Calculating the bearing to a coordinate/location from another coordinate/location in Dart using Math for Compass.

compass_bearing.dart

//This function takes two sets of values, say Coordinates and returns the angle bearing to the location.
//a1 and a2 being latitude and longitude of location a
//b1 and b2 being latitude and longitude of location b
double bearing(double a1, double a2, double b1, double b2) {
  const double TWOPI = 6.2831853071795865;
  const double RAD2DEG = 57.2957795130823209;
  // if (a1 = b1 and a2 = b2) throw an error
  //double theta = atan2(b1 - a1, a2 - b2);
  double theta = math.atan2(b1 - a1, a2 - b2);
  if (theta < 0.0)
    theta += TWOPI;
  return RAD2DEG * theta;
}

//Now when you want to use it in pointing/rotating your compass to a direction, you may use the Transform widget
Transform.rotate(
  angle: ((_direction ?? 0) * (math.pi / 180) * -1) - (_adjustmentDirection / 180), //Divide the bearing value with 180 and subtract it from the compass value.
  child: new Image.asset('lib/assets/qibla-compass.png', width: 250,),
)