Throw error from PreSet event callback

Hi everyone,

I have a little problem with a PreSet event callback which I would like to implement. My goal is to check whether my property "locked" is true. If this is the case, Matlab should throw an error. Unfortunately, it seems Matlab converts my error to a warning, which means that the code continues to execute. Is there any way to make an error inside an event callback an actual error?

For clarification ... You may wonder why I do not implement that via a set function. The reason is that this implementation is supposed to affect quite a few properties over several levels of inheritance. If I would use set functions, I would have to define them for all properties individually and they would all do absolutely the same. Right now, my top level class has a function which looks like that:

function add_listeners(obj)

    mc = metaclass(obj);
    mp = mc.PropertyList;
               
    for run = 1:length(mp)
        if mp(run).SetObservable
            addlistener(obj,mp(run).Name,'PreSet',@obj.handle_preset_event);
            addlistener(obj,mp(run).Name,'PostSet',@obj.handle_postset_event);
        end
    end

end

It is called inside the constructor method and adds listeners for all properties that are "SetObservable". My "handle_preset_event" function currently looks like that:

function handle_preset_event(obj, prop, ~)
    if obj.locked
        error('myclass:property_locked', ...
            ['Property ' prop.Name ' cannot be changed; class instance is locked.'])             
    end
end

But as mentioned above, this does not work as indended because Matlab converts the error to a warning.

Any ideas on that?

Thanks a lot in advance.

PS: Other solutions are welcome as well, of course. I just don't want to copy the same part of my code 20 times.

Answer 1

[sanidhyak]

Hi @Christian Vetter,

I understand that you are facing an issue where MATLAB converts the error inside your “PreSet” event callback into a warning instead of stopping execution. This occurs because MATLAB handles errors inside event callbacks differently, often preventing them from stopping property changes.

To resolve this, you can explicitly revert the property change inside the callback before throwing the error. Kindly refer to the following corrected implementation:

classdef MyClass < handle 
    properties(SetObservable) 
        Value 
    end 
    properties 
        locked = false; 
    end 
    methods 
        function obj = MyClass() 
            add_listeners(obj); 
        end 
        function add_listeners(obj) 
            mc = metaclass(obj); 
            mp = mc.PropertyList; 
            
            for run = 1:length(mp) 
                if mp(run).SetObservable 
                    addlistener(obj, mp(run).Name, 'PreSet', @obj.handle_preset_event); 
                end 
            end 
        end 
        function handle_preset_event(obj, prop, event) 
            if obj.locked 
                event.Source.(prop.Name) = event.AffectedObject.(prop.Name); % Revert change 
                error('myclass:property_locked', ... 
                    ['Property ' prop.Name ' cannot be changed; class instance is locked.']); 
            end 
        end 
    end 
end 

Here, the “PreSet” listener is added for all “SetObservable” properties in the constructor. Also, the “handle_preset_event” function checks if locked is true. If locked, it explicitly reverts the property change before throwing an error, ensuring MATLAB does not override it with a warning.

With these changes, MATLAB will prevent modifications when the object is locked, and an actual error will be thrown instead of a warning.

For further reference on MATLAB “event listeners”, kindly refer to following documentation:

• https://www.mathworks.com/help/matlab/matlab_oop/learning-to-use-events-and-listeners.html

Cheers & Happy Coding!