QSensor Class | QtSensors 5.3

- Qt 5.3
- Qt Sensors
- C++ Classes
- QSensor
- Qt 5.3.2 Reference Documentation

# QSensor Class

The QSensor class represents a single hardware sensor. More...

Header: | `#include <QSensor>`
qmake: | `QT += sensors`
Inherited By: | QAccelerometer, QAltimeter, QAmbientLightSensor, QAmbientTemperatureSensor, QCompass, QGyroscope, QHolsterSensor, QIRProximitySensor, QLightSensor, QMagnetometer, QOrientationSensor, QPressureSensor, QProximitySensor, QRotationSensor, QTapSensor, and QTiltSensor.

- List of all members, including inherited members

## Public Types

enum | AxesOrientationMode { FixedOrientation, AutomaticOrientation, UserOrientation }
enum | Feature { Buffering, AlwaysOn, SkipDuplicates, GeoValues, ..., AxesOrientation }

## Properties

active : bool alwaysOn : bool availableDataRates : const qrangelist axesOrientationMode : AxesOrientationMode bufferSize : int busy : const bool connectedToBackend : const bool currentOrientation : const int dataRate : int description : const QString | efficientBufferSize : const int error : const int identifier : QByteArray maxBufferSize : const int outputRange : int outputRanges : const qoutputrangelist reading : QSensorReading * const skipDuplicates : bool type : const QByteArray userOrientation : int

## Public Functions

| QSensor(const QByteArray & type, QObject * parent = 0)
virtual | ~QSensor()
void | addFilter(QSensorFilter * filter)
qrangelist | availableDataRates() const
AxesOrientationMode | axesOrientationMode() const
int | bufferSize() const
bool | connectToBackend()
int | currentOrientation() const
int | dataRate() const
QString | description() const
int | efficientBufferSize() const
int | error() const
QList<QSensorFilter *> | filters() const
QByteArray | identifier() const
bool | isActive() const
bool | isAlwaysOn() const
bool | isBusy() const
bool | isConnectedToBackend() const
bool | isFeatureSupported(Feature feature) const
int | maxBufferSize() const
int | outputRange() const
qoutputrangelist | outputRanges() const
QSensorReading * | reading() const
void | removeFilter(QSensorFilter * filter)
void | setActive(bool active)
void | setAlwaysOn(bool alwaysOn)
void | setAxesOrientationMode(AxesOrientationMode axesOrientationMode)
void | setBufferSize(int bufferSize)
void | setCurrentOrientation(int currentOrientation)
void | setDataRate(int rate)
void | setEfficientBufferSize(int efficientBufferSize)
void | setIdentifier(const QByteArray & identifier)
void | setMaxBufferSize(int maxBufferSize)
void | setOutputRange(int index)
void | setSkipDuplicates(bool skipDuplicates)
void | setUserOrientation(int userOrientation)
bool | skipDuplicates() const
QByteArray | type() const
int | userOrientation() const

## Public Slots

bool | start()
void | stop()

## Signals

void | activeChanged()
void | alwaysOnChanged()
void | availableSensorsChanged()
void | axesOrientationModeChanged(AxesOrientationMode axesOrientationMode)
void | bufferSizeChanged(int bufferSize)
void | busyChanged()
void | currentOrientationChanged(int currentOrientation)
void | dataRateChanged()
void | efficientBufferSizeChanged(int efficientBufferSize)
void | maxBufferSizeChanged(int maxBufferSize)
void | readingChanged()
void | sensorError(int error)
void | skipDuplicatesChanged(bool skipDuplicates)
void | userOrientationChanged(int userOrientation)

## Static Public Members

QByteArray | defaultSensorForType(const QByteArray & type)
QList<QByteArray> | sensorTypes()
QList<QByteArray> | sensorsForType(const QByteArray & type)

## Related Non-Members

class | qoutputrange
typedef | qoutputrangelist
typedef | qrange
typedef | qrangelist

## Detailed Description

The QSensor class represents a single hardware sensor.

The life cycle of a sensor is typically:

- Create a sub-class of QSensor on the stack or heap.
- Setup as required by the application.
- Start receiving values.
- Sensor data is used by the application.
- Stop receiving values.

The sensor data is delivered via QSensorReading and its sub-classes.

### Orientation

Some sensors react to screen orientation changes, such as QAccelerometer, QMagnetometer and QRotationSensor. These are so called orientable sensors. For orientable sensors, QSensor supports changing the reporting of the reading values based on the orientation of the screen.

For orientable sensors, the axesOrientationMode property controls how the orientation affects the reading values.

In the default mode, QSensor::FixedOrientation, the reading values remain unaffected by the orientation. In the QSensor::AutomaticOrientation mode, the reading values are automatically rotated by taking the current screen orientation into account. And finally, in the QSensor::UserOrientation mode, the reading values are rotated according to a user-specified orientation.

The functionality of this is only available if it is supported by the backend and if the sensor is orientable, which can be checked by calling QSensor::isFeatureSupported() with the QSensor::AxesOrientation flag.

The orientation values here are always of the screen orientation, not the device orientation. The screen orientation is the orientation of the GUI. For example when rotating a device by 90 degrees counter-clockwise, the screen orientation compensates for that by rotating 90 degrees clockwise, to the effect that the GUI is still facing upright after the device has been rotated. Note that applications can lock the screen orientation, for example to force portrait or landscape mode. For locked orientations, orientable sensors will not react with reading changes if the device orientation is changed, as orientable sensors react to screen orientation changes only. This makes sense, as the purpose of orientable sensors is to keep the sensor orientation in sync with the screen orientation.

The orientation values range from 0 to 270 degrees. The orientation is applied in clockwise direction, e.g. an orientation value of 90 degrees means that the screen has been rotated 90 degress to the right from its origin position, to compensate a device rotation of 90 degrees to the left.

See also QSensorReading.

## Member Type Documentation

### enum QSensor::AxesOrientationMode

Describes how reading values are affected by the screen orientation.

Constant | Value | Description
--- | --- | ---
`QSensor::FixedOrientation` | `0` | No automatic rotation is applied to the reading values.
`QSensor::AutomaticOrientation` | `1` | The reading values are automatically rotated based on the screen orientation.
`QSensor::UserOrientation` | `2` | The reading values are rotated based on the angle of the userOrientation property.

This enum was introduced or modified in Qt 5.1.

See also QSensor::axesOrientationMode.

### enum QSensor::Feature

Lists optional features a backend might support.

The features common to all sensor types are:

Constant | Value | Description
--- | --- | ---
`QSensor::Buffering` | `0` | The backend supports buffering of readings, controlled by the QSensor::bufferSize property.
`QSensor::AlwaysOn` | `1` | The backend supports changing the policy on whether to suspend when idle, controlled by the QSensor::alwaysOn property.
`QSensor::SkipDuplicates` | `5` | The backend supports skipping of same or very similar successive readings. This can be enabled by setting the QSensor::skipDuplicates property to true.

The features of QMagnetometer are:

Constant | Value | Description
--- | --- | ---
`QSensor::GeoValues` | `2` | The backend supports returning geo values, which can be controlled with the QMagnetometer::returnGeoValues property.

The features of QLightSensor are:

Constant | Value | Description
--- | --- | ---
`QSensor::FieldOfView` | `3` | The backend specifies its field of view, which can be read from the QLightSensor::fieldOfView property.

The features of QAccelerometer are:

Constant | Value | Description
--- | --- | ---
`QSensor::AccelerationMode` | `4` | The backend supports switching the acceleration mode of the acceleromter with the QAccelerometer::accelerationMode property.

The features of QPressureSensor are:

Constant | Value | Description
--- | --- | ---
`QSensor::PressureSensorTemperature` | `7` | The backend provides the pressure sensor's die temperature

The features of all orientable sensors are:

Constant | Value | Description
--- | --- | ---
`QSensor::AxesOrientation` | `6` | The backend supports changing the axes orientation from the default of QSensor::FixedOrientation to something else.

This enum was introduced or modified in Qt 5.0.

See also QSensor::isFeatureSupported().

## Property Documentation

### active : bool

This property holds a value to indicate if the sensor is active.

This is true if the sensor is active (returning values). This is false otherwise.

Note that setting this value to true will not have an immediate effect. Instead, the sensor will be started once the event loop has been reached.

Access functions:

bool | isActive() const
void | setActive(bool active)

Notifier signal:

void | activeChanged()

### alwaysOn : bool

This property holds a value to indicate if the sensor should remain running when the screen is off.

Some platforms have a policy of suspending sensors when the screen turns off. Setting this property to true will ensure the sensor continues to run.

Access functions:

bool | isAlwaysOn() const
void | setAlwaysOn(bool alwaysOn)

Notifier signal:

void | alwaysOnChanged()

### availableDataRates : const qrangelist

This property holds the data rates that the sensor supports.

This is a list of the data rates that the sensor supports. Measured in Hertz.

Entries in the list can represent discrete rates or a continuous range of rates. A discrete rate is noted by having both values the same.

See the sensor_explorer example for an example of how to interpret and use this information.

Note that this information is not mandatory as not all sensors have a rate at which they run. In such cases, the list will be empty.

Access functions:

qrangelist | availableDataRates() const

See also QSensor::dataRate and qrangelist.

### axesOrientationMode : AxesOrientationMode

This property holds the mode that affects how the screen orientation changes reading values.

When set to FixedOrientation, which is the default mode, no automatic rotation is applied to the reading. This is the only mode available for backends that do not support the QSensor::AxesOrientation feature.

When set to AutomaticOrientation, the reading values are automatically rotated when the screen orientation changes. In effect, the screen orientation is canceled out.

As an example, assume the device is rotated by 180 degrees and therefore the screen orientation also is rotated by 180 degrees from the native orientation. Without automatic axes orientation, the reading values would now be changed: Both the X and the Y values would be negated, forcing an application developer to manually cancel out the negation in application code. Automatic axes orientation does this automatically, in this mode the X and Y values would be the same as with the default screen orientation.

This automatic rotation of the axes is handy is some usecases, for example in a bubble level application that measures how level a surface is by looking at the X axis value of an accelerometer. When the device and screen orientation change by 90 degrees, an application developer does not need to change anything, he can continue using the X axis value even though the device is rotated. Without automatic axes orientation, the application developer would need to look at the Y values instead, thereby adding code to the application that reads from a different axis depending on the screen orientation.

The UserOrientation mode is quite similar to AutomaticOrientation, only that the screen orientation is manually controlled instead of automatically determined. The angle of the userOrientation property is then used for rotating the reading values.

Since the rotation of the reading values is based on the screen orientation, Z values will never change, as the Z axis is perpendicular to the screen. As screen orientation changes in 90 degree steps, rotating the reading values is also done in steps of 90 degrees.

This property is only used for orientable sensors.

This property was introduced in Qt 5.1.

Access functions:

AxesOrientationMode | axesOrientationMode() const
void | setAxesOrientationMode(AxesOrientationMode axesOrientationMode)

Notifier signal:

void | axesOrientationModeChanged(AxesOrientationMode axesOrientationMode)

### bufferSize : int

This property holds the size of the buffer. By default, the buffer size is 1, which means no buffering. If the maximum buffer size is 1, then buffering is not supported by the sensor.

Setting bufferSize greater than maxBufferSize will cause maxBufferSize to be used.

Buffering is turned on when bufferSize is greater than 1. The sensor will collect the requested number of samples and deliver them all to the application at one time. They will be delivered to the application as a burst of changed readings so it is particularly important that the application processes each reading immediately or saves the values somewhere else.

If stop() is called when buffering is on-going, the partial buffer is not delivered.

When the sensor is started with buffering option, values are collected from that moment onwards. There is no pre-existing buffer that can be utilized.

Some backends like Blackberry only support enabling or disabling the buffer and do not give control over the size. In this case, the maxBufferSize and efficientBufferSize properties might not be set at all, even though buffering is supported. Setting the bufferSize property to any value greater than 1 will enable buffering. After the sensor has been started, the bufferSize property will be set to the actual value by the backend.

On Blackberry, buffering will not wait until the buffer is full to deliver new readings. Instead, the buffer will be used if the backend does not manage to retrieve the readings in time, for example when the event loop is blocked for too long. Without a buffer, these readings would simply be dropped.

Access functions:

int | bufferSize() const
void | setBufferSize(int bufferSize)

Notifier signal:

void | bufferSizeChanged(int bufferSize)

See also QSensor::maxBufferSize and QSensor::efficientBufferSize.

### busy : const bool

This property holds a value to indicate if the sensor is busy.

Some sensors may be on the system but unavailable for use. This function will return true if the sensor is busy. You will not be able to start() the sensor.

Note that this function does not return true if you are using the sensor, only if another process is using the sensor.

Access functions:

bool | isBusy() const

See also busyChanged().

### connectedToBackend : const bool

This property holds a value indicating if the sensor has connected to a backend.

A sensor that has not been connected to a backend cannot do anything useful.

Call the connectToBackend() method to force the sensor to connect to a backend immediately. This is automatically called if you call start() so you only need to do this if you need access to sensor properties (ie. to poll the sensor's meta-data before you use it).

Access functions:

bool | isConnectedToBackend() const

### currentOrientation : const int

This property holds the current orientation that is used for rotating the reading values.

This might not be the same as the screen orientation. For example, in the FixedOrientation mode, the reading values are not rotated, and therefore the property is 0.

In the UserOrientation mode, the readings are rotated based on the userOrientation property, and therefore this property is equal to the userOrientation property.

In the AutomaticOrientation mode, the readings are rotated based on the screen orientation, and therefore this property will be equal to the current screen orientation.

This property is set by the backend and only valid for orientable sensors.

This property was introduced in Qt 5.1.

Access functions:

int | currentOrientation() const

Notifier signal:

void | currentOrientationChanged(int currentOrientation)

### dataRate : int

This property holds the data rate that the sensor should be run at.

Measured in Hertz.

The data rate is the maximum frequency at which the sensor can detect changes.

Setting this property is not portable and can cause conflicts with other applications. Check with the sensor backend and platform documentation for any policy regarding multiple applications requesting a data rate.

The default value (0) means that the app does not care what the data rate is. Applications should consider using a timer-based poll of the current value or ensure that the code that processes values can run very quickly as the platform may provide updates hundreds of times each second.

This should be set before calling start() because the sensor may not notice changes to this value while it is running.

Note that there is no mechanism to determine the current data rate in use by the platform.

Access functions:

int | dataRate() const
void | setDataRate(int rate)

Notifier signal:

void | dataRateChanged()

See also QSensor::availableDataRates.

### description : const QString

This property holds a descriptive string for the sensor.

Access functions:

QString | description() const

### efficientBufferSize : const int

The property holds the most efficient buffer size. Normally this is 1 (which means no particular size is most efficient). Some sensor drivers have a FIFO buffer which makes it more efficient to deliver the FIFO's size worth of readings at one time.

Access functions:

int | efficientBufferSize() const

Notifier signal:

void | efficientBufferSizeChanged(int efficientBufferSize)

See also QSensor::bufferSize and QSensor::maxBufferSize.

### error : const int

This property holds the last error code set on the sensor.

Note that error codes are sensor-specific.

Access functions:

int | error() const

Notifier signal:

void | sensorError(int error)

### identifier : QByteArray

This property holds the backend identifier for the sensor.

Note that the identifier is filled out automatically when the sensor is connected to a backend. If you want to connect a specific backend, you should call setIdentifier() before connectToBackend().

Access functions:

QByteArray | identifier() const
void | setIdentifier(const QByteArray & identifier)

### maxBufferSize : const int

The property holds the maximum buffer size.

Note that this may be 1, in which case the sensor does not support any form of buffering. In that case, isFeatureSupported(QSensor::Buffering) will also return false.

Access functions:

int | maxBufferSize() const

Notifier signal:

void | maxBufferSizeChanged(int maxBufferSize)

See also QSensor::bufferSize and QSensor::efficientBufferSize.

### outputRange : int

This property holds the output range in use by the sensor.

This value represents the index in the QSensor::outputRanges list to use.

Setting this property is not portable and can cause conflicts with other applications. Check with the sensor backend and platform documentation for any policy regarding multiple applications requesting an output range.

The default value (-1) means that the app does not care what the output range is.

Note that there is no mechanism to determine the current output range in use by the platform.

Access functions:

int | outputRange() const
void | setOutputRange(int index)

See also QSensor::outputRanges.

### outputRanges : const qoutputrangelist

This property holds a list of output ranges the sensor supports.

A sensor may have more than one output range. Typically this is done to give a greater measurement range at the cost of lowering accuracy.

Note that this information is not mandatory. This information is typically only available for sensors that have selectable output ranges (such as typical accelerometers).

Access functions:

qoutputrangelist | outputRanges() const

See also QSensor::outputRange and qoutputrangelist.

### reading : QSensorReading * const

This property holds the reading class.

The reading class provides access to sensor readings. The reading object is a volatile cache of the most recent sensor reading that has been received so the application should process readings immediately or save the values somewhere for later processing.

Note that this will return 0 until a sensor backend is connected to a backend.

Also note that readings are not immediately available after start() is called. Applications must wait for the readingChanged() signal to be emitted.

Access functions:

QSensorReading * | reading() const

Notifier signal:

void | readingChanged()

See also isConnectedToBackend() and start().

### skipDuplicates : bool

This property holds indicates whether duplicate reading values should be omitted.

When duplicate skipping is enabled, successive readings with the same or very similar values are omitted. This helps reducing the amount of processing done, as less sensor readings are made available. As a consequence, readings arrive at an irregular interval.

Duplicate skipping is not just enabled for readings that are exactly the same, but also for readings that are quite similar, as each sensor has a bit of jitter even if the device is not moved.

Support for this property depends on the backend. Use isFeatureSupported() to check if it is supported on the current platform.

Duplicate skipping is disabled by default.

Duplicate skipping takes effect when the sensor is started, changing the property while the sensor is active has no immediate effect.

This property was introduced in Qt 5.1.

Access functions:

bool | skipDuplicates() const
void | setSkipDuplicates(bool skipDuplicates)

Notifier signal:

void | skipDuplicatesChanged(bool skipDuplicates)

### type : const QByteArray

This property holds the type of the sensor.

Access functions:

QByteArray | type() const

### userOrientation : int

This property holds the angle used for rotating the reading values in the UserOrientation mode.

When the axesOrientationMode property is set to UserOrientation, the angle for rotating the reading values is taken from this property. In other modes, the property has no effect.

The default is 0. The only valid values are 0, 90, 180 and 270, as those are the only possible screen orientations.

This property is only valid for orientable sensors.

This property was introduced in Qt 5.1.

Access functions:

int | userOrientation() const
void | setUserOrientation(int userOrientation)

Notifier signal:

void | userOrientationChanged(int userOrientation)

## Member Function Documentation

### QSensor::QSensor(const QByteArray & type, QObject * parent = 0)

Construct the type sensor as a child of parent.

Do not use this constructor if a derived class exists for the specific sensor type.

The wrong way is to use the base class constructor:

    QSensor *magnetometer = new QSensor(QMagnetometer::type, this);

The right way is to create an instance of the derived class:

    QMagnetometer *magnetometer = new QMagnetometer(this);

The derived classes have additional properties and data members which are needed for certain features such as geo value support in QMagnetometer or acceleration mode support in QAccelerometer. These features will only work properly when creating a sensor instance from a QSensor subclass.

Only use this constructor if there is no derived sensor class available. Note that all built-in sensors have a derived class, so using this constructor should only be necessary when implementing custom sensors, like in the Grue sensor example.

### QSensor::~QSensor()`[virtual]`

Destroy the sensor. Stops the sensor if it has not already been stopped.

### void QSensor::addFilter(QSensorFilter * filter)

Add a filter to the sensor.

The sensor does not take ownership of the filter. QSensorFilter will inform the sensor if it is destroyed.

See also QSensorFilter.

### void QSensor::availableSensorsChanged()`[signal]`

This signal is emitted when the list of available sensors has changed. The sensors available to a program will not generally change over time however some of the available sensors may represent hardware that is not permanently connected. For example, a game controller that is connected via bluetooth would become available when it was on and would become unavailable when it was off.

See also QSensor::sensorTypes() and QSensor::sensorsForType().

### void QSensor::busyChanged()`[signal]`

This signal is emitted when the sensor is no longer busy. This can be used to grab a sensor when it becomes available.

    sensor.start();
    if (sensor.isBusy()) {
        // need to wait for busyChanged signal and try again
    }

### bool QSensor::connectToBackend()

Try to connect to a sensor backend.

Returns true if a suitable backend could be found, false otherwise.

The type must be set before calling this method if you are using QSensor directly.

See also isConnectedToBackend().

### QByteArray QSensor::defaultSensorForType(const QByteArray & type)`[static]`

Returns the default sensor identifier for type. This is set in a config file and can be overridden if required. If no default is available the system will return the first registered sensor for type.

Note that there is special case logic to prevent the generic plugin's backends from becoming the default when another backend is registered for the same type. This logic means that a backend identifier starting with `generic.` will only be the default if no other backends have been registered for that type or if it is specified in `Sensors.conf`.

See also Determining the default sensor for a type.

### QList<QSensorFilter *> QSensor::filters() const

Returns the filters currently attached to the sensor.

See also QSensorFilter.

### bool QSensor::isFeatureSupported(Feature feature) const

Checks if a specific feature is supported by the backend.

QtSensors supports a rich API for controlling and providing information about sensors. Naturally, not all of this functionality can be supported by all of the backends.

To check if the current backend supports the feature feature, call this function.

The backend needs to be connected, otherwise false will be returned. Calling connectToBackend() or start() will create a connection to the backend.

Backends have to implement QSensorBackend::isFeatureSupported() to make this work.

Returns whether or not the feature is supported if the backend is connected, or false if the backend is not connected.

This function was introduced in Qt 5.0.

### void QSensor::removeFilter(QSensorFilter * filter)

Remove filter from the sensor.

See also QSensorFilter.

### QList<QByteArray> QSensor::sensorTypes()`[static]`

Returns a list of all sensor types.

### QList<QByteArray> QSensor::sensorsForType(const QByteArray & type)`[static]`

Returns a list of ids for each of the sensors for type. If there are no sensors of that type available the list will be empty.

### void QSensor::setCurrentOrientation(int currentOrientation)

Sets the current screen orientation to currentOrientation. This is to be called from the backend whenever the screen orientation or the userOrientation property changes.

This function was introduced in Qt 5.1.

See also currentOrientation().

### void QSensor::setEfficientBufferSize(int efficientBufferSize)

Sets the efficient buffer size to efficientBufferSize. This is to be called from the backend.

This function was introduced in Qt 5.1.

See also efficientBufferSize().

### void QSensor::setMaxBufferSize(int maxBufferSize)

Sets the maximum buffer size to maxBufferSize. This is to be called from the backend.

This function was introduced in Qt 5.1.

See also maxBufferSize().

### bool QSensor::start()`[slot]`

Start retrieving values from the sensor. Returns true if the sensor was started, false otherwise.

The sensor may fail to start for several reasons.

Once an application has started a sensor it must wait until the sensor receives a new value before it can query the sensor's values. This is due to how the sensor receives values from the system. Sensors do not (in general) poll for new values, rather new values are pushed to the sensors as they happen.

For example, this code will not work as intended.

    sensor->start();
    sensor->reading()->x(); // no data available

To work correctly, the code that accesses the reading should ensure the readingChanged() signal has been emitted.

        connect(sensor, SIGNAL(readingChanged()), this, SLOT(checkReading()));
        sensor->start();
    }
    void MyClass::checkReading() {
        sensor->reading()->x();

See also QSensor::busy.

### void QSensor::stop()`[slot]`

Stop retrieving values from the sensor.

This releases the sensor so that other processes can use it.

See also QSensor::busy.

## Related Non-Members

### class qoutputrange

The qoutputrange class holds the specifics of an output range. More...

### typedef qoutputrangelist

This type is defined as a list of qoutputrange values.

    typedef QList<qoutputrange> qoutputrangelist;

See also QList, qoutputrange, and QSensor::outputRanges.

### typedef qrange

This type is defined as a QPair.

    typedef QPair<int,int> qrange;

See also QPair, qrangelist, and QSensor::availableDataRates.

### typedef qrangelist

This type is defined as a list of qrange values.

    typedef QList<qrange> qrangelist;

See also QList, qrange, and QSensor::availableDataRates.