What is the purpose of arguments 'src' and 'evnt' in callback functions?

For callback functions, for example, a listener function of an event, the syntax is

function my_callback(obj,src,evnt)
...
end

However, even if the arguments 'src' and 'evnt' are not used in the body of 'my_callback' function, the unused arguments are still required to appear in the parentheses. I can replace them with '~':

function my_callback(obj,~,~)
...
end

But the unused arguments always need 2 positions when declaring the function. If I write 'my_callback' without the two '~':

function my_callback(obj,~,~)
...
end

Matlab will throw an error stating 'too many arguments'. What is the reason? Thank you!

Answer 1

[OCDER]

Matlab is essentially passing on a standard set of information for any object that has a callback feature: 1) the object that is affected, 2) the event that took place, and 3) the GUI data. Even if you don't use the 2nd and 3rd inputs, Matlab still has to pass on these information for consistency. By removing your 2nd and 3rd input, Matlab will complain as it tries to do this:

@your_callback_fcn(obj, event, data)   %ERROR if your_callback_fcn accepts only 1 input.

This link could help explain callback functions in general: https://www.mathworks.com/help/matlab/creating_guis/write-callbacks-using-the-guide-workflow.html

Here's a copy of the relevant text from that link:

function pushbutton1_Callback(hObject,eventdata,handles)
% hObject    handle to pushbutton1 (see GCBO)
% eventdata  reserved - to be defined in a future version of MATLAB
% handles    structure with handles and user data (see GUIDATA)
end

All callbacks must accept at least three input arguments:

hObject — The UI component that triggered the callback.
eventdata — A variable that contains detailed information about specific mouse or keyboard actions.
handles — A struct that contains all the objects in the UI. GUIDE uses the guidata function to store and maintain this structure.

[Guanyu Wang]

Thank you Donald! You actually already answered my question. But I want be a little more greedy: So what is the reason that Matlab does not deal with number of arguments of callback functions more flexibly? Is it because the two seemingly unused arguments are used by background threads implicitly? In your sentence "Even if you don't use the 2nd and 3rd inputs, Matlab still has to pass on these information for consistency." What does 'consistency' refer to? to some underlying legacy code that running in the background?

Thanks again!

[OCDER]

MathWorks staff may know why this was designed the way it is, but if I were designing callbacks, I would do something similar too. For instance, I'd want the callback to be "robust enough" to able to 1) know that it was summoned, 2) do a special action in response to a special event (mouse click, keyboard click, etc), and 3) use existing GUI data to do some calculation. Without these information being passed on by default, that would make it hard for users because then they have to write extra code to figure out what happened and what data is currently there.

If you need flexibility, you could do this in your callback and not worry about all that extra input/output:

function my_callback(varargin)

If you're concerned that unused inputs are causing performance issues because they seem to be "passed on by value" and copied into your function for no reason, then be relieved by the clever way MathWorks prevents that issues (read article):

https://blogs.mathworks.com/loren/2006/05/10/memory-management-for-functions-and-variables/

[Guanyu Wang]

Thank you a lot Donald! Wish I can vote you twice haha.

[OCDER]

You're welcome!