# This example draws a green ball.  You must click on the canvas to
 # start it moving. click again to stop it.

 # Copyright 2005 Boley

 use strict; use warnings;

 do 'gdraw.pl';

 ## build all the basic objects.
 my $circle=fill_oval(20,40,60,80,'green');  ## the ball
 stall();                                    ## draw it now.
 my @CanvasDim=get_width(0);                 ## where's the right wall?
 my $counter = 0;                            ## count the iterations
 my $paused = 1;                             ## global 'paused' state.

 sub move_ball {                             ## move the ball 1 step
     my @position = get_coords($circle); 
     $position[0] += 2;
     $position[2] += 2;
     if ($position[2] >= $CanvasDim[0])      ## check for wrap-around
          { $position[0]=20;   $position[2]=60   };
     set_coords($circle,@position);
     print "\r$counter    ";
     $counter++;
     stall();
     return @position }
 
 sub Move_Loop {                            ## Outer loop to move ball
     ## The 'Move_Loop' procedure checks the paused state, moves the ball, 
     ### and finally repeats itself after 50 msec.
     if ($paused) { print "paused\n";  return  };
     move_ball() ;
     after(50,\&Move_Loop) };

 sub toggle { $paused = not $paused ; Move_Loop() }  ## toggle 'paused' state.
     ## toggle state, then try main task
 
 set_binding(0,\&toggle);                   ## what to do upon mouse click.

 my $textbox = draw_text('Click here to start and stop',200,300);
 
 MainLoop();                                ## Tell Tk to start processing.