MATLAB: Scroll function inside an app does not work scroll(app.ListBox, ‘bottom’), but it works when called on the app object from the command line. Why

appMATLABscroll

I have a simple app, where user can input words in an EditField or in a DropDown.
The entered words are then displayed in a ListBox.
Problem 1:
scroll(app.ListBox, 'bottom')
does not work when called from "DropDownValueChanged" function.
However, if I create the app as an object, and call the scroll function from the command line:
h = histroryscroll
scroll(h.ListBox, 'bottom')
then it works as expected. Is this a bug or am I missing something?
Problem 2:
I want to be able to enter the same words several times in a row, i.e. anytime I press Enter from within DropDown componenet. However, the ValueChanged function DropDownValueChanged gets triggered only when the value is actually changed, so i cannot enter the same word twice in a row. Any idea for a solution?
The test code (histroryscroll.m)
%
classdef historyscroll < matlab.apps.AppBase
    % usage:
    % h = histrorycall
    % scroll(h.ListBox, 'bottom')
    
    % Properties that correspond to app components
    properties (Access = public)
        UIFigure        matlab.ui.Figure
        EditFieldLabel  matlab.ui.control.Label
        EditField       matlab.ui.control.EditField
        ListBoxLabel    matlab.ui.control.Label
        ListBox         matlab.ui.control.ListBox
        DropDownLabel   matlab.ui.control.Label
        DropDown        matlab.ui.control.DropDown
        Label           matlab.ui.control.Label
    end
    methods (Access = private)
        % Value changed function: EditField
        function EditFieldValueChanged(app, event)
            value = app.EditField.Value;
            app.EditField.Value = '';
            app.ListBox.Items(end+1) = {value};
            scroll(app.ListBox, 'bottom');      % This does not work

        end
        % Value changed function: DropDown
        function DropDownValueChanged(app, event) 
            value = app.DropDown.Value;
            app.DropDown.Items(end+1) = {value};
            app.DropDown.Value = '';
            app.ListBox.Items(end+1) = {value};
            scroll(app.ListBox, 'bottom');      % This does not work
        end
    end
    % App initialization and construction
    methods (Access = private)
        % Create UIFigure and components
        function createComponents(app)
            % Create UIFigure
            app.UIFigure = uifigure;
            app.UIFigure.Position = [100 100 249 294];
            app.UIFigure.Name = 'UI Figure';
            % Create EditFieldLabel
            app.EditFieldLabel = uilabel(app.UIFigure);
            app.EditFieldLabel.HorizontalAlignment = 'right';
            app.EditFieldLabel.Position = [9 176 56 22];
            app.EditFieldLabel.Text = 'Edit Field';
            % Create EditField
            app.EditField = uieditfield(app.UIFigure, 'text');
            app.EditField.ValueChangedFcn = createCallbackFcn(app, @EditFieldValueChanged, true);
            app.EditField.Position = [80 176 151 22];
            app.EditField.Value = 'Enter Cmds here ...';
            % Create DropDownLabel
            app.DropDownLabel = uilabel(app.UIFigure);
            app.DropDownLabel.HorizontalAlignment = 'right';
            app.DropDownLabel.Position = [8 144 66 22];
            app.DropDownLabel.Text = 'Drop Down';
            % Create DropDown
            app.DropDown = uidropdown(app.UIFigure);
            app.DropDown.Items = {'... or here ...'};
            app.DropDown.Editable = 'on';
            app.DropDown.ValueChangedFcn = createCallbackFcn(app, @DropDownValueChanged, true);
            app.DropDown.BackgroundColor = [1 1 1];
            app.DropDown.Position = [89 144 142 22];
            app.DropDown.Value = {'... or here ...'};
            % Create ListBoxLabel
            app.ListBoxLabel = uilabel(app.UIFigure);
            app.ListBoxLabel.HorizontalAlignment = 'right';
            app.ListBoxLabel.Position = [19 259 48 22];
            app.ListBoxLabel.Text = 'List Box';
            % Create ListBox
            app.ListBox = uilistbox(app.UIFigure);
            app.ListBox.Items = {'Commands History', ''};
            app.ListBox.Position = [82 209 149 74];
            app.ListBox.Value = 'Commands History';
            % Create Label
            app.Label = uilabel(app.UIFigure);
            app.Label.VerticalAlignment = 'top';
            app.Label.Position = [19 1 212 110];
            app.Label.Text = {'Problem1: List Box does not '; ' scroll to the bottom i.e.'; 'scroll(app.ListBox'; '''bottom'') does not work'; ''; 'Problem2: I cannot enter the same '; 'command twice'; ''; ''};
        end
    end
    methods (Access = public)
        % Construct app
        function app = historyscroll
            % Create and configure components
            createComponents(app)
            % Register the app with App Designer
            registerApp(app, app.UIFigure)
            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

Best Answer

I had the same problem when trying to write a GUI with App Designer that included a scrolling status window that would scroll up when a new message was displayed. After reading other posts I implemented this as a listbox, however I found that scrolling didn't work if I called:
scroll(app.ListBox, 'bottom');
within the function that added the message to the list. However, it would work if I called the same function from another callback or from the command line, neither of which solved my problem. The solution turned out to be to call pause with a short delay between updating the list box entries and calling scroll, e.g.:
{Code to update list box entries}
pause(0.01);
scroll(app.ListBox, 'bottom');
Presumably the call to pause() gives Matlab a chance to do something important in the background - the same strategy has got me out of trouble with other graphics issues in the past.
I hope this helps someone,
Alec
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