///withoutPrompt 
function plot3d_uicontrol()
  global('a_slider','a_text','b_slider','b_text','a','b','activate','n_fact');
  if activate == %t then 
    a=get(a_slider,'Value')/n_fact;
    b=get(b_slider,'Value')/n_fact;
    set(a_text,'String','a='+string(a))
    set(b_text,'String','b='+string(b))
    t=linspace(0,2,200);
    xbasc();t=linspace(-b*%pi,b*%pi,20);plot3d(t,t,a*sin(t)'*cos(t));
  end
endfunction

function myuidialog()
  global('a_slider','a_text','b_slider','b_text','a','b','activate','n_fact');
  n_fact=100; /// \sleftarrow{\normalfont scaling because sliders work with integers }
  activate=%f ;  /// \sleftarrow{\normalfont delay callback execution}
  f = figure("Position",[50 50 200 60],"Unit", "pixel");

  a_text = uicontrol(f,"Position",[140 10 55 15],"Style","text",...
                     "String"    , "a=5")

  a_slider = uicontrol(f, "Position"  , [10 10 120 15],..
                       "Style"     , "slider",...
                       "Min"       , 1*n_fact,...
                       "Max"       , 10*n_fact,...
                       "Value"     , 5*n_fact,...
                       "SliderStep", [1 1],...
                       "callback"  , "plot3d_uicontrol()")

  b_text = uicontrol(f, "Position",[140 35 55 15],"Style","text",...
                     "String"    , "b=2")

  b_slider = uicontrol(f, "Position"  , [10 35 120 15],...
                       "Style"     , "slider",...
                       "Min"       , 1*n_fact,...
                       "Max"       , 3*n_fact,...
                       "Value"     , 2*n_fact,...
                       "SliderStep", [1 1],...
                       "callback"  , "plot3d_uicontrol()")
  activate=%t; /// \sleftarrow{\normalfont now we accept callback calls}
endfunction ///\withPrompt{}