Callback function not found for dynamically created app UI component

I would like to dynamically/programmtically create a uidropdown component that calls a function when its value is changed. However, even though I provide a function handle as the ValueChangedFcn property, the uidropdown is unable to find the function. I receive the error below.

Undefined function 'DropDownChange' for input arguments of type 'matlab.ui.control.DropDown'.

Error using matlab.ui.control.internal.controller.ComponentController/executeUserCallback (line 305)

Error while evaluating DropDown PrivateValueChangedFcn

I am attaching a simple app created in R2016b that exhibits this behavior. Why can't the function be found? How should I define it to resolve the issue?

[Jan]

Please post code as text, such that writing an answer allows to copy&paste.

[Monika Jaskolka]

The code was in the attached mlapp, but here it is again:

classdef App_Test < matlab.apps.AppBase

    % Properties that correspond to app components
    properties (Access = public)
        UIFigure  matlab.ui.Figure
    end


    properties (Access = private)
        DropDown
    end

    methods (Access = public)
        
        
        function DropDownChange(app)
            disp('Works!')
        end
        
    end


    methods (Access = private)

        % Code that executes after component creation
        function startupFcn(app)
            app.DropDown = uidropdown(app.UIFigure, ...
                'Position', [80, 100, 100 ,24], ...
                'ValueChangedFcn', @DropDownChange);
        end
    end

    % App initialization and construction
    methods (Access = private)

        % Create UIFigure and components
        function createComponents(app)

            % Create UIFigure
            app.UIFigure = uifigure;
            app.UIFigure.Position = [100 100 274 165];
            app.UIFigure.Name = 'UI Figure';
            setAutoResize(app, app.UIFigure, true)
        end
    end

    methods (Access = public)

        % Construct app
        function app = App_Test()

            % Create and configure components
            createComponents(app)

            % Register the app with App Designer
            registerApp(app, app.UIFigure)

            % Execute the startup function
            runStartupFcn(app, @startupFcn)

            if nargout == 0
                clear app
            end
        end

        % Code that executes before app deletion
        function delete(app)

            % Delete UIFigure when app is deleted
            delete(app.UIFigure)
        end
    end
end

Answer 1

[Monika Jaskolka]

I ended up contacting MathWorks support for this. The solution is to call the function like so:

app.DropDown = uidropdown(app.UIFigure, ...
    'Position', [80, 100, 100 ,24], ...
    'ValueChangedFcn', @(~,~)app.DropDownChange);

The first argument passed to the callback is the uidropdown, so I don't want to name it "app" and shadow the "app" argument. Additionally, since I want to invoke a method of the app, I need to call either "app.DropDownChange" or "DropDownChange(app)".

Answer 2

[Jan]

As far as I can see, Matlab searchs for a matching function with the input (app, event), but your callback accepts (app) only. So try to append a 2nd (unused...) input to your CropDownChange function.

[EDITED] No, this does not work.

[Monika Jaskolka]

The following still gives the same error. I believe I am following the format outlined here correctly, unless I am missing something!

app.DropDown = uidropdown(app.UIFigure, ...
    'Position', [80, 100, 100 ,24], ...
    'ValueChangedFcn', @(app,event)DropDownChange);

Answer 3

[Bear]

I have a similar issue.

I'm creating a submenu voice with a file path and I want to associate a function. The idea is the add a voice every time the user selects a directory in order to have a history of selected folders.

I would like to catch also the event when the user selects a folder in order to retrieve the selected path and move to that folder.

mitem = uimenu(app.RecentpathsMenu,"Text",user_dir,"MenuSelectedFcn",@(~,~)app.ChangePath);