How can I retrieve data from a callback of an in-built function?

Hello. I am trying to write a script that, when executed, allows the user to click on a button in a dialog box and select a directory. Furthermore, I want to store the directory path as a string. The problem I have is that the 'callback' I am using is to an in-built function and the 'help' I have looked at seems to be for user-created functions.

For example: https://uk.mathworks.com/help/matlab/creating_guis/share-data-among-callbacks.html#bt9p4qp.

I believe the advice on 'guidata' and 'guihandles' is for cases where you can write 'guihandles' into a separate section of code where the function is defined, which I don't have. I saw the same thing in the section for storing data in UserData.

Here is what I have so far:

% Setting up a dialog box
box = dialog('WindowStyle','normal','Name','My Dialog');
% Setting up 'uicontrol'
uicontrol(box,'Style','pushbutton','String','Click here','Position',[50 50 50 50],'Callback',@(src,event)uigetdir('C:\','Pick a folder:'));

So, how can I retrieve the directory path from the callback of the 'uigetdir' function? Any solutions or links to documentation that already has the answer would be greatly appreciated.

Many Thanks.

[Image Analyst]

Why not simply use GUIDE or App Designer? Why do it the hard way by going into all these gory details yourself?

[Manoj Abhishetty]

I actually only discovered GUIDE after going down the above route. GUIDE does seem easier, but this exercise is mainly for interest. I would just like to know if and how it is possible to do it without using GUIDE.

Answer 1

[Walter Roberson]

You cannot retrieve any value at all from the result of a Callback. Any value returned by a Callback will be discarded, always.

The only exception to this rule is that there are limited number of circumstances in which a callback function is used as a filter, such as providing constraint information for the imroi class of functions. Also, the UpdateFcn of a datecursormode() object must return a character vector or cell array of character vectors (or perhaps string array these days; I haven't looked into that detail.)

What you need to do is code the callback function so that it saves information in a known location, such as using guidata() or setappdata(), or set the UserData property of an object.

http://matlab.wikia.com/wiki/FAQ#How_can_I_share_data_between_callback_functions_in_my_GUI.28s.29.3F

[Manoj Abhishetty]

Thanks for the answer. Just to clarify, does this mean that I need to write a separate piece of code for a new function that does what 'uigetdir' does and also saves the information? So it is not possible with 'uigetdir' alone?

[Walter Roberson]

Correct, you need to write some extra code to save the results of the uigetdir() call.

If you were only interested in the first output value from uigetdir() then there are potentially methods you could use within an anonymous function. However, there is no method that can be used in an expression to collect multiple outputs of a function: at some point there would have to be an assignment statement of the form

[filename, pathname] = uigetdir(....)

so at least one non-anonymous function would have to be used, even if it was just an auxillary function

function result = call2(f, varargin)
  [val1, val2] = f(varargin{:});
  result = {val1, val2};

with the result of the work done through expression syntax.

The alternative instead of anonymous function would be to specify the callback as a character vector, in which case it will be executed as if by evalin('base', ...)

so you could code the callback as

'[filename, pathname] = uigetdir(....); set(gcbo, ''UserData'', {filename, pathname});'

[Manoj Abhishetty]

Thanks so much Walter - I ended up using the UserData property and it worked fine.

Sorry for how late this response is but I wanted to thank you!