Trackball inside the slider was added

[diegoveloper]

Based on this issue #1 it's interesting to have a visual indicator apart from the curve of the line to have more feedback about the current position.

I created a new property for WaveSlider , it's a bool value displayTrackball (false by default).

Also I created a sample project inside the current project (just as all packages have)

This is a demo

Usage

  WaveSlider(
            displayTrackball: true,
            onChanged: (double dragUpdate) {
              setState(() {
                _dragPercentage = dragUpdate *
                    100; // dragUpdate is a fractional value between 0 and 1
              });
            },
          ),

[funwithflutter]

Great job. Like the changes!