Qt Sensors C++ Overview

Sensor Types

On a device there can be many types of sensors. Not all of the types that the Qt Sensors API supports may be available. There may also be types available that are not defined in the Qt Sensors API. The types of sensors available on a device is found using the QSensor::sensorTypes() function.

For a list of built-in sensor types, see the Sensor Classes section below.

Common Conventions

Unless specified otherwise, Qt Sensors uses the Right Hand Cartesian coordinate system.

To allow for measurements in all 6 directions, negative values are used.

Where rotation around an axis is used, the rotation shall be expressed as a Right Hand rotation.

In general, sensor data is oriented relative to QPlatformScreen::nativeOrientation, that is to the top of the device when the device is held in its natural orientation (normally when the device logo appears the right side up). If values are to be displayed on the screen, the values may need to be transformed so that they match the user interface orientation. A sensor may define its data as being oriented to the UI. This will be noted in the documentation for the sensor.

Using a Sensor

The life cycle of a QSensor is typically:

• Create an instance of QSensor or one of its sub-classes on the stack or heap.
• Setup as required by the application.
• Start receiving values.
• Sensor data is used by the application.
• Stop receiving values.

Here is an example of creating a sensor on the heap and on the stack.

  // On the heap (deleted when this object is deleted)
  QAccelerometer *sensor = new QAccelerometer(this);

  // On the stack (deleted when the current scope ends)
  QOrientationSensor orient_sensor;

Accessing Sensor Data in a Generic Fashion

The preferred way to deal with sensor data is via the Reading Classes. However, sometimes this may not be possible. For example, you may be deploying an application to a device that has a new sensor type but no C++ header describing the reading class is available.

Thanks to Qt's property system you can still access the sensor data. You need to know 3 pieces of information in order to do this:

• The sensor type.
• The property name or index.
• The property type or a comparable type.

For example, here is an example of how you can access a property of the accelerometer. This code does not require any compile-time links to QAccelerometer or QAccelerometerReading.

  // start the sensor
  QSensor sensor("QAccelerometer");
  sensor.start();

  // later
  QSensorReading *reading = sensor.reading();
  qreal x = reading->property("x").value<qreal>();
  qreal y = reading->value(1).value<qreal>();

You can discover all of this information at runtime too. The sensor_explorer example shows you information about available sensors.

Front End, Back End

The Qt Sensors API has a front end, for application developers to use and a back end, where device implementors write code to access their hardware. As an application developer you do not need to access the back end though it may be useful to understand how it works.

Commands from the application are delivered through QSensor and then down to the device plugin. Data comes back through the QSensorReading class.

More information about the back end can be found in Qt Sensors Backend.

Main Classes

The primary classes that make up the Qt Sensors API.

QSensor	Represents a single hardware sensor
QSensorFilter	Efficient callback facility for asynchronous notifications of sensor changes
QSensorReading	Holds the readings from the sensor

Reading Classes

The best way to access sensor data is via one of these classes.

QAccelerometerReading	Reports on linear acceleration along the X, Y and Z axes
QAltimeterReading	Holds readings from the altimeter sensor
QAmbientLightReading	Represents one reading from the ambient light sensor
QAmbientTemperatureReading	Holds readings of the ambient temperature
QCompassReading	Represents one reading from a compass
QDistanceReading	Holds distance reading in cm from the proximity sensor
QGyroscopeReading	Represents one reading from the gyroscope sensor
QHolsterReading	Holds readings from the holster sensor
QHumidityReading	Holds readings from the humidity sensor
QIRProximityReading	Holds readings from the IR proximity sensor
QLidReading	Holds readings from the Lid sensor
QLightReading	Represents one reading from the light sensor
QMagnetometerReading	Represents one reading from the magnetometer
QOrientationReading	Represents one reading from the orientation sensor
QPressureReading	Holds readings from the pressure sensor
QProximityReading	Represents one reading from the proximity sensor
QRotationReading	Represents one reading from the rotation sensor
QTapReading	Represents one reading from the tap sensor
QTiltReading	Holds readings from the tilt sensor

Sensor Classes

These classes provide convenience wrappers that reduce the need for casting. Each of these classes represents a sensor type that the Qt Sensors API knows about. Note that additional types may be made available at run-time. See Sensor Types for more information.

QAccelerometer	Convenience wrapper around QSensor
QAltimeter	Convenience wrapper around QSensor
QAmbientLightSensor	Convenience wrapper around QSensor
QAmbientTemperatureSensor	Convenience wrapper around QSensor
QCompass	Convenience wrapper around QSensor
QDistanceSensor	Convenience wrapper around QSensor
QGyroscope	Convenience wrapper around QSensor
QHolsterSensor	Convenience wrapper around QSensor
QHumiditySensor	Convenience wrapper around QSensor
QIRProximitySensor	Convenience wrapper around QSensor
QLidSensor	Convenience wrapper around QSensor
QLightSensor	Convenience wrapper around QSensor
QMagnetometer	Convenience wrapper around QSensor
QOrientationSensor	Convenience wrapper around QSensor
QPressureSensor	Convenience wrapper around QSensor
QProximitySensor	Convenience wrapper around QSensor
QRotationSensor	Convenience wrapper around QSensor
QTapSensor	Convenience wrapper around QSensor
QTiltSensor	Convenience wrapper around QSensor

Filter Classes

As with the sensor classes, these provide convenience wrappers that reduce the need for casting.

QAccelerometerFilter	Convenience wrapper around QSensorFilter
QAltimeterFilter	Convenience wrapper around QSensorFilter
QAmbientLightFilter	Convenience wrapper around QSensorFilter
QAmbientTemperatureFilter	Convenience wrapper around QSensorFilter
QCompassFilter	Convenience wrapper around QSensorFilter
QDistanceFilter	Convenience wrapper around QSensorFilter
QGyroscopeFilter	Convenience wrapper around QSensorFilter
QHolsterFilter	Convenience wrapper around QSensorFilter
QHumidityFilter	Convenience wrapper around QSensorFilter
QIRProximityFilter	Convenience wrapper around QSensorFilter
QLidFilter	Convenience wrapper around QSensorFilter
QLightFilter	Convenience wrapper around QSensorFilter
QMagnetometerFilter	Convenience wrapper around QSensorFilter
QOrientationFilter	Convenience wrapper around QSensorFilter
QPressureFilter	Convenience wrapper around QSensorFilter
QProximityFilter	Convenience wrapper around QSensorFilter
QRotationFilter	Convenience wrapper around QSensorFilter
QTapFilter	Convenience wrapper around QSensorFilter
QTiltFilter	Convenience wrapper around QSensorFilter