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/*!
\ingroup technology-apis
\title Bluetooth Low Energy Overview
\page qtbluetooth-le-overview.html
\brief The Qt Bluetooth Low Energy API enables communication between Bluetooth
Low Energy devices.

\tableofcontents

    The Qt Bluetooth Low Energy API was introduced by Qt 5.4. Since Qt 5.5 the API
    is final and a compatibility guarantee is given for future releases.
    At the moment, Qt supports the Bluetooth Low Energy central role.
    For more details on this limitation see below.

    \section1 What Is Bluetooth Low Energy

    Bluetooth Low Energy, also known as Bluetooth Smart, is a wireless computer
    network technology, which was officially introduced in 2011. It works on the same
    2.4 GHz frequency as ”classic” Bluetooth. The main difference is, as stated by its technology name,
    low energy consumption. It provides an opportunity for Bluetooth Low Energy devices to
    operate for months, even years, on coin-cell batteries. The technology was introduced by
    \l {https://www.bluetooth.org/en-us/specification/adopted-specifications}{Bluetooth v4.0}.
    Devices which support this technology are called Bluetooth Smart Ready Devices.
    The key features of the technology are:

    \list
    \li Ultra-low peak, average and idle mode power consumption
    \li Ability to run for years on standard, coin-cell batteries
    \li Low cost
    \li Multi-vendor interoperability
    \li Enhanced range
    \endlist

    Bluetooth Low Energy uses a client-server architecture. The server (also
    known as peripheral) offers services such as temperature or heart rate
    and advertises them. The client (known as central
    device) connects to the server and reads the values advertised by the server.
    An example might be an apartment with Bluetooth Smart Ready sensors such
    as a thermostat, humidity or pressure sensor. Those sensors are peripheral
    devices advertising the environment values of the apartment. At the same time
    a mobile phone or computer might connect to those sensors, retrieve their
    values and present them as part of a larger environment control application
    to the user.

    \section1 Basic Service Structure

    Bluetooth Low Energy is based on two protocols: ATT (Attribute Protocol)
    and GATT (Generic Attribute Profile). They specify the communication layers
    used by every Bluetooth Smart Ready device.

    \section2 ATT Protocol

    The basic building block of ATT is an \e attribute. Each attribute consists of
    three elements:

    \list
        \li a value - the payload or desirable piece of information
        \li a UUID - the type of attribute (used by GATT)
        \li a 16-bit handle - a unique identifier for the attribute
    \endlist

    The server stores the attributes and the client uses the ATT protocol to
    read and write values on the server.

    \section2 GATT Profile

    GATT defines grouping for a set of attributes by applying a meaning to predefined
    UUIDs. The table below shows an example service exposing a heart rate
    on a particular day. The actual values are stored inside the two characteristics:

    \table
    \header
        \li Handle
        \li UUID
        \li Value
        \li Description
    \row
        \li 0x0001
        \li 0x2800
        \li UUID 0x180D
        \li Begin Heart Rate service
    \row
        \li 0x0002
        \li 0x2803
        \li UUID 0x2A37, Value handle: 0x0003
        \li Characteristic of type \e {Heart Rate Measurement (HRM)}
    \row
        \li 0x0003
        \li 0x2A37
        \li 65 bpm
        \li Heart rate value
    \row
        \li 0x0004
        \li 0x2803
        \li UUID 0x2A08, Value handle: 0x0006
        \li Characteristic of type Date Time
    \row
        \li 0x0005
        \li 0x2A08
        \li 18/08/2014 11:00
        \li Date and Time of the measurement
    \row
        \li 0x0006
        \li 0x2800
        \li UUID xxxxxx
        \li Begin next service
    \row
        \li ...
        \li ...
        \li ...
        \li ...
    \endtable

    GATT specifies that the above used UUID \c 0x2800 marks the begin of a service definition.
    Every attribute following \c 0x2800 is part of the service until the next \c 0x2800 or the
    end is encountered. In similar ways the well known UUID \c 0x2803 states that a characteristic
    is to be found and each of the characteristics has a type defining the nature of the value.
    The example above uses the UUIDs \c 0x2A08 (Date Time) and \c 0x2A37 (Heart Rate Measurement).
    Each of the above UUIDs is defined by the \l {https://bluetooth.org}{Bluetooth Special Interest Group}.
    and can be found in the
    \l{https://developer.bluetooth.org/gatt/Pages/default.aspx}{GATT specification}. While it
    is advisable to use pre-defined UUIDs where available it is entirely possible to use new and not
    yet used UUIDs for characteristic and service types.

    In general, each service may consist of one or more characteristics. A characteristic
    contains data and can be further described by descriptors, which provide additional
    information or means of manipulating the characteristic. All services, characteristics and
    descriptors are recognized by their 128-bit UUID. Finally, it is possible to include
    services inside of services (see picture below).

    \image peripheral-structure.png

    \section1 Using Qt Bluetooth Low Energy API

    This section describes how to use the Bluetooth Low Energy API provided by Qt. Currently the API
    permits creating connections to peripheral devices, discovering their services, as well as reading
    and writing data stored on the device. The example code below is taken from the
    \l {heartlistener}{Heart Listener} example.

    \section2 Establishing a Connection

    To be able to read and write the characteristics of the Bluetooth Low Energy peripheral device,
    it is necessary to find and connect the device. This requires the peripheral device to advertise
    its presence and services. We start the device discovery with the help of the
    \l QBluetoothDeviceDiscoveryAgent class. We connect to its \l {QBluetoothDeviceDiscoveryAgent::deviceDiscovered()}
    signal and start the search with \l {QBluetoothDeviceDiscoveryAgent::start()}{start()}:

    \snippet heartlistener/heartrate.cpp devicediscovery-1
    \snippet heartlistener/heartrate.cpp devicediscovery-2

    Since we are only interested in Low Energy devices we filter the device type within the
    receiving slot. The device type can be ascertained using the \l QBluetoothDeviceInfo::coreConfigurations()
    flag:

    \snippet heartlistener/heartrate.cpp devicediscovery-3
    \snippet heartlistener/heartrate.cpp devicediscovery-4

    Once the address of the peripheral device is known we use the \l QLowEnergyController class.
    This class is the entry point for all Bluetooth Low Energy development. The  constructor of the class
    accepts the remote device's \l QBluetoothAddress. Finally we set up the customary slots and
    directly connect to the device using
    \l {QLowEnergyController::connectToDevice()}{connectToDevice()}:

    \snippet heartlistener/heartrate.cpp Connect signals

    \section2 Service Search

    As soon as the connection is established we initiate the service discovery:

    \snippet heartlistener/heartrate.cpp Connecting to service

    The \c serviceDiscovered() slot below is triggered as a result of the
    \l {QLowEnergyController::serviceDiscovered()} signal and provides an intermittent progress report.
    Since we are talking about the heart listener app which monitors HeartRate devices in the vicinity
    we ignore any service that is not of type \l QBluetoothUuid::HeartRate.

    \snippet heartlistener/heartrate.cpp Filter HeartRate service 1

    Eventually the \l {QLowEnergyController::discoveryFinished()} signal is emitted to indicate
    the successful completion of the service discovery. Provided a HeartRate service was found,
    a \l QLowEnergyService instance is created to represent the service. The returned service object
    provides the required signals for update notifications and the discovery of service details
    is triggered using \l QLowEnergyService::discoverDetails():

    \snippet heartlistener/heartrate.cpp Filter HeartRate service 2

    During the detail search the service's \l {QLowEnergyService::state()}{state()} transitions
    from \l {QLowEnergyService::DiscoveryRequired}{DiscoveryRequired} to
    \l {QLowEnergyService::DiscoveringServices}{DiscoveringServices} and eventually ends with
    \l {QLowEnergyService::ServiceDiscovered}{ServiceDiscovered}:

    \snippet heartlistener/heartrate.cpp Find HRM characteristic

    \section2 Interaction with the Peripheral Device

    In the code example above, the desired characteristic is of type
    \l {QBluetoothUuid::HeartRateMeasurement}{HeartRateMeasurement}. Since the application measures
    the heart rate changes, it must enable change notifications for the characteristic.
    Note that not all characteristics provide change notifications. Since the HeartRate characteristic
    has been standardized it is possible to assume that notifications can be received. Ultimately
    \l QLowEnergyCharacteristic::properties() must have the \l {QLowEnergyCharacteristic::Notify} flag
    set and a descriptor of type \l {QBluetoothUuid::ClientCharacteristicConfiguration} must exist to confirm
    the availability of an appropriate notification.

    Finally, we process the value of the HeartRate characteristic, as per Bluetooth Low Energy standard:

    \snippet heartlistener/heartrate.cpp Reading value 1
    \snippet heartlistener/heartrate.cpp Reading value 2

    In general a characteristic value is a series of bytes. The precise interpretation of
    those bytes depends on the characteristic type and value structure.
    A significant number has been standardized by the
    \l {https://developer.bluetooth.org/gatt/services/Pages/ServicesHome.aspx}{Bluetooth SIG} whereas others
    may follow a custom protocol. The above code snippet demonstrates how to the read the standardized
    HeartRate value.
*/
