get(i).getVendor().contains("Google Inc.")) && |
| (gravSensors.get(i).getVersion() == 3)){ |
| // Use the version 3 gravity sensor. |
| mSensor = gravSensors.get(i); |
| } |
| } |
| } |
| else{ |
| // Use the accelerometer. |
| if (mSensorManager.getDefaultSensor(Sensor.TYPE_ACCELEROMETER) != null){ |
| mSensor = mSensorManager.getDefaultSensor(Sensor.TYPE_ACCELEROMETER); |
| } |
| else{ |
| // Sorry, there are no accelerometers on your device. |
| // You can't play this game. |
| } |
| } |
| </pre> |
| |
| <p>Another useful method is the {@link android.hardware.Sensor#getMinDelay getMinDelay()} method, |
| which returns the minimum time interval (in microseconds) a sensor can use to sense data. Any sensor |
| that returns a non-zero value for the {@link android.hardware.Sensor#getMinDelay getMinDelay()} |
| method is a streaming |
| sensor. Streaming sensors sense data at regular intervals and were introduced in Android 2.3 (API |
| Level 9). If a sensor returns zero when you call the {@link android.hardware.Sensor#getMinDelay |
| getMinDelay()} method, it means the |
| sensor is not a streaming sensor because it reports data only when there is a change in the |
| parameters it is sensing.</p> |
| |
| <p>The {@link android.hardware.Sensor#getMinDelay getMinDelay()} method is useful because it lets |
| you determine the maximum rate |
| at which a sensor can acquire data. If certain features in your application require high data |
| acquisition rates or a streaming sensor, you can use this method to determine whether a sensor |
| meets those requirements and then enable or disable the relevant features in your application |
| accordingly.</p> |
| |
| <p class="caution"><strong>Caution:</strong> A sensor's maximum data acquisition rate is not |
| necessarily the rate at which the sensor framework delivers sensor data to your application. The |
| sensor framework reports data through sensor events, and several factors influence the rate at which |
| your application receives sensor events. For more information, see <a |
| href="#sensors-monitor">Monitoring Sensor Events</a>.</p> |
| |
| <h2 id="sensors-monitor">Monitoring Sensor Events</h2> |
| |
| <p>To monitor raw sensor data you need to implement two callback methods that are exposed through |
| the {@link android.hardware.SensorEventListener} interface: {@link |
| android.hardware.SensorEventListener#onAccuracyChanged onAccuracyChanged()} and {@link |
| android.hardware.SensorEventListener#onSensorChanged onSensorChanged()}. The Android system calls |
| these methods whenever the following occurs:</p> |
| |
| <ul> |
| <li><strong>A sensor's accuracy changes.</strong> |
| <p>In this case the system invokes the {@link |
| android.hardware.SensorEventListener#onAccuracyChanged onAccuracyChanged()} method, providing |
| you with a reference to the {@link android.hardware.Sensor Sensor} object that changed and the |
| new accuracy of the sensor. Accuracy is represented by one of four status constants: |
| {@link android.hardware.SensorManager#SENSOR_STATUS_ACCURACY_LOW}, |
| {@link android.hardware.SensorManager#SENSOR_STATUS_ACCURACY_MEDIUM}, |
| {@link android.hardware.SensorManager#SENSOR_STATUS_ACCURACY_HIGH}, |
| or {@link android.hardware.SensorManager#SENSOR_STATUS_UNRELIABLE}.</p> |
| </li> |
| <li><strong>A sensor reports a new value.</strong> |
| <p>In this case the system invokes the {@link |
| android.hardware.SensorEventListener#onSensorChanged onSensorChanged()} method, providing you with |
| a {@link android.hardware.SensorEvent SensorEvent} object. A {@link android.hardware.SensorEvent |
| SensorEvent} object |
| contains information about the new sensor data, including: the accuracy of the data, the |
| sensor that generated the data, the timestamp at which the data was generated, and the new |
| data that the sensor recorded.</p> |
| </li> |
| </ul> |
| |
| <p>The following code shows how to use the {@link |
| android.hardware.SensorEventListener#onSensorChanged onSensorChanged()} method to monitor data from |
| the light sensor. This example displays the raw sensor data in a {@link android.widget.TextView} |
| that is |
| defined in the main.xml file as <code>sensor_data</code>.</p> |
| |
| <pre> |
| public class SensorActivity extends Activity implements SensorEventListener { |
| private SensorManager mSensorManager; |
| private Sensor mLight; |
| |
| @Override |
| public final void onCreate(Bundle savedInstanceState) { |
| super.onCreate(savedInstanceState); |
| setContentView(R.layout.main); |
| |
| mSensorManager = (SensorManager) getSystemService(Context.SENSOR_SERVICE); |
| mLight = mSensorManager.getDefaultSensor(Sensor.TYPE_LIGHT); |
| } |
| |
| @Override |
| public final void onAccuracyChanged(Sensor sensor, int accuracy) { |
| // Do something here if sensor accuracy changes. |
| } |
| |
| @Override |
| public final void onSensorChanged(SensorEvent event) { |
| // The light sensor returns a single value. |
| // Many sensors return 3 values, one for each axis. |
| float lux = event.values[0]; |
| // Do something with this sensor value. |
| } |
| |
| @Override |
| protected void onResume() { |
| super.onResume(); |
| mSensorManager.registerListener(this, mLight, SensorManager.SENSOR_DELAY_NORMAL); |
| } |
| |
| @Override |
| protected void onPause() { |
| super.onPause(); |
| mSensorManager.unregisterListener(this); |
| } |
| } |
| </pre> |
| |
| <p>In this example, the default data delay ({@link |
| android.hardware.SensorManager#SENSOR_DELAY_NORMAL}) is specified when the {@link |
| android.hardware.SensorManager#registerListener registerListener()} method is invoked. The data |
| delay (or sampling rate) controls the interval at which sensor events are sent to your application |
| via the {@link |
| android.hardware.SensorEventListener#onSensorChanged onSensorChanged()} callback method. The default |
| data delay is suitable for monitoring |
| typical screen orientation changes and uses a delay of 200,000 microseconds. You can specify other |
| data delays, such as {@link android.hardware.SensorManager#SENSOR_DELAY_GAME} (20,000 microsecond |
| delay), {@link android.hardware.SensorManager#SENSOR_DELAY_UI} (60,000 microsecond delay), or {@link |
| android.hardware.SensorManager#SENSOR_DELAY_FASTEST} (0 microsecond delay). As of Android 3.0 (API |
| Level 11) you can also specify the delay as an absolute value (in microseconds).</p> |
| |
| <p>The delay that you specify is only a suggested delay. The Android system and other applications |
| can alter this delay. As a best practice, you should specify the largest delay that you can because |
| the system typically uses a smaller delay than the one you specify (that is, you should choose the |
| slowest sampling rate that still meets the needs of your application). Using a larger delay imposes |
| a lower load on the processor and therefore uses less power.</p> |
| |
| <p>There is no public method for determining the rate at which the sensor framework is sending |
| sensor events to your application; however, you can use the timestamps that are associated with each |
| sensor event to calculate the sampling rate over several events. You should not have to change the |
| sampling rate (delay) once you set it. If for some reason you do need to change the delay, you will |
| have to unregister and reregister the sensor listener.</p> |
| |
| <p>It's also important to note that this example uses the {@link android.app.Activity#onResume} and |
| {@link android.app.Activity#onPause} callback methods to register and unregister the sensor event |
| listener. As a best practice you should always disable sensors you don't need, especially when your |
| activity is paused. Failing to do so can drain the battery in just a few hours because some sensors |
| have substantial power requirements and can use up battery power quickly. The system |
| will not disable sensors automatically when the screen turns off.</p> |
| |
| <h2 id="sensors-configs">Handling Different Sensor Configurations</h2> |
| |
| <p>Android does not specify a standard sensor configuration for devices, |
| which means device manufacturers can incorporate any sensor configuration that they want into their |
| Android-powered devices. As a result, devices can include a variety |
| of sensors in a wide range of configurations. For example, the Motorola Xoom has a pressure sensor, |
| but the Samsung Nexus S does not. Likewise, the Xoom and Nexus S have gyroscopes, but the HTC Nexus |
| One does not. If your application relies on a specific type of sensor, you have to ensure that the |
| sensor is present on a device so your app can run successfully. You have two options for ensuring |
| that a given sensor is present on a device:</p> |
| <ul> |
| <li>Detect sensors at runtime and enable or disable application features as appropriate.</li> |
| <li>Use Google Play filters to target devices with specific sensor configurations.</li> |
| </ul> |
| |
| <p>Each option is discussed in the following sections.</p> |
| |
| <h4><strong>Detecting sensors at runtime</strong></h4> |
| |
| <p>If your application uses a specific type of sensor, but doesn't rely on it, you can use the |
| sensor framework to detect the sensor at runtime and then disable or enable application features |
| as appropriate. For example, a navigation application might use the temperature sensor, |
| pressure sensor, GPS sensor, and geomagnetic field sensor to display the temperature, barometric |
| pressure, location, and compass bearing. If a device doesn't have a pressure sensor, you can use the |
| sensor framework to detect the absence of the pressure sensor at runtime and then disable the |
| portion of your application's UI that displays pressure. For example, the following code checks |
| whether there's a pressure sensor on a device:</p> |
| <pre> |
| private SensorManager mSensorManager; |
| ... |
| mSensorManager = (SensorManager) getSystemService(Context.SENSOR_SERVICE); |
| if (mSensorManager.getDefaultSensor(Sensor.TYPE_PRESSURE) != null){ |
| // Success! There's a pressure sensor. |
| } |
| else { |
| // Failure! No pressure sensor. |
| } |
| </pre> |
| |
| <h4>Using Google Play filters to target specific sensor configurations</h4> |
| |
| <p>If you are publishing your application on Google Play you can use the |
| <a href="{@docRoot}guide/topics/manifest/uses-feature-element.html"><code><uses-feature> |
| </code></a> element in your manifest file to filter your application from devices that do not |
| have the appropriate sensor configuration for your application. The |
| <code><uses-feature></code> element has several hardware descriptors that let you filter |
| applications based on the presence of specific sensors. The sensors you can list include: |
| accelerometer, barometer, compass (geomagnetic field), gyroscope, light, and proximity. The |
| following is an example manifest entry that filters apps that do not have an accelerometer:</p> |
| |
| <pre> |
| <uses-feature android:name="android.hardware.sensor.accelerometer" |
| android:required="true" /> |
| </pre> |
| |
| <p>If you add this element and descriptor to your application's manifest, users will see your |
| application on Google Play only if their device has an accelerometer.</p> |
| |
| <p>You should set the descriptor to <code>android:required="true"</code> only if your application |
| relies entirely on a specific sensor. If your application uses a sensor for some functionality, but |
| still runs without the sensor, you should list the sensor in the <code><uses-feature></code> |
| element, but set the descriptor to <code>android:required="false"</code>. This helps ensure that |
| devices can install your app even if they do not have that particular sensor. This is also a |
| project management best practice that helps you keep track of the features your application uses. |
| Keep in mind, if your application uses a particular sensor, but still runs without the sensor, |
| then you should detect the sensor at runtime and disable or enable application features as |
| appropriate.</p> |
| |
| <h2 id="sensors-coords">Sensor Coordinate System</h2> |
| |
| <p>In general, the sensor framework uses a standard 3-axis coordinate system to express data values. |
| For most sensors, the coordinate system is defined relative to the device's screen when the device |
| is held in its default orientation (see figure 1). When a device is held in its default orientation, |
| the X axis is horizontal and points to the right, the Y axis is vertical and points up, and the Z |
| axis points toward the outside of the screen face. In this system, coordinates behind the screen |
| have negative Z values. This coordinate system is used by the following sensors:</p> |
| |
| <div class="figure" style="width:269px"> |
| <img src="{@docRoot}images/axis_device.png" alt="" height="225" /> |
| <p class="img-caption"> |
| <strong>Figure 1.</strong> Coordinate system (relative to a device) that's used by the Sensor |
| API. |
| </p> |
| </div> |
| |
| <ul> |
| <li><a |
| href="{@docRoot}guide/topics/sensors/sensors_motion.html#sensors-motion-accel">Acceleration |
| sensor</a></li> |
| <li><a |
| href="{@docRoot}guide/topics/sensors/sensors_motion.html#sensors-motion-gravity">Gravity |
| sensor</a></li> |
| <li><a |
| href="{@docRoot}guide/topics/sensors/sensors_motion.html#sensors-motion-gyro">Gyroscope</a></li> |
| <li><a |
| href="{@docRoot}guide/topics/sensors/sensors_motion.html#sensors-motion-linear">Linear acceleration |
| sensor</a></li> |
| <li><a |
| href="{@docRoot}guide/topics/sensors/sensors_position.html#sensors-pos-mag">Geomagnetic field |
| sensor</a></li> |
| </ul> |
| |
| <p>The most important point to understand about this coordinate system is that the axes are not |
| swapped when the device's screen orientation changes—that is, the sensor's coordinate system |
| never changes as the device moves. This behavior is the same as the behavior of the OpenGL |
| coordinate system.</p> |
| |
| <p>Another point to understand is that your application must not assume that a device's natural |
| (default) orientation is portrait. The natural orientation for many tablet devices is landscape. And |
| the sensor coordinate system is always based on the natural orientation of a device.</p> |
| |
| <p>Finally, if your application matches sensor data to the on-screen display, you need to use the |
| {@link android.view.Display#getRotation} method to determine screen rotation, and then use the |
| {@link android.hardware.SensorManager#remapCoordinateSystem remapCoordinateSystem()} method to map |
| sensor coordinates to screen coordinates. You need to do this even if your manifest specifies |
| portrait-only display.</p> |
| |
| <p>For more information about the sensor coordinate system, including information about how to |
| handle screen rotations, see <a |
| href="http://android-developers.blogspot.com/2010/09/one-screen-turn-deserves-another.html">One |
| Screen Turn Deserves Another</a>.</p> |
| |
| <p class="note"><strong>Note:</strong> Some sensors and methods use a coordinate system that is |
| relative to the world's frame of reference (as opposed to the device's frame of reference). These |
| sensors and methods return data that represent device motion or device position relative to the |
| earth. For more information, see the {@link android.hardware.SensorManager#getOrientation |
| getOrientation()} method, the {@link android.hardware.SensorManager#getRotationMatrix |
| getRotationMatrix()} method, <a |
| href="{@docRoot}guide/topics/sensors/sensors_position.html#sensors-pos-orient">Orientation |
| Sensor</a>, and <a |
| href="{@docRoot}guide/topics/sensors/sensors_motion.html#sensors-motion-rotate">Rotation Vector |
| Sensor</a>.</p> |
| |
| <h2 id="sensors-practices">Best Practices for Accessing and Using Sensors</h2> |
| |
| <p>As you design your sensor implementation, be sure to follow the guidelines that are discussed in |
| this section. These guidelines are recommended best practices for anyone who is using the sensor |
| framework to access sensors and acquire sensor data.</p> |
| |
| <h4>Unregister sensor listeners</h4> |
| |
| <p>Be sure to unregister a sensor's listener when you are done using the sensor or when the sensor |
| activity pauses. If a sensor listener is registered and its activity is paused, the sensor will |
| continue to acquire data and use battery resources unless you unregister the sensor. The following |
| code shows how to use the {@link android.app.Activity#onPause} method to unregister a listener:</p> |
| |
| <pre> |
| private SensorManager mSensorManager; |
| ... |
| @Override |
| protected void onPause() { |
| super.onPause(); |
| mSensorManager.unregisterListener(this); |
| } |
| </pre> |
| |
| <p>For more information, see {@link android.hardware.SensorManager#unregisterListener}.</p> |
| |
| <h4>Don't test your code on the emulator</h4> |
| |
| <p>You currently can't test sensor code on the emulator because the emulator cannot emulate sensors. |
| You must test your sensor code on a physical device. There are, however, sensor simulators that you |
| can use to simulate sensor output.</p> |
| |
| <h4>Don't block the onSensorChanged() method</h4> |
| |
| <p>Sensor data can change at a high rate, which means the system may call the {@link |
| android.hardware.SensorEventListener#onSensorChanged} method quite often. As a best practice, you |
| should do as little as possible within the {@link |
| android.hardware.SensorEventListener#onSensorChanged} method so you don't block it. If your |
| application requires you to do any data filtering or reduction of sensor data, you should perform |
| that work outside of the {@link android.hardware.SensorEventListener#onSensorChanged} method.</p> |
| |
| <h4>Avoid using deprecated methods or sensor types</h4> |
| |
| <p>Several methods and constants have been deprecated. |
| In particular, the {@link android.hardware.Sensor#TYPE_ORIENTATION} |
| sensor type has been deprecated. To get orientation data you should use the {@link |
| android.hardware.SensorManager#getOrientation getOrientation()} method instead. Likewise, the |
| {@link android.hardware.Sensor#TYPE_TEMPERATURE} sensor type has been deprecated. You should use |
| the {@link android.hardware.Sensor#TYPE_AMBIENT_TEMPERATURE} sensor type instead on devices |
| that are running Android 4.0.</p> |
| |
| <h4>Verify sensors before you use them</h4> |
| |
| <p>Always verify that a sensor exists on a device before you attempt to acquire data from it. Don't |
| assume that a sensor exists simply because it's a frequently-used sensor. Device manufacturers are |
| not required to provide any particular sensors in their devices.</p> |
| |
| <h4>Choose sensor delays carefully</h4> |
| |
| <p>When you register a sensor with the {@link android.hardware.SensorManager#registerListener |
| registerListener()} method, be sure you choose a delivery rate that is suitable for your |
| application or use-case. Sensors can provide data at very high rates. Allowing the system to send |
| extra data that you don't need wastes system resources and uses battery power.</p> |