From the documentation:
addNewPositionCallback — Add new-position callback to ROI object
id = addNewPositionCallback(h,fcn) adds the function handle fcn to the list of new-position callback functions of the ROI object h. Whenever the ROI object changes its position each function in the list is called with the syntax:
fcn(pos)
where pos is of the form returned by the object's getPosition method.
The return value, id, is used only with removeNewPositionCallback.
This indicates that only 1 input is being passed to hpos, and you've define it to require 2 inputs. I'm not sure it'll work, but I would try:
handles.h = imline(gca,[-1 -1],[-2 2]);
id = addNewPositionCallback(handles.h,{@hpos,handles.iminfo});
% Update handles structure
guidata(hObject, handles);
function hpos(pos,iminfo_obj)
% Display image info
set(iminfo_obj, 'String','It works!');
% I'm assuming 'It works!' is a placeholder and you want to use pos above
