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I am attempting to set an EventHandler to change the radius of a circle by dragging on its circumference. But the response to the dragging is terribly slow. What is wrong with the code?

cnt[p_] := Return[Round /@ p]; grids[min_, max_] := Join[Range[Ceiling[min], Floor[max]], Table[{j + 1, Lighter@Lighter@Lighter@Lighter@Blue}, {j, Round[min], Round[max - 1], 1}]]; DynamicModule[{pnt = {1, 1}, VPOS = {1, 1}, rad = 1}, Dynamic@Graphics[ {Thick, EventHandler[ Circle[ cnt[pnt] , rad], { "MouseDragged" :> ( VPOS = MousePosition["Graphics"]; rad = EuclideanDistance[pnt, VPOS]; ) }, PassEventsDown -> True ], Locator[Dynamic[pnt], None] }, Axes -> True, GridLines -> grids, PlotRange -> {{-10, 10}, {-10, 10}}] ]

enter image description here

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  • $\begingroup$ Is EventHandler a need? Using Locator might be easier. $\endgroup$ Commented Aug 1, 2014 at 10:42
  • $\begingroup$ Yes, I am trying to test how to correctly set an EventHandler. $\endgroup$ Commented Aug 1, 2014 at 10:50
  • $\begingroup$ It's probably slow because of the EuclideanDistance. $\endgroup$ Commented Aug 1, 2014 at 10:56
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    $\begingroup$ Is it again the problem of out of focus? Seems it only response when the mouse is right on the top of the circumference. Is there a way of work-around? $\endgroup$ Commented Aug 1, 2014 at 11:06
  • $\begingroup$ Can you clarify what you are trying to do? I can move the circle when I click the mouse on its center, and when I mouseclick on the circumference, the size changes. Are you interested in being able to both move the circle and change its size with the mouse? $\endgroup$ Commented Aug 1, 2014 at 11:38

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The front end's event handling mechanism seems to have a hard time deciding which event should be passed to the locator. Therefore, I think it's best to dispense with the locator. This seems to work fairly well:

cnt[p_] := Return[Round /@ p]; grids[min_, max_] := Join[ Range[Ceiling[min], Floor[max]], Table[{j + 1, Lighter @ Lighter @ Lighter @ Lighter @ Blue}, {j, Round[min], Round[max - 1], 1}]]; DynamicModule[{pnt = {1, 1}, VPOS = {1, 1}, rad = 1, d}, EventHandler[ Dynamic @ Graphics[{{PointSize[Medium], Point[cnt[pnt]]}, {Thick, Circle[cnt[pnt], rad]}}, Axes -> True, GridLines -> grids, PlotRange -> {{-10, 10}, {-10, 10}}], {"MouseDragged" :> ( VPOS = MousePosition["Graphics"]; d = EuclideanDistance[pnt, VPOS]; If[d < 1, pnt = VPOS, rad = d])}]]

graphics

I made the center point visible as an aid to dragging the circle from grid point to grid point. I also need to point out that, when adjusting the radius, it is best to drag from just outside the circle.

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  • $\begingroup$ It's much better than the OP's answer indeed, but moving the centre too fast catches the circle sometimes.. $\endgroup$ Commented Aug 1, 2014 at 14:22
  • $\begingroup$ @Öskå. Yeah, that's why I wrote "This seems to work fairly well" rather than "This seems to work well". It does meet the requirement of moving the circumference smoothly. $\endgroup$ Commented Aug 1, 2014 at 14:26

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