GATTCCharacteristic

The smallest concept in GATT is the Characteristic, which encapsulates a single data point (though it may contain an array of related data, such as X/Y/Z values from a 3-axis accelerometer, longitude and latitude from a GPS, etc.).

The following class allows you to manage characteristics from a Client.

Methods

characteristic.uuid()

Returns the UUID of the service. In the case of 16-bit or 32-bit long UUIDs, the value returned is an integer, but for 128-bit long UUIDs the value returned is a bytes object.

characteristic.instance()

Returns the instance ID of the service.

characteristic.properties()

Returns an integer indicating the properties of the characteristic. Properties are represented by bit values that can be OR-ed together. See the constants section for more details.

characteristic.read()

Read the value of the characteristic, sending a request to the GATT server. Returns a bytes object representing the characteristic value.

characteristic.value()

Returns the locally stored value of the characteristic without sending a read request to the GATT server. If the characteristic value hasn’t been read from the GATT server yet, the value returned will be 0.

characteristic.write(value)

Writes the given value on the characteristic. For now it only accepts bytes object representing the value to be written.

characteristic.write(b'x0f')

characteristic.callback(trigger=None, handler=None, arg=None)

Creates a callback that will be executed when any of the triggers occur. The arguments are:

• trigger can be Bluetooth.CHAR_NOTIFY_EVENT.
• handler is the function that will be executed when the callback is triggered.
• arg is the argument that gets passed to the callback. If nothing is given, the characteristic object that owns the callback will be used.

Beyond the arg a tuple (called data) is also passed to handler. The tuple consists of (event, value), where event is the triggering event and value is the value strictly belonging to the event.

We recommend getting both the event and new value of the characteristic via this tuple, and not via characteristic.event() and characteristic.value() calls in the context of the handler to make sure no event and value is lost. The reason behind this is that characteristic.event() and characteristic.value() return with the very last event received and with the current value of the characteristic, while the input parameters are always linked to the specific event triggering the handler. If the device is busy executing other operations, the handler of an incoming event may not be called before the next event occurs and is processed.

Usage example can be found under GATTSCharacteristic page.

characteristic.read_descriptor(uuid)

Returns the value of the descriptor specified by the uuid parameter. If no descriptor found for the characteristic returns None.

descriptor = char.read_descriptor(0x2900)
if(descriptor != None):
    print("Characteristic Extended Properties: " + str(binascii.hexlify((descriptor))))

descriptor = char.read_descriptor(0x2901)
if(descriptor != None):
    print("Characteristic User Description: " + str(binascii.hexlify((descriptor))))

descriptor = char.read_descriptor(0x2902)
if(descriptor != None):
    print("Client Characteristic Configuration: " + str(binascii.hexlify((descriptor))))

descriptor = char.read_descriptor(0x2904)
if(descriptor != None):
    print("Characteristic Presentation Format: " + str(binascii.hexlify((descriptor))))