How do you correctly reference contents of a 'DataAvailable' event?

I am trying to access the TimeStamps field of a 'DataAvailable' event from a NIDAQ session. I get the error:

Struct contents reference from a non-struct array object.

when I try to refer to the contents using the following code:

s = daq.createSession('ni');
ch = addAnalogInputChannel(s, 'Dev1', 1, 'Voltage');
ch(1).TerminalConfig = 'SingleEnded';
s.Rate = 1000;
s.DurationInSeconds = 10;

dataListener1 = addlistener(s, 'DataAvailable', @(src, event) datamanager(src, event));
startBackground(s)
datamanager(s, 'DataAvailable')

Which refers to the function datamanager:

function datamanager(src, event)
  realData=[realData, event.Data, event.TimeStamps];

The "event" is of the class "daq.DataAvailableInfo," which seems to behave like a struct (see below), but I get this problem nonetheless.

The output for "event" is the following:

event = 

    DataAvailableInfo with properties:

      TriggerTime: 7.3659e+05
             Data: [100x32 double]
       TimeStamps: [100x1 double]
           Source: [1x1 daq.ni.Session]
        EventName: 'DataAvailable'

The presence of a TimeStamps field inside event makes this error confusing to me. Any thoughts on how I could avoid or fix this would be appreciated. Thanks!

[Walter Roberson]

I suggest experimenting with

 realData=[realData, get(event, 'Data'), get(event, 'TimeStamps')];

[Eamon]

Hi Walter, I've tried that, but

get(event, 'Data');

results in:

Error using get
Invalid handle

I think this is because Matlab thinks that "event" refers to a graphics object, but it doesn't find a graphics object named "event." I'm not sure why it would think this, given that this link shows how "get" can be used to access values of properties in structs. Any ideas?

[Walter Roberson]

The addlistener for DataAvailable documentation shows an example

lh = s.addlistener('DataAvailable', ...
        @(src,event) plot(event.TimeStamps, event.Data));

Could I suggest you try that, to see if you can replicate what is documented?

Which MATLAB release are you using?

[Eamon]

Running the code you suggest launches a new window with correct plot, however it does not plot on the axes inside the GUI, which is where I would like to plot. Maybe I can modify the listener to plot on the desired axes. I'm using Matlab 2016a.

[Walter Roberson]

The first argument to plot() can be the handle to the axes to plot on to.

[Eamon]

Hi Walter. I used

findobj(gcf,'type','axes')

to find the axes. However, when I incorporate the axes into the plot argument like so:

dataListener2 = addlistener(s, 'DataAvailable', @(src,event) plot(handles.axes1, event.TimeStamps, event.Data));

No plot is generated, even in a separate window, and I get the same error as before, the "Struct contents reference from a non-struct array object" error. I'm not sure why this is but I'm going to experiment with different axes and see if that might work.

[Walter Roberson]

You would have to have handles defined and have an axes1 field at the time you created the listener for that to be valid. When you reference a variable in an anonymous function like you are creating there, the contents of the variable are captured as of the time the anonymous function is created.

... but that might not be the difficulty you are encountering.

I do have access to that toolbox, but unfortunately I do not have a data generating device to test with, or at least not one that comes to mind.