.. highlight:: cpp

Data Signal
===========
Data signals are an abstract representation of the data produced by the sensors.  The API treats them as an event that contains data and represents 
them with the `MblMwDataSignal <https://mbientlab.com/docs/metawear/cpp/latest/datasignal__fwd_8h.html#a1ce49f0af124dfa7984a59074c11e789>`_ struct.  
They can be safely typecasted to a `MblMwEvent <https://mbientlab.com/docs/metawear/cpp/latest/event__fwd_8h.html#a569b89edd88766619bb41a2471743695>`_

Data Handling
-------------
Signal data is encapsulated by the `MblMwData <https://mbientlab.com/docs/metawear/cpp/latest/structMblMwData.html>`_ struct.  The struct contains a: 

* Timestamp of when the data was created
* Pointer to the data
* Data type id indicating how to cast the pointer

An enumeration of data types is defined by the 
`MblMwDataTypeId <https://mbientlab.com/docs/metawear/cpp/latest/data_8h.html#a8335412069204db23669001bcaed532e>`_ enum and structs wrapping non basic 
data types are defined in the `types.h <https://mbientlab.com/docs/metawear/cpp/latest/types_8h.html>`_ header file. ::

    #include "metawear/core/data.h"
    
    #include <chrono>
    #include <iomanip>
    
    using std::chrono::duration_cast;
    using std::chrono::milliseconds;
    using std::chrono::system_clock;
    using std::chrono::time_point;
    
    void data_printer(const MblMwData* data) {
        // Print data as 2 digit hex values
        uint8_t* data_bytes = (uint8_t*) data->value;
        string bytes_str("{");
        char buffer[5];
        for (uint8_t i = 0; i < data->length; i++) {
            if (i) {
                bytes_str += ", ";
            }
            sprintf(buffer, "0x%02x", data_bytes[i]);
            bytes_str += buffer;
        }
        bytes_str += "}";
    
        // Format time as YYYYMMDD-HH:MM:SS.LL
        time_point<system_clock> then(milliseconds(data->epoch));
        auto time_c = system_clock::to_time_t(then);
        auto rem_ms= data->epoch % 1000;
    
        cout << "{timestamp: " << put_time(localtime(&time_c), "%Y%m%d-%T") << "." << rem_ms << ", "
            << "type_id: " << data->type_id << ", "
            << "bytes: " << bytes_str.c_str() << "}"
            << endl;
    }

Streaming
---------
To stream data live to your device, call 
`mbl_mw_datasignal_subscribe <https://mbientlab.com/docs/metawear/cpp/latest/datasignal_8h.html#ab2708a821b8cca7c0d67cf61acec42c3>`_  with the 
desired data signal and a callback function for handling the received data.  Terminating the live stream is done by calling 
`mbl_mw_datasignal_unsubscribe <https://mbientlab.com/docs/metawear/cpp/latest/datasignal_8h.html#ab2708a821b8cca7c0d67cf61acec42c3>`_. ::

    #include "metawear/core/datasignal.h"
    
    #include "metawear/sensor/switch.h"
    
    void subscribe_switch(MblMwMetaWearBoard* board) {
        static auto data_handler = [](const MblMwData* data) -> void {
            if (*((uint32_t*)data->value)) {
                cout << "Switch Pressed" << endl;
            } else {
                cout << "Switch Released" << endl;
            }
        };
    
        auto switch_signal = mbl_mw_switch_get_state_data_signal(board);
        mbl_mw_datasignal_subscribe(switch_signal, data_handler);
    }

Logging
-------
Alternatively, data can be logged and retrieved at a later time using the functions outlined in the :doc:`logging` section.  To direct your signal 
data to the logger, call 
`mbl_mw_datasignal_log <https://mbientlab.com/docs/metawear/cpp/latest/datasignal_8h.html#a9c2fdb156bb2ac424bebb6ecc55df3f7>`_.  This function is 
asynchronous and will alert the caller when it is completed via the 2nd callback function.  Removing your signal from the logger is done by calling  
`mbl_mw_datasignal_remove_logger <https://mbientlab.com/docs/metawear/cpp/latest/datasignal_8h.html#a4e9c13deb114bfd7a8c035e20bd2c9ac>`_. ::

    #include "metawear/core/datasignal.h"
    #include "metawear/sensor/multichanneltemperature.h"
    
    void log_temperature(MblMwMetaWearBoard* board) {
        static auto data_handler = [](const MblMwData* data) -> void {
            printf("temperature= %.3fC\n", *((float*) data->value));
        };
        static auto logger_ready = [](void) -> void {
            printf("logger ready\n");
        };
    
        auto temp_signal = mbl_mw_multi_chnl_temp_get_temperature_data_signal(board, 0);
        mbl_mw_datasignal_log(temp_signal, data_handler, logger_ready);
    }

Data Processing
---------------
Data signals can be fed through the on-board data processors to filter and/or transform the data in the firmware.  By performing computations on the 
MetaWear side, you can reduce the amount of data that is sent over the radio and the amount of postprocessing that is done on your mobile device.  Data processors can also be chained together to perform more complex tasks, such as using the rss, average, and threshold processors to determine if the 
board is in freefall based on the XYZ acceleration data. 

See the :doc:`mblmwdataprocessor` section for more details on the data processing system. ::

    #include "metawear/core/dataprocessor_fwd.h"
    #include "metawear/processor/rss.h"
    #include "metawear/sensor/accelerometer.h"
    
    void rss_accelerometer(MblMwMetaWearBoard* board) {
        static auto data_handler = [](const MblMwData* data) -> void {
            printf("acc rss= %.3fg\n", (float*) data->value);
        };
        static auto rss_ready = [](MblMwDataProcessor* processor) -> void {
            // subscribe to the rss processor
            mbl_mw_datasignal_subscribe((MblMwDataSignal*) processor, data_handler);
        };
    
        // Create an rss processor to transform the XYZ values into vector magnitude
        // Do not need to compute rss on your device and less data is transmitted
        auto acc_signal = mbl_mw_acc_get_acceleration_data_signal(board);
        mbl_mw_dataprocessor_rss_create(acc_signal, rss_ready);
    }