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diff --git a/doc/src/sensors.qdoc b/doc/src/sensors.qdoc new file mode 100644 index 00000000..6dc1efbd --- /dev/null +++ b/doc/src/sensors.qdoc @@ -0,0 +1,524 @@ +/**************************************************************************** +** +** Copyright (C) 2010 Nokia Corporation and/or its subsidiary(-ies). +** All rights reserved. +** Contact: Nokia Corporation (qt-info@nokia.com) +** +** This file is part of the documentation of the Qt Toolkit. +** +** $QT_BEGIN_LICENSE:FDL$ +** No Commercial Usage +** This file contains pre-release code and may not be distributed. +** You may use this file in accordance with the terms and conditions +** contained in the Technology Preview License Agreement accompanying +** this package. +** +** GNU Free Documentation License +** Alternatively, this file may be used under the terms of the GNU Free +** Documentation License version 1.3 as published by the Free Software +** Foundation and appearing in the file included in the packaging of this +** file. +** +** If you have questions regarding the use of this file, please contact +** Nokia at qt-info@nokia.com. +** $QT_END_LICENSE$ +** +****************************************************************************/ + +/*! +\page sensors-api.html +\title Sensors +\brief The Sensors API provides access to sensors. +\ingroup mobility +\ingroup technology-apis + + +The Sensors API is primarily concerned with low-level, real-time sensors such as +the accelerometer although there are higher-level, event-driven sensors represented too. + +\tableofcontents + +\section1 Sensor Types + +On a device there can be many types of sensors. Not all of the types that the Sensors API +supports may be available. There may also be types available that are not defined in the +Sensors API. You can find the sensor types available on a device using the +\l QSensor::sensorTypes() function. + +For a list of built-in sensor types, see the \l{Sensor Classes} section below. + +\section1 Common Conventions + +Unless otherwise specified, sensors shall use the +\l{http://en.wikipedia.org/wiki/Cartesian_coordinate_system}{Right Hand Cartesian coordinate system}. + +\image sensors-coordinates.jpg + +To allow for measurements in all 6 directions, negative values are used. + +\image sensors-coordinates2.jpg + +Where rotation around an axis is used, the rotation shall be expressed as a Right Hand rotation. + +\image sensors-coordinates3.jpg + +In general, sensor data is oriented to the top of the device. 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. + +\image sensors-sides2.jpg + +\section1 Using a Sensor + +The life cycle of a sensor is typically: + +\list +\o Create an instance of QSensor or one of its sub-classes on the stack or heap. +\o Setup as required by the application. +\o Start receiving values. +\o Sensor data is used by the application. +\o Stop receiving values. +\endlist + +Here is an example of creating a sensor on the heap and on the stack. + +\snippet snippets/sensors/creating.cpp Creating a sensor + +\section1 Accessing sensor data in a generic fashion + +The preferred way to deal with sensor data is via the \l{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: + +\list +\o The sensor type. +\o The property name or index. +\o The property type or a comparable type. +\endlist + +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 \l QAccelerometer or +\l QAccelerometerReading. + +\snippet snippets/sensors/start.cpp Starting a sensor + +You can discover all of this information at runtime too. The sensor_explorer example +shows you information about available sensors. + +\section1 Platform notes + +\section2 S60 3rd Edition + +Note that support for sensors in S60 3.1 device is extremely limited due to the native API. +Only the accelerometer is supported and only a few devices. + +Some devices running S60 3.2 support a newer native API and therefore support more sensors. + +More information about these platforms can be found \l{http://wiki.forum.nokia.com/index.php/Nokia_Sensor_APIs}{here}. + +Note that timestamps on this platform come from the system clock. +Applications need to handle shifts in time caused by the user manually setting the clock or +from the automatic time synchronization feature setting the clock. + +\section2 Symbian + +Most Symbian devices have their sensor data read via the Sensor Framework API. Some limitations +appear in the Sensors API as a result. + +Only specific data rates can be selected. Setting an invalid data rate has no effect so applications +that need to influence the used data rate should connect to the sensor, check the available data rates +and select one as appropriate. + +Readings are delivered to the application via a queue. If the application blocks the event loop or otherwise +interferes with the ability of the system to deliver readings (eg. by using up too much CPU time), they can +get blocked in this queue. Since delayed readings are not useful, the system will drop readings as needed +so that the application is always dealing with the most recent reading available. The application can tweak +the policy by setting properties on the sensor. + +The default policy is to accept up to 100 readings from the system at once and to discard all but the last one. + +\code +QAccelerometer sensor; +sensor.setProperty("maximumReadingCount", 100); +sensor.setProperty("processAllReadings", false); +\endcode + +Applications that desire the original behaviour can set the maximumReadingCount to 1. Note that this does not +guarantee that readings will not be dropped by the system. If the queue fills up, readings will be dropped. + +\code +QAccelerometer sensor; +sensor.setProperty("maximumReadingCount", 1); +\endcode + +Larger maximumReadingCount values reduce the need for the lower-priority sensor daemon to get CPU timeslices. +If the application is using lots of CPU but is still able to process readings quickly, it can request that +all the fetched readings are processed. + +\code +QAccelerometer sensor; +sensor.setProperty("maximumReadingCount", 10); +sensor.setProperty("processAllReadings", true); +\endcode + +More information about the native API can be found \l{http://wiki.forum.nokia.com/index.php/Nokia_Sensor_APIs}{here}. + +Note that timestamps on this platform come from the system clock. +Applications need to handle shifts in time caused by the user manually setting the clock or +from the automatic time synchronization feature setting the clock. + +The ambient light sensor can only detect changes. Unlike all other sensors, it cannot report the "current value" +so it is not possible to determine the current ambient light level. + +\section2 Maemo 5 + +The N900 represents a unique device for the Sensors API. Unlike the Symbian and MeeGo platforms, sensor data is +retrieved directly from the kernel and this has implications on the API. + +Axes are rotated when compared to Symbian or MeeGo devices. While Symbian and MeeGo devices orient their +hardware sensors towards a portrait orientation, the N900 does not do this. Instead, it orients the hardware sensors +towards its default landscape orientation. This has portability implications for applications that want to force the +use of a particular screen orientation and use sensors. The following code shows how accelerometer values can be +interpreted to ensure consistent results on the N900 as well as Symbian and MeeGo devices. + +\code +#ifdef Q_WS_MAEMO_5 + qreal x = reading->y(); + qreal y = -reading->x(); +#else + qreal x = reading->x(); + qreal y = reading->y(); +#endif + qreal z = reading->z(); +\endcode + +Alternatively, applications can set the environment variable \c N900_PORTRAIT_SENSORS to 1. This must be done +before any Sensors API calls are made so the beginning of the main function is a good place to do it. + +\code +int main(int argc, char **argv) +{ + qputenv("N900_PORTRAIT_SENSORS", "1"); + ... +\endcode + +Despite hardware that allows for multiple data rates and output ranges, the Sensors API does not allow access to +these due to permissions issues. + +Readings are polled using a timer. If the application blocks the event loop or otherwise interferes with the +ability of the timer to fire, readings will be missed. There are no queues so applications must ensure that +they process the readings promptly (possibly saving them into a buffer for later processing if required). + +\section2 MeeGo + +The data rates offered by MeeGo are not tied to how fast the hardware runs. + +The default data rate for a sensor is likely to be low when compared to Symbian or Maemo 5. Applications should +request a suitable data rate, taking care to avoid selecting invalid rates on other devices. + +Sensors may be suspended by the system in order to save power. Applications can avoid this by setting a property +on the sensor object. + +\code +QAccelerometer *accelerometer = new QAccelerometer(this); +accelerometer->setProperty("alwaysOn", true); +accelerometer->start(); +\endcode + +Unlike Symbian and N900, MeeGo does not currently provide initial readings. Thus, certain sensors must detect +a change in value before a value can be reported. Examples include the orientation sensor and ambient light +sensor. + +\section1 Front end, back end + +The 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. + +\image sensors-overview.png + +More information about the back end can be found in \l{Sensors Backend}. + +\section1 Main Classes + +The primary classes that make up the Sensors API. + +\annotatedlist sensors_main + +\section1 Reading Classes + +The best way to access sensor data is via one of these classes. + +\annotatedlist sensors_reading + +\section1 Sensor Classes + +These classes provide convenience wrappers that reduce the need for casting. +Each of these classes represents a sensor type that the Sensors API knows +about. Note that additional types may be made available at run-time. See +\l{Sensor Types} for more information. + +\annotatedlist sensors_type + +\section1 Filter Classes + +As with the sensor classes, these provide convenience wrappers that reduce +the need for casting. + +\annotatedlist sensors_filter + +*/ + +/*! +\page sensors-backend.html +\title Sensors Backend +\brief The Sensors Backend connects the Sensors API to the platform services or hardware sensors. + +The Sensors Backend connects the Sensors API to the platform services or hardware sensors. + +\tableofcontents + +\section1 Overview + +\section1 Backend API + +QSensor instances talk to a backend object. Backends are usually supplied +with the QtSensors library for a specific device although third party +backends may be used as well. A backend may talk +directly to hardware or it may talk to a system service. In some instances +it may even talk to another sensor. +An example of this is the orientation sensor backend that talks to an +accelerometer to determine the orientation. + +There are also some \l{Sensors Backend Topics}{topics} specific to backend +implementors. + +\section1 Backend Classes +If you are making sensors available through the Sensors API, these are the +classes to use. +\annotatedlist sensors_backend + +\sa {Sensors Backend Topics} + +*/ + +/*! +\group sensors_backend_topics +\title Sensors Backend Topics +\generatelist related +*/ + +/*! +\page creating-a-sensor-plugin.html +\title Creating a sensor plugin +\ingroup sensors_backend_topics + +\section1 How a sensor plugin is loaded + +Since sensor backends are created on demand, the sensor plugin is loaded and asked +to register the sensor backends it handles. The plugin should implement +QSensorPluginInterface::registerSensors() and call QSensorManager::registerBackend() +to register available backends. Typically the plugin will also inherit from +QSensorBackendFactory and implement +QSensorBackendFactory::createBackend() in order to instantiate backends it has registered. + +The simplest plugin will have just once sensor backend although there is no reason +that multiple sensor backends cannot be in a plugin. + +An example follows. + +\snippet snippets/sensors/plugin.cpp Plugin + +If you would like to build a backend into a library or application you can use the +REGISTER_STATIC_PLUGIN() macro although it may not work in all situations as it +uses static initialization. + +*/ + +/*! +\page determining-the-default-sensor-for-a-type.html +\title Determining the default sensor for a type +\ingroup sensors_backend_topics + +\section1 Multiple sensors can exist for a type + +Sensors was designed so that multiple sensors could exist for a given type. Why? +Consider this example. + +The N900 has an accelerometer built-in. It also features bluetooth and can pair +with a gaming controller that features an accelerometer. To a developer writing +a game these two devices are conceptually the same type. + +\section1 Default sensor for a type + +To avoid the need to know (or check) what the default sensor for a type is, the system will +use the default sensor for a type. Most of the time this is what the app developer wants to +do. In cases where the app developer wants to select a specific sensor they must call the +QSensor::setIdentifier() method before they start the sensor so that the appropriate backend +is used. + +From a system perspective though, selecting which sensor should be the default gets tricky. +The sensors library uses the first registered identifier as the default. This means that the +order in which sensor backends are registered is important so the system will allow a config +file to determine the default instead. + +\section1 Sensors.conf + +The config file that determines the default sensor for a type is called Sensors.conf. If present, +it is located in /etc/xdg/Nokia. It is read using QSettings so it has the standard formatting +of a QSettings .conf file. + +The settings live in the Default group and the general format is: +\code +type = identifier +\endcode + +An example Sensors.conf that ensures the N900 accelerometer is used as the default no matter the +order in which backends were registered is presented here. + +\code +[Default] +QAccelerometer = n900.accelerometer +\endcode + +If Sensors.conf specifies an identifier that is not registered then the system will fall back to +the first registered identifier as the default. + +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 \c{generic.} will only be the default if no other backends have been +registered for that type or if it is specified in \c{Sensors.conf}. + +*/ + +/*! +\page dynamic-sensor-backend-registration.html +\title Dynamic Sensor Backend Registration +\ingroup sensors_backend_topics + +\section1 Static Backend Registration + +Sensor backends are generally registered statically. The registration happens when the sensors +library is first used and the registration remains in effect while the program runs. + +\image sensors-static.png + +Statically registered backends may still exhibit some dynamic behaviour as the +QSensorBackendFactory is free to return 0 to indicate that a backend cannot be created. + +\section1 Dynamic Backend Registration + +While static registration is fine for most backends there are some situations where this is +problematic. + +The clearest example is backends that represent non-fixed hardware. As an example, lets consider +a game controller that is connected via Bluetooth. As there may be more than one game controller +in range of the phone, the program wants to record that a specific game controller should be used. +If the backend had been registered statically there would have been no unique information about +the controller. Instead, the registration is delayed until the controller is seen. + +\image sensors-dynamic.png + +\section1 Suggested Registration Policy + +A backend for fixed hardware should be registered immediately. Applications can see that the +sensor can be used. + +A backend for remote hardware should not be registered immediately. Applications can see that +the sensor cannot be used. When the remote hardware becomes available the backend should be +registered. Applications can see that the sensor is now available. + +If it is necessary to return 0 from a factory for a backend that was registered, the backend +should be unregistered. Applications can see that the sensor is no longer available. If the +factory can create the backend again it should be registered. Applications can see that the +sensor is available again. + +When the underlying hardware is no longer available, the backend should be deregistered. +Existing instances of the backend should report error states to the application but should +handle the situation gracefully. + +*/ + +/*! +\page qml-sensors.html +\title Sensors QML Plugin +\brief A QML plugin for the QtMobility Project Sensors API. + +\section1 Overview + +The identifying string for this component is \e {"QtMobility.sensors"}. +Use this in the QML \e {import} statement. + +The Sensors QML Plugin registers the C++ Sensors classes directly to the QML environment. +This causes some limitations due to the use of types that do not work in the QML environment. +See \l{Sensors QML Limitations}{below} for a list of the known limitations. + +See \l Sensors for more information about the Sensors API. + +\section1 Sensors QML Limitations + +The following limitations affect the Sensors QML bindings for Qt Mobility 1.1 and 1.2. + +\list 1 +\o The QSensor::sensorid property cannot be set because QML does not support QByteArray. + This means that it is not possible to specify a particular sensor when two or more have + been registered with the same type. +\o The QSensor::availableDataRates property cannot be used because QML does not support \l qrangelist. +\o The QSensor::outputRanges property cannot be used because QML does not support \l qoutputrangelist. +\o The QLightSensor::fieldOfView property cannot be used because QML cannot access dynamic properties. +\o The QMagnetometer::returnGeoValues property cannot be used because QML cannot access dynamic properties. +\o The QRotationSensor::hasZ property cannot be used because QML cannot access dynamic properties. +\o The QTapSensor::returnDoubleTapEvents property cannot be used because QML cannot access dynamic properties. +\endlist + +\section1 QML Sensor Elements + +These elements represent specific types of sensors. + +\annotatedlist qml-sensors_type + +\section1 QML Reading Elements + +The data from a sensor comes through a reading class. + +\annotatedlist qml-sensors_reading + +*/ + +/*! +\page meego-integration-notes.html +\title MeeGo Integration Notes +\ingroup sensors_backend_topics + +\section1 MeeGo Integration Notes + +The implementation of the API builds on top of the MeeGo Sensor Framework +that provides all the sensors specified in 1.2 API version. + +\section2 Available sensors + +If HW sensor is missing, the configuration file "Sensors.conf" +must be updated and sensor removed. The file +has the following format: + +\code +[Default] +QAccelerometer=meego.accelerometer +QAmbientLightSensor=meego.als +\endcode + +It lists sensor types and type's default implementation by giving the sensor id. +If the type is omitted then the backend does not support it in this device; this +gives a way of controlling and differentiating the supported sensor set. + +*/ + |