MATLAB: Basic implementation of events and listeners.

classMATLABoop

I am trying to write a class with all properties depending on eachother It seems events and listeners can be used for this, but I can't get the basic implementation of these to work.
% when property a or b is altered, sum will automatically be recalculated
classdef myclass < handle
    properties
        a = 1;
        b = 2;
        sum
    end
    events
        valuechange
    end
    methods
        function obj = myclass()
            addlistener(obj, valuechange, calc_sum(obj))
        end
        function obj = set.a(obj, input)
            obj.a = input;
            notify(obj, valuechange)
        end
        function obj = set.b(obj, input)
            obj.b = input;
            notify(obj, valuechange)
        end
        function calc_sum(obj)
            obj.sum = obj.a + obj.b
        end
    end
end
an attempt to create an object returns following error:
Undefined function or variable 'valuechange'.
Error in myclass (line 18)
            addlistener(obj, valuechange, calc_sum(obj))
What am I not understanding?
Thanks

Best Answer

You have three minor problems and one major problem. In the addlistener and notify calls you need to make valuechange into a string 'valuechange'. You also need the callback in addlistener to be a function handle that takes two arguments:
addlistener(obj, 'valuechange', @(obj, evt)calc_sum(obj));
The major problem is that this is not the correct way to do what you are trying to do. The calc_sum callback will only be initiated when the event queue is flushed. This means that there is a period of time in which the class is in an intermediate state (i.e., obj.a+obj.b will not equal obj.sum). You should look into dependent properties. You could also have set.a and set.b call calc_sum directly.
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