SwiftUI Map 使用 MapPolyline 无法显示

参考:SwiftUI Map MapPolyline not showing

原因是没有为 Polyline 设置颜色：

MapPolyline(coordinates: deviceLocations)
     .stroke(.blue, lineWidth: 2.0)

或者

struct CustomShapeStyle: ShapeStyle {
    func resolve(in environment: EnvironmentValues) -> some ShapeStyle {
        if environment.colorScheme == .light {
            return Color.blue.blendMode(.lighten)
        } else {
            return Color.blue.blendMode(.darken)
        }
    }
}
...
MapPolyline(coordinates: deviceLocations)
     .stroke(CustomShapeStyle(), lineWidth: 2.0)