Clearing object from base workspace on closing a GUI

I'm buidling a GUI with the following structure:

classdef App < handle

properties
    gui
    other_properties
end

methods
    function obj = App()       
        % Initialize window
        obj.gui = obj.main_window_layout('reuse');  
        
        % Assign callbacks
        obj.assign_callbacks();
    end
    
end

end

The main windows layout is defined in:

function h1 = main_window_layout(obj,policy)
    %% policy - create a new figure or use a singleton. 'new' or 'reuse'.
    persistent hsingleton;
    if strcmpi(policy, 'reuse') & ishandle(hsingleton)
        h1 = hsingleton;
        return;
    end   
    
    %% Create the figure instance
    % Define position [x bottom left, y bottom left, width, height]
    figure_position = [10 10 112 32.3076923076923]; 
    h1 = figure(...
    'PaperUnits','inches',...
    'Units','characters',...
    'Position',figure_position);

    %% Additional uicontrols etc..
    
end

I'd like the GUI to close when I press the close button in the window, that's why I defined the following callback:

function assign_callbacks(obj)
%assign_callbacks Assigns the callbacks of the GUI

% Closing event
obj.gui_handles.figure.CloseRequestFcn = @(hObject,eventdata)close_figure_Callback(hObject,eventdata,obj);

%% --- Executes on close figure
function close_figure_Callback(~,~,obj)
    delete(obj)
    delete(gcf)   
end

end

I start the GUI by:

%% Only start the app if it is not running yet
if exist('app_object','var') == 0
    app_object = App();
else
    clear app_object
    app_object = App();    
end

When I close the figure, the window closes and the GUI is cleared. However, the app_object is still existing in the base workspace, it is empty though. I'd like to clear the object from the base workspace. The reason for this is that I want to prevent my GUI from initializing twice by calling the start script.. Is this the right way to achieve this?

Answer 1

[Walter Roberson]

function close_figure_Callback(~,~,obj)
    delete(obj)
    delete(gcf)
    clear main_window_layout   %get rid of the persistent
    evalin('base', 'clear app_object;');
end

[Jan Loof]

I think this can help to solve the problem but relies on the object instance being called 'app_object', what if it's called different?

[Walter Roberson]

Then you are going to have problems with your code that checks the existence of app_object .

If your question is, "How can an object know the name of the variable it was assigned to", the answer is, "It can't".

Value objects have a different name in every workspace they exist in, and might have several names in the same workspace, since they can be assigned to different variables.

You could whos() the base workspace and look for values listed as being of the correct datatype... but are you sure that the user even stored the object in the top level of a variable? And if it is a value object, can you be sure that you have found the right object?

Handle objects have a different name in every workspace they exist in, and might have several names in the same workspace, since they can be assigned to different variables. The difference is for handle objects, you can use isequal() to test whether the handle is the same as the handle of the object you are working with... if you can find the handle (are you sure it is even stored in the top level of a variable?)

I would suggest to you that rather than just checking the existance of the variable, that you also check isvalid() of the handle.

[Jan Loof]

How about this solution then:

function close_figure_Callback(~,~,obj)
    delete(obj)
    delete(gcf)   
    clear main_window_layout   %get rid of the persistent
    
    % Clean up base workspace
    evalin('base','vars = whos()');
    evalin('base','name = vars(strcmp({vars.class},''App'')).name');
    evalin('base','clearvars(name)');
end

This searches for my class instances in the base workspace and clears them. In my case the user will always start the app in the base workspace and the goals is to have only one instance of the app open at all times.

I don't completely understand what you mean with the isvalid() of the handle and why it is necessary?

Edit: only clear name and not name{:} because name is a character.

[Walter Roberson]

%% Only start the app if it is not running yet
if ~exist('app_object','var') || ~isvalid(app_object)
    app_object = App();
else
    clear app_object
    app_object = App();    
end

Mind you, I do not understand why you clear app_object and start a new one if the existing one is still valid ??

[Jan Loof]

Because this re-initializes the app and adds default values to possibly already present values.