Matlab anonymous callback function arguments

Question

Following this example of a Model View Controller GUI in Matlab, I have a question regarding the anonymous function callback input arguments

Here is the a view function that creates the gui handles and passes them as an input argument to the onChanged callback function.

function handles = View_TimeDomain(m)
    %VIEW  a GUI representation of the signal model

    % build the GUI
    handles = initGUI();
    onChangedF(handles, m);    % populate with initial values

    % observe on model changes and update view accordingly
    % (tie listener to model object lifecycle)
    addlistener(m, 'f', 'PostSet', ...
        @(o,e) onChangedF(handles,e.AffectedObject));
end

The first thing that I don't quite understand is that according to the Matlab documentation the first argument must be the source of the event and the second arument must be the event data (Matlab doc1, Matlab doc2) but in this case it is handles. When the event is triggered, the following onChangedF function is called as expected.

function onChangedF(handles, model)
    % respond to model changes by updating view
    if ~ishghandle(handles.fig), return, end
    set(handles.line, 'XData',model.t, 'YData',model.data)
    set(handles.slider, 'Value',model.f);
end

However, in this case handles is the struct containing the handles defined using initGui() and not the event source.

I guess this stems from the definition of the anonymous function:

@(o,e) onChangedF(handles, e.AffectedObject)

Am I correct, assuming that o is the source which is not used in the onChangedF function input. Can someone explain why this syntax of an anonymous function is valid? I thought that o needs to be an argument of this specific callback function as well. Something like this:

@(o,e) onChangedF(o, handles, e.AffectedObject)

where additional arguments are appended (at the end) of the function. And then just avoid this unused argument using ~:

function onChangedF(~, handles, model)
    % respond to model changes by updating view
    if ~ishghandle(handles.fig), return, end
    set(handles.line, 'XData',model.t, 'YData',model.data)
    set(handles.slider, 'Value',model.f);
end

You don't need put `o` as argument of onChangedF because, this function isn't your callback. The calback function is the anonymous function, and it calls your function with the parameters you want. For example you can have an anonymous function that is `@(o, e) data = sqrt(5)`, I don't know if you see my point but the thing is that in your callback (anonymous or not) you can call any function with any parameters because the function you call aren't callback's function so they don't need any parameters link to the callback event. — Théo P., Jul 25 '17 at 09:40
Thanks for your help. So `@(o, e)` is some sort of placeholder for the source and evend data, which must be provided according to the matlab documentation? — evolved, Jul 25 '17 at 10:18

score 1 · Accepted Answer · answered Jul 25 '17 at 11:46

Anonymous functions are a subset of function handles that allow you to fully define functions inline, as opposed to function handles that execute a function that exists elsewhere.

The syntax of an anonymous function is af = @(arglist)anonymous_function, which is functionally the same as:

function af(arglist)
    anonymous_function
end

This means your PostSet callback is functionally equivalent to:

function PostSet(o,e) 
    onChangedF(handles, e.AffectedObject)
end

Which satisfies MATLAB's callback definition requirements.

Because handles is in the function scope when you are creating your PostSet anonymous function, it is available in the scope of the anonymous function callback as well. This is explained in the 'Variables in the Expression' section of the anonymous function documentation. This can also be visualized using the functions function, which provides information about a function handle:

z = 50;
fh = @(x, y) thing(y, z);

fhinfo = functions(fh);
fhworkspace = fhinfo.workspace{1}

Which returns:

fhworkspace = 

  struct with fields:

    z: 50

Matlab anonymous callback function arguments

1 Answers1