How to define a ButtonDownFcn function inside a GUI app ?

Hello All,

I have a gui at which I am drawing an image, named 'im', inside a UIAxes object.

I am able to define the ButtonDownFcn property, called fcn, of im:

im = imagesc(app.UIAxes2,Plot_Entities.WK{1},Plot_Entities.WK{2},Plot_Entities.WK{3});
im.ButtonDownFcn = @fcn;

and it indeed calls it whenever I click on im. but I am getting an error saying the following (although i defined fcn inside the gui code):

(( Undefined function 'fcn' for input arguments of type 'matlab.graphics.primitive.Image'.

Error while evaluating Image ButtonDownFcn. ))

which is solved when I define fcn outside the gui app.

my question here: can I define fcn inside the code of GUI app itself ? or it should always be defined in the path ?