Usage Examples for DynamicWrapper

Suppose we have a Graphics object which depends on some parameters and a controller with which we want to control these parameters. This could be done easily enough using the second argument of Dynamic, for example

gr[pts_, col_, radius_] := Graphics[{col, Disk[#, radius] & /@ pts}, 
   PlotRange -> {{0, 3}, {0, 3}}, ImageSize -> 200];

contrl = {1, radius};
col = Blue; radius = .1;
pts = RandomReal[3, {10, 2}];

Grid[{{Dynamic[Framed[gr[pts, col, radius]]], 
  Slider2D[Dynamic[cntrl, 
   (cntrl = #; col = Blend[{Red, Blue}, cntrl[[1]]]; radius = cntrl[[2]])&]]
}}]

However, suppose that we also want a switch which switches the coupling between the controller and the plot on and off. With DynamicWrapper this can be done by doing something like this

Grid[{{Dynamic[Framed[gr[pts, col, radius]]], 
  Slider2D[Dynamic[cntrl]], 
  Toggler["Off", {DynamicWrapper["On", 
      col = Blend[{Red, Blue}, cntrl[[1]]]; radius = cntrl[[2]]], 
     "Off"}]
 }}]

By clicking on the label of the slider you can then toggle between coupling or no coupling. The same effect can be achieved without using DynamicWrapper for example

DynamicModule[{state = "Off"}, 
  Grid[{{Dynamic[Framed[gr[pts, col, radius]]], 
    Labeled[Slider2D[Dynamic[cntrl,(cntrl = #; 
         If[state === "On", col = Blend[{Red, Blue}, cntrl[[1]]]; 
         radius = cntrl[[2]]]) &]], 
     Toggler[Dynamic[state], {"On", "Off"}], Top]
  }}]
]

but imho the DynamicWrapper solution is more elegant in this case.

How about a 'Next page' button that becomes active only if the user has seen or at least scrolled to a particular part of a page, a disclaimer for instance?

---

EDIT
As requested: this was what I had in mind

texts = ExampleData["Text"];
i = 1;
imax = texts // Length;
bottomSeen = False;
Panel[
 Column[
  {
   Button["Next page", If[i <= imax, i++, i = 1]; bottomSeen = False;,
     Enabled -> Dynamic[bottomSeen]],
   Dynamic[
    Pane[
     Column[
      {
       ExampleData[texts[[i]]],
       ,
       DynamicWrapper["SEEN THIS", bottomSeen = True]
       }],
     ImageSize -> {500, 150}, Scrollbars -> True, 
     ScrollPosition -> {1, 1}
     ]
    ]
   }
  ]
 ]

There's one problem I didn't anticipate: a DynamicWrapper placed in a Pane is activated as soon as the Pane is visible even when the DynamicWrapper content is scrolled outside the visible window of the Pane. I haven't solved that yet.

It seems to me that it is not so much that something would be hard to implement without DynamicWrapper but that using that function offers an alternative and perhaps cosmetically/aesthetically better option.

My typical usage would be where I need to have an expression or compound expression dynamically evaluate (things that come to mind are evaluations that determine the list for a popup menu). Dynamic needs to display so you could just stick a spacer at the end of Dynamic:

DynamicModule[{a, b},

 Column[{
   Style["Heading", "Section"],
   Dynamic[b = 2 a + 1; Spacer[0]],
   Slider[Dynamic[a]],
   Dynamic[{a, b}]
   }]
 ]

or do something like this:

DynamicModule[{a, b},

 Column[{
   DynamicWrapper[Style["Heading", "Section"], b = 2 a + 1],
   Slider[Dynamic[a]],
   Dynamic[{a, b}]
   }]
 ]

but as per documentation this is the same as writing

DynamicModule[{a, b},

 Column[{
   Dynamic[b = 2 a + 1; Refresh[Style["Heading", "Section"], None]],
   Slider[Dynamic[a]],
   Dynamic[{a, b}]
   }]
 ]

So using DynamicWrapper is just a personal preference (IMO).