Building a calculator, confused with GUI ButtonDownFct

Hello, I am building a calculator from scratch and Im having trouble with the ButtonPressFct. I am trying to make it so I store a variable ONLY IF you press a certain button. In this case, Im trying to do it for the '+' button. Heres what I have and it is not working

 function togglebutton12_Callback(hObject, eventdata, handles)
% hObject    handle to togglebutton12 (see GCBO)
% eventdata  reserved - to be defined in a future version of MATLAB
% handles    structure with handles and user data (see GUIDATA)

 % Hint: get(hObject,'Value') returns toggle state of togglebutton12

 global n1
if (get(handles.text1,'ButtonDownFct') ~= 'on'
    n1 = get(handles.text1,'String');
else
    n1 = 0;
end
textString = get(handles.text1,'String');
textString = '';
set(handles.text1,'String',textString)