title: Flutter: Disabling Multi-Touch/Drag with Gesture Detector
body: <p>Im currently building a Slider widget which multiple handle and am currently implementing the Gesture Detector. I am having an issue where if you touch/drag the screen with a second finger it will be recognised by the detector and call the onDragUpdate function, which is what I am trying to disable - whilst one finger is dragging the handle around, the gesture detector shouldn't call updates for a second finger but should still allow the first finger to call the updates.</p>
  
  <p>For example, if you're sliding the handle around with finger 1 (fine) and then add a second finger, finger 2, the gesture detector should only recognise finger 1 even though both fingers are on screen.</p>
  
  <p>I understand there is a function in the Listener class which allows you to specify a device (using details.device) but is there a way to achieve this using the Gesture Detector class?</p>
  
  <p>Currently looking into different implementers for the Gesture Detector, including looking at this answer <a href="https://stackoverflow.com/a/56037327/12147590">https://stackoverflow.com/a/56037327/12147590</a> but i'm still struggling. If anyone could point me in the right direction or explain the answer further in simple terms for me that would be great, cheers :)</p>