Creating a slider that instead of setting, increases/decreases a variable

Question

The objective: Create a slider that, when displaced from the center, increases-decreases the value of another variable at a rate proportional to the displacement.

So, I create a slider that automatically comes back to the center

Slider[Dynamic[var, {(var = #) &, (var = 0.) &}], {-1, 1}]

Now I want a second variable to be dynamically increased/decreased when var!=0.

My ideas are:

1. insert in the previous dynamic, something that does the updating
2. use an animator with its speed set by var when var!=0. that updates the value

Problems with the above:

1. How to control the speed of the updating given that I can't know for sure the intervals in which val will be updated, and UpdateIntervals sets an upper bound?
2. How to make it work without it showing in the screen?

Any other alternatives are welcome (ideally I'd like an elegant solution).

Answer 1

Maybe you could do something like this,

var = 0.; p = 0.; interval = .2;
task = RunScheduledTask[p += var, interval];
{Slider[Dynamic[var, {(var = #) &, (var = 0.) &}], {-1, 1}], Dynamic[p]}

The downside is that rather than increasing p by a fixed amount at variable time intervals, this increases p by a variable amount at a fixed frequency. Also, you need to remember to remove task (using RemoveScheduledTask) once you're done with it.

Answer 2

Here's another way using Refresh that's perhaps a bit cleaner:

DynamicModule[{x = 0, y = 0},
    {Slider[Dynamic[x, {(x = #) &, (x = 0.) &}], {-1, 1}], 
     Dynamic[Refresh[y += x, TrackedSymbols :> {}, UpdateInterval -> 0.1]]}
]