Remotely change the value of any number slider in Grasshopper with C#

The slider in Grasshopper doesn’t have an input. That means that only a user with a mouse can change it, right?

A slider with some kind of input is an ongoing topic on the Grasshopper forum. I know myself I’ve been in the position many times where I’ve defined a number with a slider, only to realise I later need Grasshopper to be changing that number for me. In lieu of an updated slider, I have come up with a solution…

Following on from my earlier post on adding a coloured progress bar to Grasshopper sliders, it became apparent that it’s in fact very easy to change the values of a slider with a bit of code. Watch the video below to see what I mean:

The only hard bit is figuring out how to ‘connect’ to the slider you want to modify. Basically, we look at the objects connected to our C# component, and try to convert them to a slider. If this is successful, then we know we have a slider, and we can start changing its properties.

Modify the slider value

This is the same code as what was in the video above. It simply reads in a value, finds a slider connected to x, and changes the ‘x’ slider to the ‘y’ value.

  private void RunScript(object x, double y, ref object A)
  {
    var input = Component.Params.Input[0].Sources[0]; //get the first thing connected to the first input of this component
    var slider = input as Grasshopper.Kernel.Special.GH_NumberSlider; //try to cast that thing as a slider

    if(slider != null) //if the component was successfully cast as a slider
    {
      slider.SetSliderValue((decimal) y);
    }
  }

Change the slider min and max values

Changing the minimum and maximum values on the slider is also easy. To do this, we need to dig into the slider a little more. Inside slider.Slider, there are lots more hidden properties which are fun to play with:

  private void RunScript(object x, double y, ref object A)
  {
    var input = Component.Params.Input[0].Sources[0]; //get the first thing connected to the first input of this component
    var slider = input as Grasshopper.Kernel.Special.GH_NumberSlider; //try to cast that thing as a slider

    if(slider != null) //if the component was successfully cast as a slider
    {
      slider.Slider.Minimum = 0;
      slider.Slider.Maximum = y;
    }
  }

Change the appearance of a slider

There are lots of options hidden away for changing the appearance of a slider – my last post being one, adding progress bar by changing the slider’s ‘rail’.

There are lots of further options for changing the background colour, adding a border, or adding shadows.

  private void RunScript(object x, double y, ref object A)
  {
    var input = Component.Params.Input[0].Sources[0]; //get the first thing connected to the first input of this component
    var slider = (Grasshopper.Kernel.Special.GH_NumberSlider) input; //try to cast that thing as a slider

    if(slider != null) //if the component was successfully cast as a slider
    {
      slider.Slider.DrawControlBackground = true;
      slider.Slider.DrawControlBorder = true;
      slider.Slider.ControlEdgeColour = Color.Blue;
      slider.Slider.ControlBackColour = Color.Aquamarine;
    }
  }

However, like changing the rail type, these changes are always lost when a user manually changes the slider. Obviously, what’s happening in the background is there is a separate process being called to ‘clean’ the slider whenever the slider value is changed via the UI. Finding it is a challenge though. If anyone knows the solution, I’d be glad to hear!