Singleton

Normally, you want to have just one single instance of a GUI window running. This is the so-called “singleton” concept.

There are several reasons why you normally want to make sure that only one instance of a particular GUI window can exist at a given time:

• GUI windows are quite expensive in terms of resources.
• The user gets easily confused if several instances are open at the same time.
• You will easily get confused (or particularly your code) if several instances of a GUI window try to write data back to the main GUI.

The solution to these problems is to make sure that only one instance of a GUI window will be open at a given time.

The good news is: It is quite easy to make use of the singleton concept within Matlab™. Here is how:

function mySingletonGUI
% MYSINGLETONGUI A GUI window stub demonstrating the singleton concept
% in Matlab(tm).
 
% (c) 2012, Till Biskup
% 2012-05-31
 
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
%  Construct the components
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
 
% Make GUI effectively a singleton
singleton = findobj('Tag',mfilename);
if (singleton)
    figure(singleton);
    return;
end
 
%  Construct the components
hMainFigure = figure('Tag',mfilename,...
    'Name','My singleton GUI',...
    'Units','Pixels',...
    'Position',[300,300,350,250],...
    'Resize','off',...
    'NumberTitle','off', ...
    'Menu','none','Toolbar','none'...
    );

Although it appears to be quite a bit of code, the crucial part is only a very few lines. But besides that, what you see in the listing above is an example of a generic GUI function.

Please note: Although the above example will just create and open a pretty boring (empty) figure window, it is a working and tested example.

The basic idea is to give your main GUI window (the one that you create with the figure command) a unique name, called “Tag”.

At the beginning of your GUI function, you just search for an object with that tag (via findobj). If the search returns a value, just bring that figure to focus (simply calling figure() with the respective figure handle as argument) and return.

That's it.