How do I determine which Locator is selected in a LocatorPane with multiple Locators?

Question

This is a cross-post of http://community.wolfram.com/groups/-/m/t/1186782

I am working on something where I need to know which locator out of many is currently being changed.

Here is some working code that uses the second argument to Dynamic to identify which locator is active for a limited set of Locators. I'd like to extend this to many Locators. In this code, the Locators are hard-coded. I'd like to extend this programmatically so I can slot in the Locators and the Manipulator controls. I've tried various combinations of Hold* without really knowing what I am doing.

Does anyone know how I can achieve my goal of finding which is the active locator?

See some additional comments below the working code about an attempt to do this with LocatorPane.

DynamicModule[
 {active = 0, p},
 Manipulate[
  Column[
   {active,
     Deploy@Graphics[
      {
       {
        Locator@
         Dynamic[p[1, 1, 
           1], {((p[1, 1, 1] = {-1, Last[#]}; 
              active = {1, 1, 1}) &), ((active = 0) &)}],
        Locator@
         Dynamic[p[1, 1, 
           2], {((p[1, 1, 2] = #; 
              active = {1, 1, 2}) &), ((active = 0) &)}],
        Locator@
         Dynamic[p[1, 1, 
           3], {((p[1, 1, 3] = {1, Last[#]}; 
              active = {1, 1, 3}) &), (active = 0) &}]
        },
       Line[{p[1, 1, 1], p[1, 1, 2], p[1, 1, 3]}] 
       },
      PlotRange -> 2
      ]
    }
   ],
  {{p[1, 1, 1], {-1, -1}}, ControlType -> None},
  {{p[1, 1, 2], {0, 0}}, ControlType -> None},
  {{p[1, 1, 3], {1, 1}}, ControlType -> None}
  ]
 ]

Trying to do the same thing with a LocatorPane and Nearest. It is not clear to me that the 2 argument form of Dynamic is working here:

DynamicModule[{active = False, pt1 = {1, 1}/2, pt2 = {-1, 1}/2, 
  pt3 = {1, -1}/2},
 Row[
  {active,
   LocatorPane[
    {
     Dynamic[
      pt1, {((active = Nearest[pt1, #, 1][[1]]); 
         pt1 = {0, 1} #) &, ((active = 0) &)}],
     Dynamic[
      pt2, {((active = Nearest[pt2, #, 1][[1]]); 
         pt2 = #) &, ((active = 0) &)}]
     },
    Graphics[{Yellow, Disk[{0, 0}, 2]},
     PlotRange -> 2
     ]
    ]
   }
  ]
 ]

Answer 1

You may use MousePosition set to the "Graphics" of the LocatorPane with Nearest.

pts = {{1, 1}/2, {-1, 1}/2, {1, -1}/2};
selectedLocator = None;
LocatorPane[
 Dynamic[pts,
  {
   (selectedLocator = 
      FirstPosition[pts, First@Nearest[pts, MousePosition[{"Graphics", LocatorPane}]]]) &,
   Automatic,
   None
   }],
 Graphics[{Yellow, Disk[{0, 0}, 2]}, PlotRange -> 2],
 LocatorAutoCreate -> True
 ]

Dynamic[selectedLocator]

When the LocatorPane beings to update locator the MousePosition in the pane's "Graphics" coordinates will be used to calculate the index of the nearest locator in pts and store this position in selectedLocator. This calculation only occurs at the start of updating a locator which reduces the load on the system.

With LocatorAutoCreate set to True additional locators can be added by holding Alt and clicking on the LocatorPane.

selectedLocator can be set to None in the third item of the second parameter of Dynamic if needed; (selectedLocator = None)&.

Hope this helps.

Answer 2

Does this work for you?

DynamicModule[
 {pts = Array[pt, 5], active},
 Column@{
   Dynamic@active,
   Dynamic@pts,
   LocatorPane[
    Dynamic[
     pts,
     {
      (active = First@FirstPosition[pts - #, _?(# != {0, 0} &)]; pts = #)&,
      (active = None) &
     }
    ],
    Graphics[{Yellow, Disk[{0, 0}, 2]}, PlotRange -> 2]
    ]
   }
 ]

This simply finds the currently updating point by checking which value has changed.

Update

Seeing @user6014's solution, I thought I'd point out some issues I see with both apporaches:

• My approach fails if you don't actually change the position of the locator (you can do this by clicking once to get the locator directly under your cursor and then clicking again without moving the mouse)
• @user6014's approach fails if you manage to put two locators on top of each other (or at least it might not find the correct locator)

Answer 3

Does this work for you?

DynamicModule[{pts = {{-1, 1}/2, {1, 1}/2}, a = False, 
  pointNames = Table["Point " <> ToString@i, {i, 10}]},
 EventHandler[
  {LocatorPane[Dynamic[pts], Framed@Graphics[{}], 
    LocatorAutoCreate -> True], Dynamic[a]}
  ,
  {"MouseUp" :> {a = "Null"}, 
   "MouseDragged" :> {a = 
      pointNames[[Position[pts, 
          Nearest[pts, MousePosition["Graphics"]][[1]]][[1, 1]]]]}}
  , PassEventsDown -> True]
 ]

This simply finds the point closest to your current mouse position when pressed down.

Use Alt+Click to create new locators.