Matlab WindowButtonMotion callback stops working when image is changed

Question

In my GUIDE generated gui I have an axes object into which I imshow() a bitmap when the gui initializes. I have a WindowButtonMotion callback defined as:

function theGui_WindowButtonMotionFcn(hObject, eventdata, handles)
  % handles.mode_manager is initialized in the gui startup code
  if (isempty(handles.mode_manager.CurrentMode))
    obj_han=overobj('axes');
    if ~isempty(obj_han)
      set(handles.theGui, 'Pointer','cross');
    else
      set(handles.theGui, 'Pointer','arrow');
    end
  end
end

I have a callback on the open file button on the toolbar that does this:

function openFile_ClickedCallback(hObject, eventdata, handles)
  % handles.image_handle received the handle from the imshow that 
  % opened the initial image
  tmp_handle = handles.image_handle;
  [name, path] = uigetfile({'*.bmp'; '*.jpg'});
  if (path == 0)
    return
  else
    filename = strcat(path, name);
  end

  % read the image into the axes panel
  hold on
  handles.image_handle = imshow(filename);
  set(handles.image_handle, 'ButtonDownFcn', @imageMouseDown);
  handles.mode_manager = uigetmodemanager();
  delete(tmp_handle);
  guidata(hObject, handles);
end

After the new image is displayed in the axes object of the gui, the pointer no longer changes to a cross over the axes object. The problem has something to do with the newly displayed image, because if I comment out the part of the code that actually displays the new image, the pointer appears as a cross after calling the openFile callback.

Answer 1

The callback stops working, because using imshow replaces the axes object.

The following code demonstrates the problem:

imshow(zeros(100));
h = gca;
h.UserData = 123;  %Set UserData property value to 123
imshow(ones(100)); %Use imshow again.
h2 = gca;

Now:

h2.UserData

ans =

     []

h.UserData
Invalid or deleted object.

As you can see, using imshow again replaced the axes object, with new axes object.

---

The following example, modifies only the image data, without modifying the axes:

image_handle = imshow(zeros(100));
h = gca;
h.UserData = 123;  %Set UserData property value to 123
%imshow(ones(100), 'Parent', h); %Use imshow again.
image_handle.CData = ones(100); %Modify only image data, without modifying the axes. 
h2 = gca;

Now:

h2.UserData

ans =

   123

---

Modify your handles.image_handle = imshow(filename); code to:

I = imread(filename);
handles.image_handle.CData = I;