CODEWITHSUNDEEP
matlabclassevents
SergioABP
SergioABP

Reputation: 345

Matlab: how to access object values inside an eventhandler?

I am trying to raise an event in Matlab. It is working, but not as i expected. What is going on is that the source object is passed as parameter do the event handler and i can't access my object properties inside the event handler.

Here is my class:

classdef MyClass < handle
    properties
        Name = 'a';
    end
    events
        Changed
    end
    methods
        function obj = MyClass(current)
            if exist('current','var')
                addlistener(current,'Changed',@ChangedHandle);
            end
        end
        function obj = Change(obj,value)
            notify(obj,'Changed');
        end
        function obj = ChangedHandle(obj,value)
            disp(obj.Name);
        end
    end
end

These are the command lines to reproduce whats going on:

a = MyClass();
b = MyClass(a);
b.Name = 'b';
a.Change(3);

This is returning "a" and i want it to return "b".

Regards

Upvotes: 1

Views: 128

Answers (2)

matlabgui
matlabgui

Reputation: 5672

In your addlistener code you are not telling the listener which object you want to call the method on, so what Matlab does it will apply the listener to the current object since thats all that was passed into the function, you can check this by changing the code to:

addlistener(current,'Changed',@(h,evt)current.ChangedHandle);

if you want it to run the code for the other class b (obj), then you use:

addlistener(current,'Changed',@(h,evt)obj.ChangedHandle);

Upvotes: 1

Will
Will

Reputation: 1880

The listener callback you specify receives details of the object raising the event. The fact that the listener has been created during the creation of a different object is not an intrinsic property of the listener – unless you effectively embed this in the design of your callback.

In your example, @ChangedHandle only works as a callback because it happens to be a method of the class current belongs to. If a and b belonged to different classes the problem would be more obvious: if ChangedHandle was a method only of a, it wouldn't know anything about the Name property of b. And if only a method of b, the listener bound to a would have only the function handle without any reference to the instance b of the class to which ChangeHandle belongs.

As described in the listener callback documentation you can use a method of the specific object instance current as an event listener using the syntax @current.ChangedHandle.

This method then receives callback arguments in the format callbackMethod(obj,src,evnt), so you need to add an argument to the definition of ChangedHandle. The first argument obj will be the instance referenced by current when the listener is created, so the line disp(obj.Name) will then produce the intended result without modification.

The reference to a received by the callback in the example will still be passed to the new callback – this is a fundamental behaviour of listener callbacks – but it will now be accessible from the second argument src.

Upvotes: 1

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