How can I check if a MATLAB handle is valid?

Question

I manipulate data on a plot using its handle:

x = 1:10; y = sin(x);
h1 = line(x,y);

However, if the figure gets closed before the script actually terminates, doing something like this returns an error.

>>set(h1,'Color','green') % line is green
??? Error using ==> set
Invalid handle object.

Is there a way to check if h1 is a valid handle before doing any manipulations with it?

Answer 1

You can use the ishandle function to check first if a graphics handle is valid:

if ishandle(h1)
  set(h1, 'Color', 'green');
end

UPDATE:

For newer versions of MATLAB, handle objects are actual objects, not just numeric values. The better option is to use the isvalid method for handle objects:

if isvalid(h1)
  set(h1, 'Color', 'green');
end

Answer 2

Note that ishandle has a drawback in that it also accepts common numeric values like 0 (=desktop handle) and 1 (=the first open figure by default) which are often also valid handles although possibly not the expected handle. You will then still see an error if you try to set a non-existent property.

To handle such cases, simply place your code within an exception-handling block:

try
   set(myHandle,propName,propValue);
catch
   % do something useful... (recreate the GUI?)
end