.. highlight:: javascript

Logger
======
The MetaWear board can log sensor data and store it in the internal memory of the device using ``loggers`` to be retrieved at a later time.

Loggers record data from a data signal and are represented by the 
`MblMwDataLogger <https://mbientlab.com/docs/metawear/cpp/0/logging__fwd_8h.html#a84a99b569b691df5017c03721645b49d>`_ struct.  

Create an 
``MblMwDataLogger`` object by calling 
`mbl_mw_datasignal_log <https://mbientlab.com/docs/metawear/cpp/latest/datasignal_8h.html#aa7ec82a61e31616ff2eaedb0a96160d8>`_ with the data signal 
you want to log.  

If successful, the callback function will be executed with a  
`MblMwDataLogger <https://mbientlab.com/docs/metawear/cpp/0/logging__fwd_8h.html#a84a99b569b691df5017c03721645b49d>`_ pointer and if creating the 
logger failed, a ``null`` pointer will be returned.  ::

    var accSignal = MetaWear.mbl_mw_acc_get_acceleration_data_signal(device.board);
    
    MetaWear.mbl_mw_datasignal_log(accSignal, ref.NULL, MetaWear.FnVoid_VoidP_DataLoggerP.toPointer(function (context, logger) {
        accelLogger = logger;
        callback(logger.address() ? null : new Error('failed to start logging accel'));
    }))

MblMwDataLogger objects only interact with the specific data signal, they do not control the logging features.  Logging control functions are detailed in the :doc:`logging` section.

ID
--
MblMwDataLogger objects are identified by a numerical id. 

This id can be used to keep track of loggers when there is considerable time between the start of a log and the download of a log. It is also useful to get the state of the device (i.e is my device still downloading?).

You can retrieve the id by calling 
`mbl_mw_logger_get_id <https://mbientlab.com/docs/metawear/cpp/0/logging_8h.html#ab32e4ae06e057cbb0180558ef8ec8165>`_.  

The id is used to retrieve existing loggers from the API with the 
`mbl_mw_logger_lookup_id <https://mbientlab.com/docs/metawear/cpp/0/logging_8h.html#a1b95ca107021c1e8f6ddaef0fbc85c4b>`_ function.

Handling Data
-------------
Like a data signal, you can subscribe to an MblMwDataLogger to process the downloaded data.  

Call `mbl_mw_logger_subscribe <https://mbientlab.com/docs/metawear/cpp/0/logging_8h.html#ac1fa6f2a678f61d86ccc80b092e8c098>`_ to attach a callback 
function to the MblMwDataLogger which handles all received data.  ::

    // Subscribe to the accelerometer data
    MetaWear.mbl_mw_logger_subscribe(accelLogger, ref.NULL, MetaWear.FnVoid_VoidP_DataP.toPointer(function onSignal(context, dataPtr) {
        var data = dataPtr.deref();
        var pt = data.parseValue();
        console.log(data.epoch + ' ' + pt.x + ',' + pt.y + ',' + pt.z);
    }));

Removal
-------
When you no longer want to log the values from a data signal, call 
`mbl_mw_logger_remove <https://mbientlab.com/docs/metawear/cpp/0/logging_8h.html#a8877b9a3f6c8571c41c21cda4a9c90cb>`_ to remove the logger.  ::

    MetaWear.mbl_mw_logger_remove(accelLogger);