Restrict Sensitivity of Locators in LocatorPane

Question

We can restrict the movement of locators in a LocatorPane as follows:

In the following example, the first locator's movement is confined to the x-axis and the second locator's movement is confined to the y-axis.

Manipulate[LocatorPane[Dynamic[pts], 
 Graphics[{}, PlotRange -> max, Axes -> True, ImageSize -> 350],
 {{{-max, 0}, {max, 0}},
 {{0, -max}, {0, max}}}],
{{pts, {{9, 0}, {0, 11}}}, ControlType -> None}, Initialization :> {max = 25}]

However, a locator can be dragged without necessarily being selected. You can begin dragging a locator by clicking down on just about any point in the four quadrants and dragging from there.

I would like to have the locator respond only to drags that are initiated by "mousedown"ing on the locator itself.

Any ideas as to how I might achieve this?

---

Addendum In my the more complicated example I've been working with, I've made use of jump parameters to confine locators to integer values:

{{{-maxX, 0}, {maxX, 0}, {1, 0}},
{{0, -maxY}, {0, maxY}, {0, 1}}}

I'm not sure whether such constraints (to integers) can be imposed on locators outside of LocatorPanes.

Answer 1

That is the default (and expected) behaviour of LocatorPane. This is useful in implementing things like colour pickers, for example, where it is convenient to simply click on any point to select that colour and have the locator move there automatically to indicate selection.

To create locators that move only when explicitly clicked and dragged, use a Locator. Here's an example for the scenario in the question:

DynamicModule[{pt1 = {-5, 0}, pt2 = {0, 5}},
    With[{max = 25},
        Graphics[{
            Locator[Dynamic[pt1, (pt1 = {First@#, 0}) &]], 
            Locator[Dynamic[pt2, (pt2 = {0, Last@#}) &]]
            }, PlotRange -> max, Axes -> True, ImageSize -> 350
        ]
    ]
]

Answer 2

This is the bare bone implementation with direct control of mouse events:

DynamicModule[{p = {0, 0}}, EventHandler[Framed@Dynamic[Style[
  Graphics[{Red, Disk[p, 0.2]}, PlotRange -> 2], Selectable -> False]], 
      {"MouseDragged" :> (p = MousePosition["Graphics"])}]]

And this is more or less what you need:

DynamicModule[{p1 = {0, 2}, p2 = {2, 0}}, EventHandler[
  Dynamic[Style[Graphics[{{Red, Disk[{0, Round@p1[[2]]}, .2]}, {Blue, 
       Disk[{Round@p2[[1]], 0}, .2]}}, PlotRange -> 5, Axes -> True], 
    Selectable -> False]],{"MouseDragged" :> (If[
       EuclideanDistance[p1, MousePosition["Graphics"]] < 
        EuclideanDistance[p2, MousePosition["Graphics"]], 
       p1 = MousePosition["Graphics"], p2 = MousePosition["Graphics"]];)}]]

It is a bit hacky, but because of integer settings it works.