Mouse Drag One Example

to setup
  clear-all
  ask n-of 30 patches [ sprout 1 ]
end 

to go
  if mouse-down? [
    let candidate min-one-of turtles [distancexy mouse-xcor mouse-ycor]
    if [distancexy mouse-xcor mouse-ycor] of candidate < 1 [
      ;; The WATCH primitive puts a "halo" around the watched turtle.
      watch candidate
      while [mouse-down?] [
        ;; If we don't force the view to update, the user won't
        ;; be able to see the turtle moving around.
        display
        ;; The SUBJECT primitive reports the turtle being watched.
        ask subject [ setxy mouse-xcor mouse-ycor ]
      ]
      ;; Undoes the effects of WATCH.  Can be abbreviated RP.
      reset-perspective
    ]
  ]
end 


; Public Domain:
; To the extent possible under law, Uri Wilensky has waived all
; copyright and related or neighboring rights to this model.