Change color of polylines on it's click in flutter

Here is how you can change the Polyline color on tap:

First, you will need to define your Polyline object as a collection of key/value pairs using the Map<> class, this will help you later on identifying which polyline is being clicked/tapped on, specially if you have many Polyline objects.

  Map<PolylineId, Polyline> _polylines = <PolylineId, Polyline>{};

  var pointArray; // var to hold the PolylineOverview
  int _polylineIdCounter = 1; // polylineId that you will use for your polylines

Then, you can set your polylines like this:

void _setPolyline() {
   pointArray = myPoints.decode(overviewPolylinePointsExample); // use PolyUtil(); to decode polylines

   final String polylineIdVal = 'marker_id_$_polylineIdCounter';
   _polylineIdCounter++;
   final PolylineId polylineId = PolylineId(polylineIdVal);

   final Polyline polyline = Polyline(
       polylineId: polylineId,
       consumeTapEvents: true, // Set to true to make polylines recognize tap events
       points: pointArray,
       color: Colors.red,
       onTap: () {
         _handlePolylineTap(polylineId); // function that will handle the color change and will be triggered when the polyline was tapped
    });

   setState(() {
     _polylines[polylineId] = polyline; // Add the polyline to your collection
   });
}

From the above code, consumeTapEvents is set to true to make polylines recognize tap events. Then, on the onTap method you will need a function that will handle the changing of color which should be like this:

_handlePolylineTap(PolylineId polylineId) {
    setState(() {
       final Polyline newPolyline =
            _polylines[polylineId].copyWith(colorParam: Colors.blue); // create a new polyline object which has a different color using the colorParam property
       _polylines[polylineId] = newPolyline; // add that new polyline object to the list
     });
}

Lastly, you can add your polyline list to the GoogleMap() widget like this:

GoogleMap(
    ...
    polylines: Set<Polyline>.of(_polylines.values),
    ...
),