Why is my HandleCompatible class out-of-date when accessed in callback?

Hello,

I'm trying to write a MATLAB (2019b) class to encompass a group of UI components. Despite setting my class to "HandleCompatible", which I thought meant it would effectively be passed by reference rather than by value, I find that when using an class method as a callback to a UI event, my instance is out of date, as though it had been cached or copied at the time the callback was registered. Help!

I created a minimal working example to explain what I mean. Here's the class - it consists of a gridlayout and two edit fields. The first edit field has a callback that updates the second edit field when the first edit field value is changed.

classdef (HandleCompatible) HandleExample
    properties
        layout          matlab.ui.container.GridLayout
        field1          matlab.ui.control.EditField
        field2          matlab.ui.control.EditField
    end
    methods  %(Access = public)
        function obj = HandleExample(parentLayout)
            % Create GridLayout
            obj.layout = uigridlayout(parentLayout);
            obj.layout.ColumnWidth = {'1x', '1x'};
            obj.layout.RowHeight = {'1x'};

            % Create field 1
            obj.field1 = uieditfield(obj.layout);
            obj.field1.Layout.Row = 1;
            obj.field1.Layout.Column = 1;
            obj.field1.ValueChangedFcn = @obj.field1ChangeHandler;

            % Create field 2
            obj.field2 = uieditfield(obj.layout);
            obj.field2.Layout.Row = 1;
            obj.field2.Layout.Column = 2;            
        end
        function field1ChangeHandler(obj, thrower, event)
            disp(obj)
            disp(thrower)
            disp(event)
            obj.field2.Text = 'This won''t work :(';
        end
    end
end

Here's how I instantiate the class and here's the error:

>> x = HandleExample(uifigure)

x = 

  HandleExample with properties:

    layout: [1×1 GridLayout]
    field1: [1×1 EditField]
    field2: [1×1 EditField]

After I change the text in edit field 1, I get this output/error:

  HandleExample with properties:

    layout: [1×1 GridLayout]
    field1: [1×1 EditField]
    field2: [0×0 EditField]

  EditField (hi) with properties:

               Value: 'hi'
     ValueChangedFcn: @(varargin)obj.field1ChangeHandler(varargin{:})
    ValueChangingFcn: ''
            Position: [11 11 265 400]

  Show all properties

  ValueChangedData with properties:

            Value: 'hi'
    PreviousValue: ''
           Source: [1×1 EditField]
        EventName: 'ValueChanged'

Property assignment is not allowed when the object is empty.  Use subscripted assignment to
create an array element.

Error in HandleExample/field1ChangeHandler (line 29)
            obj.field2.Text = 'This won''t work :(';

Error in HandleExample>@(varargin)obj.field1ChangeHandler(varargin{:}) (line 18)
            obj.field1.ValueChangedFcn = @obj.field1ChangeHandler;
 
Error using matlab.ui.control.internal.controller.ComponentController/executeUserCallback (line 378)
Error while evaluating EditField PrivateValueChangedFcn.

Notice that inside the callback. obj.field2 is 0x0, as though my class instance was frozen by value at the line where I register the callback! I thought this is exactly what the "HandleCompatible" keyword was supposed to avoid, but I must be misunderstanding something. This is my first MATLAB classdef.

Thank you in advance for the help!

Answer 1

[Brian Kardon]

Wow, ok, I searched a lot for the answer to this question, and minutes after I finally asked, I seem to have found the answer:

https://www.mathworks.com/help/matlab/matlab_oop/class-methods-for-graphics-callbacks.html

Apparently setting the class to "HandleCompatible" is not enough. In fact, I'm not sure it's even necessary. What I was missing is making my class a subclass of "handle". So, while this doesn't work,

classdef (HandleCompatible) HandleExample

this does:

classdef (HandleCompatible) HandleExample < handle

(also, incidentally, the correct property name for the text of an uieditfield is "Value", not "Text".

[per isakson]

"In fact, I'm not sure it's even necessary." No it is neither necessary nor meaningful. (Why doesn't the Code Analyzer show a warning? (R2018b)) HandleCompatible doesn't add anything to a class that subclasses the handle class.

The documentation, Handle Compatible Classes , says:

Handle-compatible class — a class that you can include with handle classes in a class hierarchy, even if the class is not a handle class.