Updating a Dynamic variable based on another variable's change

Question

In a dynamic interface, I am using a custom replacement for InputField that tracks two things separately:

1. The value of the variable that is input.
2. The boxes that comprise the user-entered expression.

This way, the user-entered formatting is kept (e.g., scientific notation), incomplete entry is handled smoothly, and the boxes can be initialized to a given number of digits.

Here is a pared-down version:

holdIF[Dynamic[var_], Dynamic[boxes_]] := 
  DynamicModule[{updateFunction}, 
    If[boxes == Null, 
       boxes = ToBoxes[SetPrecision[var, 5]]]; 
    InputField[Dynamic[boxes, {Temporary, updateFunction}], Boxes], 
      Initialization :> (SetAttributes[updateFunction, HoldRest]; 
        updateFunction[new_, boxVar_] := 
          With[{expr = Quiet@ToExpression@new}, boxes = new; 
            If[expr =!= $Failed, var = expr]
          ]
       )
  ]

And here is some test code showing it at work.

testvar = 1.23456789;
testboxes = Null;
holdIF[Dynamic[testvar], Dynamic[testboxes]]
Dynamic[{NumberForm[testvar, \[Infinity]], testboxes}]

However, the value of the variable can be modified elsewhere in the dynamic interface. The idea is that the variable are estimates for parameters in nonlinear model fitting; the user enters values and looks at the result in a dynamically-updating plot, and when a suitable starting-point has been found, the user clicks a Button to initiate an automatic search for a better fit. I therefore want the values displayed in the InputField boxes to update based on changes to the variable parameters. What would be really nice is if I could signal that the holdIF function should update its box representation by simply setting the boxes variable to Null.

This works if I wrap the holdIF call in a Dynamic wrapper, because that then regenerates the DynamicModule when testboxes changes.

testvar = 1.23456789;
testboxes = Null;
Dynamic[holdIF[Dynamic[testvar], Dynamic[testboxes]]]
Dynamic[{NumberForm[testvar, \[Infinity]], testboxes}]

But it really seems that this shouldn't be necessary. Both var and boxes are Dynamic in the call, so shouldn't there be a way within the holdIF function to notice that boxes has been set to Null and as a result reinitialize var without having to redraw the InputBox? Thoughts?

Answer 1

Pardon me if I miss the point, as this looks rather complicated but I'm only giving it a quick look-over. Does it work to move your first If expression inside the Dynamic as follows?

holdIF[Dynamic[var_], Dynamic[boxes_]] := DynamicModule[{updateFunction},
  InputField[
   Dynamic[If[boxes === Null, boxes = ToBoxes[SetPrecision[var, 5]]]; 
    boxes, {Temporary, updateFunction}], Boxes], 
  Initialization :> (SetAttributes[updateFunction, HoldRest];
    updateFunction[new_, boxVar_] := 
     With[{expr = Quiet@ToExpression@new}, boxes = new;
      If[expr =!= $Failed, var = expr]])]