Using WindowButtonDownFcn callback to track mouse moves in figure

In Matlab 2013a I must track mouse moves in a figure to let user position vertical markers (timeline limits).

After creating my figure with hfig = figure('Position',rect, ...

I call set(hfig,'WindowButtonDownFcn',@ButtonDown);

and declare

function ButtonDown(hObject, eventdata, handles)

But when I click the mouse in the figure, I get "Undefined function 'ButtonDown' for input arguments of type 'double'. Error while evaluating figure WindowButtonDownFcn".

What is the right function prototype for this callback?

Answer 1

[Geoff Hayes]

Luc - the correct signature for this function would be

 function ButtonDown(hObject, eventdata)

since you are not supplying any additional parameters when you assign this callback to the WindowsButtonDownFcn

 set(hfig,'WindowButtonDownFcn',@ButtonDown);

However, I suspect that isn't the source of the error. Where is this function, ButtonDown, defined relative to your function that assigns it as a callback? Are they both in the same file?

[Geoff Hayes]

Luc's answer moved here

Both functions were in same .m file, but the callback was defined after its invocation. I fixed the prototype and moved it up and it now works. Thanks!

If I want to have access to handles in the callback, what must I change in the invocation?

[Geoff Hayes]

Are you using GUIDE to create your GUI or are you programmatically creating a GUI? I think the latter and so handles must be a local variable in your main script. If so, then you can nest ButtonDown callback in your main function so that it has access to the local variables defined there. For example,

 function myMainFunc

     % some code

     % create the figure
     handles.hFig = = figure('Position',rect, ...);

     % assign the callback
      set(hfig,'WindowButtonDownFcn',@ButtonDown);

      % nest the callback
      function ButtonDown(hObject, eventdata)
          % make use of handles
          get(handles.hFig);  % or whatever
      end
 end

[Geoff Hayes]

Luc's answer moved here

Yes, the figure is created programmatically.

Thanks for the answer.

[Rik]

If you are not nesting functions, you can use guidata as well to access the handles struct.