A problem with Deploy and Locator in a Manipulate

Question

I recently asked a question about Locators not behaving properly (see Locators and Table within a Manipulate are not behaving) and thought I had figured out the answer. However, as I played around to figure out why the problem existed in the first place I discovered that using Deploy caused the problem to return. Here is as simple of an example as I can come up with that reproduces the problem and maintains some of the features that I need in the Manipulate. Particularly, I need to link the two sliders with the Locator. The following works fine:

Manipulate[
 vector = Graphics[{Green, Arrow[{{0, 0}, p}], 
    Locator[Dynamic[p, (p = #; x = p[[1]]; y = p[[2]]) &]]}]; 
 Show[{vector}, PlotRange -> {{-2.1, 2.1}, {-2.1, 2.1}}, 
  ImageSize -> 500], 
 Row[{"Ax", Manipulator[Dynamic[x, (x = #; p[[1]] = x) &], {-2, 2}], 
   Spacer[4], Dynamic[x]}], 
 Row[{"Ay", Manipulator[Dynamic[y, (y = #; p[[2]] = y) &], {-2, 2}], 
   Spacer[4], Dynamic[y]}],
 {{p, {1, 1}}, None},
 {{x, 1}, None},
 {{y, 1}, None},
 TrackedSymbols -> {x, y, p}]

You get a green arrow that you can move with either the Locator or the Sliders (one component at a time).

However, if the user clicks slightly off of the Locator they highlight the whole graphics image and go into an editing mode. Users unfamiliar with this may not know what to do so I want to prevent this by using Deploy.

Adding Deployed->True as an option to the Manipulate doesn't work. The documentation for Manipulate says that it should, but for some reason it does nothing here.

So instead I wrapped Deploy around Show:

 Deploy@Show[{vector}, PlotRange -> {{-2.1, 2.1}, {-2.1, 2.1}}, 
  ImageSize -> 500],

Now the user doesn't accidentally go into editing mode, but the Locator doesn't work. The Sliders do move the vector around and the Locator moves with them, but I can't move the locator with the mouse except for a VERY small distance at a time.

I'm wondering if this might have something to do with how I have linked the various control variables using the second argument of Dynamic. Perhaps the solution to this is also ultimately the solution to the question I referenced above. Thanks!

Answer 1

This seems to work:

Manipulate[
 DynamicModule[{vector},
  vector = 
   Graphics[{Green, Arrow[{{0, 0}, Dynamic[p]}], 
     Locator[Dynamic[p, (p = #; x = p[[1]]; y = p[[2]]) &]]}];
  Deploy@Show[vector,
    PlotRange -> {{-2.1, 2.1}, {-2.1, 2.1}}, ImageSize -> 500]],

 Row[{"Ax", Manipulator[Dynamic[x, (x = #; p[[1]] = x) &], {-2, 2}], 
   Spacer[4], Dynamic[x]}], 
 Row[{"Ay", Manipulator[Dynamic[y, (y = #; p[[2]] = y) &], {-2, 2}], 
   Spacer[4], Dynamic[y]}],
 {{p, {1, 1}}, None},
 {{x, 1}, None},
 {{y, 1}, None}, TrackedSymbols -> {x, y, p}]

The only difference with the original code is that I've wrapped p with Dynamic in Arrow.

By the way, since p == {x,y}, you can actually replace p with {x, y} making the code a bit more elegant in this case:

Manipulate[
 DynamicModule[{vector},
  vector = 
   Graphics[{Green, Arrow[{{0, 0}, Dynamic[{x, y}]}], 
     Locator[Dynamic[{x, y}]]}];
  Deploy@Show[vector,
    PlotRange -> {{-2.1, 2.1}, {-2.1, 2.1}}, ImageSize -> 500]],
 {{x, 1, "Ax"}, -2, 2, Appearance -> "Labeled"},
 {{y, 1, "Ay"}, -2, 2, Appearance -> "Labeled"}]

Answer 2

The answer of Heike fixes your problem. I want to add, that your use of Manipulate is rather unusual, because like you did it, it would be more appropriate to use DynamicModule. This is because the documentation an all tutorials suggest, that Manipulate is for not too complex situations. It can be used there, but the main area of Manipulate are situations where you display something and control it with Sliders,... which are give as separate argument to Manipulate.

As Heike showed, there is often a way to make it work too, but the mixture of display and control in your code, was what led to the error in the first place. Let me explain this in more detail and start with a very simple example. This

Manipulate[Deploy[Graphics[{Arrow[{{0, 0}, p}], 
         Locator[Dynamic[p]]}, PlotRange -> 2]], 
   {{p, {1, 1}}, None}]

shows the same behavior you described and I believe it is, because Manipulate cannot handle the control inside the Graphics when it is combined with Deploy and the controlled variable appears in the graphics without Dynamic. Usually, no explicit Dynamic is needed inside Manipulate for such a simple situation. If you remove the Deploy it works. Try this the far more simpler version

Manipulate[Deploy[Graphics[{Arrow[{{0, 0}, p}]}, 
       PlotRange -> 2]], {{p, {1, 1}}, Locator}]

and you'll see, that it works without complaint, although we used p without Dynamic. Let's see whether we can adapt this to your piece of code. You will instantly object, that you need the special set-function inside your Locator to ensure, the sliders are moved with the locator. But look, you did the similar thing with vector: you just used = and it got dynamically updated:

Manipulate[
  vector = Graphics[{Green, Arrow[{{0, 0}, p}]}]; 
  {x, y} = p;
  Deploy[Show[{vector}, PlotRange -> {{-2.1, 2.1}, {-2.1, 2.1}}, 
        ImageSize -> 500]], 
  Row[{"Ax", Manipulator[Dynamic[x, (x = #1; p[[1]] = x) & ], {-2, 2}], 
       Spacer[4], Dynamic[x]}], 
  Row[{"Ay", Manipulator[Dynamic[y, (y = #1; p[[2]] = y) & ], {-2, 2}], 
       Spacer[4], Dynamic[y]}], 
  {{p, {1, 1}}, Locator}, 
  {{x, 1}, None}, 
  {{y, 1}, None}
]

This works as you would have expected it in the first place. Since you need an explicit Manipulator to make the x and y sliders work and since the {x,y}=p is not really nice, you should write a DynamicModule which is in my opinion the simpler version and clearer solution here:

DynamicModule[{p = {1, 1}, x = 1, y = 1},
 Panel[Column[{
    Row[{"Ax", Spacer[4], 
      Manipulator[Dynamic[x, (x = #1; p[[1]] = x) & ], {-2, 2}], 
      Spacer[4], Dynamic[x]}], 
    Row[{"Ay", Spacer[4], 
      Manipulator[Dynamic[y, (y = #1; p[[2]] = y) & ], {-2, 2}], 
      Spacer[4], Dynamic[y]}],
    LocatorPane[Dynamic[p, (p = #; x = p[[1]]; y = p[[2]]) &],
     Graphics[{Green, Arrow[{{0, 0}, Dynamic[p]}]}, 
      PlotRange -> {{-2.1, 2.1}, {-2.1, 2.1}}, 
           ImageSize -> 500, Background -> White]
     ]
    }]
  ]
 ]

Note, that in all above examples I used the pieces of your code to make it more clear what I changed and what I put where. It is not necessary to use p and x,y and therefore a shorter version is

DynamicModule[{x = 1, y = 1}, 
  Panel[Column[{
    Row[{"Ax", Spacer[4], Manipulator[Dynamic[x], {-2, 2}], 
      Spacer[4], Dynamic[x]}], 
    Row[{"Ay", Spacer[4], Manipulator[Dynamic[y], {-2, 2}], 
      Spacer[4], Dynamic[y]}], 
    LocatorPane[Dynamic[{x, y}], 
      Graphics[{Green, Arrow[{{0, 0}, Dynamic[{x,y}]}]}, 
        PlotRange -> {{-2.1, 2.1},{-2.1, 2.1}}, 
        ImageSize -> 500, Background -> White]
    ]}]
  ]
]